Unsteady aerodynamic force mechanisms of a hoverfly hovering with a short stroke-amplitude Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 Hao Jie Zhu, Mao Sun
Hovering insects require a rather large lift coefficient. Many insects hover with a large stroke amplitude (120°-170°), and it has been found that the high lift is mainly produced by the delayed-stall mechanism. However, some insects hover with a small stroke amplitude (e.g., 65°). The delayed-stall mechanism might not work for these insects because the wings travel only a very short distance in a stroke, and other aerodynamic mechanisms must be operating. Here we explore the aerodynamic mechanisms of a hoverfly hovering with an inclined stroke plane and a small stroke amplitude (65.6°). The Navier-Stokes equations are numerically solved to give the flows and forces and the theory of vorticity dynamics used to reveal the aerodynamic mechanisms. The majority of the weight-supporting vertical force is produced in the mid portion of the downstroke, a short period (about 26% of the stroke cycle) in which the vertical force coefficient is larger than 4. The force is produced using a new mechanism, the “paddling mechanism.” During the short period, the wing moves rapidly downward and forward at a large angle of attack (about 48°), and strong counter clockwise vorticity is produced continuously at the trailing edge and clockwise vorticity at the leading edge, resulting in a large time rate of change in the first moment of vorticity, hence the large aerodynamic force. It is interesting to note that with the well known delayed stall mechanism, the force is produced by the relative motion of two vortices of opposite sign, while in the “paddling mechanism,” it is produced by generating new vortices of opposite sign at different locations.
Why do we live for much less than 100 years? A fluid mechanics view and approach Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-18 Gerasimos A. T. Messaris, Maria Hadjinicolaou, George T. Karahalios
Blood flow in arteries induces shear stresses on the arterial walls. The present work is motivated by the implications of low shear stress on the human arterial system and its effect on the duration of the life of a subject. The low and/or bidirectional wall shear stress stiffens the arterial wall and in synergy with the fluctuating tissue stress due to the fluctuating blood pressure activates the mechanism of aging. If the shear stress were not low and/or bidirectional and if it did not contribute to local endothelium dysfunctions, the tissue stress alone would take more than 100 yr to cause a failure on the human arterial system. Applying the s-n diagram (tissue stress against the number of cycles to failure) to determine the fatigue life of the aorta, for example, we find that in the absence of other pathogenic factors, for a tissue stress 1.2 times bigger than the tissue stress of a non-stiff aorta, the potential 100 yr of life are reduced to nearly 80 yr. Calculation of the rate of variation of the tissue stress of a subject with time may lead to a possible prognosis about the evolution of wall stiffness and its impact on the arterial aging of this subject. Further patient-specific in vivo mechanistic studies complemented by molecular imaging are needed to contribute to the formation of a data base, from which improved models describing the evolution of the arterial stiffness can be developed. Accordingly, the degree of stiffness of the aorta compared with existing data from a corresponding data base may provide with information about the degree of the fatigue of the aortic wall and its possible future behavior and lead to a patient-adapted medical treatment as a means of a would-be preventive medication.
Free convection flow of some fractional nanofluids over a moving vertical plate with uniform heat flux and heat source Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-01 Waqas Ali Azhar, Dumitru Vieru, Constantin Fetecau
Free convection flow of some water based fractional nanofluids over a moving infinite vertical plate with uniform heat flux and heat source is analytically and graphically studied. Exact solutions for dimensionless temperature and velocity fields, Nusselt numbers, and skin friction coefficients are established in integral form in terms of modified Bessel functions of the first kind. These solutions satisfy all imposed initial and boundary conditions and reduce to the similar solutions for ordinary nanofluids when the fractional parameters tend to one. Furthermore, they reduce to the known solutions from the literature when the plate is fixed and the heat source is absent. The influence of fractional parameters on heat transfer and fluid motion is graphically underlined and discussed. The enhancement of heat transfer in such flows is higher for fractional nanofluids in comparison with ordinary nanofluids. Moreover, the use of fractional models allows us to choose the fractional parameters in order to get a very good agreement between experimental and theoretical results.
Optimizing electroosmotic pumping rates in a rectangular channel with vertical gratings Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-03 Anison K. R. Lai, Chien-Cheng Chang, Chang-Yi Wang
The Helmholtz-Smoluchowski (H-S) velocity is known to be an accurate and useful formula for estimating the electro-osmotic (EO) flow rates in a simple micro-channel with a thin electric-double layer. However, in case the channel cross section is not so simple, the usefulness of H-S velocity could be sharply limited. A case of fundamental interest representing this situation is a rectangular channel (comprising parallel plates) with built-in vertical gratings, in which the surfaces inside the channel may develop different normalized zeta potentials α (on the gratings) and β (on the side walls). In this study, analytical solutions are pursued under the Debye-Hückel approximation to obtain EO pumping rates in a rectangular channel with vertical gratings. In particular, we identify the conditions under which the H-S formula can be properly applied and investigate how the EO flow rates may deviate from those predicted by the H-S velocity with varying physical parameters. Moreover, a diagram of the optimal EO pumping rates on the α-β plane is introduced that accounts for the general features of the analysis, which is consistent with a mathematical model and may serve as a convenient guide for engineering design and applications.
Effects of orthogonal rotating electric fields on electrospinning process Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 M. Lauricella, F. Cipolletta, G. Pontrelli, D. Pisignano, S. Succi
Electrospinning is a nanotechnology process whereby an external electric field is used to accelerate and stretch a charged polymer jet, so as to produce fibers with nanoscale diameters. In quest of a further reduction in the cross section of electrified jets hence of a better control on the morphology of the resulting electrospun fibers, we explore the effects of an external rotating electric field orthogonal to the jet direction. Through intensive particle simulations, it is shown that by a proper tuning of the electric field amplitude and frequency, a reduction of up to a 30% in the aforementioned radius can be obtained, thereby opening new perspectives in the design of future ultra-thin electrospun fibers. Applications can be envisaged in the fields of nanophotonic components as well as for designing new and improved filtration materials.
Cross-stream migration of a surfactant-laden deformable droplet in a Poiseuille flow Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-11 Sayan Das, Shubhadeep Mandal, Suman Chakraborty
The motion of a viscous deformable droplet suspended in an unbounded Poiseuille flow in the presence of bulk-insoluble surfactants is studied analytically. Assuming the convective transport of fluid to be negligible, we perform a small-deformation perturbation analysis to obtain the droplet migration velocity. The droplet dynamics strongly depends on the distribution of surfactants along the droplet interface, which is governed by the relative strength of convective transport of surfactants as compared with the diffusive transport of surfactants. The present study is focused on the following two limits: (i) when the surfactant transport is dominated by surface diffusion and (ii) when the surfactant transport is dominated by surface convection. In the first limiting case, it is seen that the axial velocity of the droplet decreases with an increase in the advection of the surfactants along the surface. The variation of cross-stream migration velocity, on the other hand, is analyzed over three different regimes based on the ratio of the viscosity of the droplet phase to that of the carrier phase (λ). In the first regime (∼λ < 0.75), the migration velocity decreases with an increase in surface advection of the surfactants, although there is no change in the direction of droplet migration. For the second regime (∼0.75 < λ < 11), the direction of the cross-stream migration of the droplet changes (which means the droplet moves either towards the flow centerline or away from it) depending on different parameters. In the third regime (∼λ > 11), the migration velocity is merely affected by any change in the surfactant distribution. For the other limit of higher surface advection in comparison with surface diffusion of the surfactants, the droplet always moves towards the flow centerline and the axial velocity of the droplet is found to be independent of the surfactant distribution. However, the cross-stream velocity is found to decrease with an increase in nonuniformity in surfactant distribution.
A novel combined model of discrete and mixture phases for nanoparticles in convective turbulent flow Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-17 Mostafa Mahdavi, Mohsen Sharifpur, Josua P. Meyer
In this study, a new combined model is presented to study the flow and discrete phase features of nano-size particles for turbulent convection in a horizontal tube. Due to the complexity and many phenomena involved in particle-liquid turbulent flows, the conventional models are not able to properly predict some hidden aspects of the flow. Therefore, a new form of Brownian force is implemented in the discrete phase model to predict the migration of the particles as well as energy equation has modified for particles. Then, the final results are exported to the mixture equations of the flow. The effects of the mass diffusion due to thermophoresis, Brownian motion, and turbulent dispersion are implemented as source terms in equations. The results are compared with the experimental measurements from the literature and are adequately validated. The accuracy of predicted heat transfer and friction coefficients is also discussed versus measurements. The migration of the particles toward the centre of the tube is properly captured. The results show the non-uniform distribution of particles in the turbulent flow due to strong turbulent dispersion. The proposed combined model can open new viewpoints of particle-fluid interaction flows.
Acoustics of multiscale sorptive porous materials Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-23 R. Venegas, C. Boutin, O. Umnova
This paper investigates sound propagation in multiscale rigid-frame porous materials that support mass transfer processes, such as sorption and different types of diffusion, in addition to the usual visco-thermo-inertial interactions. The two-scale asymptotic method of homogenization for periodic media is successively used to derive the macroscopic equations describing sound propagation through the material. This allowed us to conclude that the macroscopic mass balance is significantly modified by sorption, inter-scale (micro- to/from nanopore scales) mass diffusion, and inter-scale (pore to/from micro- and nanopore scales) pressure diffusion. This modification is accounted for by the dynamic compressibility of the effective saturating fluid that presents atypical properties that lead to slower speed of sound and higher sound attenuation, particularly at low frequencies. In contrast, it is shown that the physical processes occurring at the micro-nano-scale do not affect the macroscopic fluid flow through the material. The developed theory is exemplified by introducing an analytical model for multiscale sorptive granular materials, which is experimentally validated by comparing its predictions with acoustic measurements on granular activated carbons. Furthermore, we provide empirical evidence supporting an alternative method for measuring sorption and mass diffusion properties of multiscale sorptive materials using sound waves.
Effect of spatial distribution of porous matrix surface charge heterogeneity on nanoparticle attachment in a packed bed Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-23 Ngoc H. Pham, Dimitrios V. Papavassiliou
In this study, the effect of spatial distribution of the porous matrix surface heterogeneity on nanoparticle deposition is numerically explored using lattice Boltzmann simulation methods and tracking of individual particles with Lagrangian algorithms. Packed beds with four different patterns of surface charge heterogeneity, on which favorable surfaces for particle attachment are located at different locations, are generated. The heterogeneity is binary, so that the porous surface can either accommodate nanoparticle attachment or not. It is found that the heterogeneity pattern has a stronger effect when the rate constant for particle attachment is high, when the particle size is small, and/or when the fraction of the surface area that is favorable to attachment is about 0.5. At fixed conditions, the heterogeneity pattern with randomly and uniformly distributed active surface area is the most favorite for particle attachment, compared to those where the active surface areas are banded perpendicularly to the flow direction. There exists a critical ratio of the Damkohler number to the Peclet number, beyond which the heterogeneity pattern effect becomes more visible.
Analytical investigation of electrokinetic effects of micropolar fluids in nanofluidic channels Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-24 Zhaodong Ding, Yongjun Jian, Lin Wang, Liangui Yang
The effects of microstructure of fluid particles on the electrokinetic phenomena are investigated analytically based on a micropolar fluid model, where micro-rotation of fluid particles and material parameters like viscosity and angular viscosity coefficients are involved. Meanwhile, the influences of velocity slip at the surface of a nanofluidic channel and overlapped electrical double layers (EDLs) are incorporated. Results indicate that the introduction of micropolarity will significantly affect the electrokinetic effects, especially in the case of overlapped EDLs. Qualitatively, it leads to evident reductions in the flow rate, streaming current, and streaming potential relative to Newtonian fluids. The velocity slip is an opposing and competitive mechanism which tends to increase the flow rate, streaming current, and potential. Furthermore, the interplay between the micropolarity and slip effects is studied in detail. The influence of micropolarity on the electrokinetic energy conversion (EKEC) efficiency depends on the ionic Peclet number R. For small values of R (e.g., R = 0.1), the EKEC efficiency for micropolar fluids may exceed that for Newtonian fluids in some range of parameter K in the case of overlapped EDLs for nanochannels. However, for R ≥ 0.2, the EKEC efficiency for micropolar fluids is always less than that for Newtonian fluids.
Numerical investigations of electrothermally actuated moving contact line dynamics: Effect of property contrasts Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-30 Golak Kunti, Anandaroop Bhattacharya, Suman Chakraborty
This article reports interfacial electro-thermo-chemical-hydrodynamics of binary fluids with contrasting viscosity, thermal conductivity, and electrical conductivity of fluids under AC electrokinetics, called alternating current electrothermal (ACET) mechanism, over wetted substrates. The interfacial kinetics of the two phases are modulated by the coupled influence of electrothermal, viscous, and capillary forces. Numerical investigations of contact line dynamics reveal that at low viscosity of displaced fluid, viscous drag force significantly reduces leading to faster progression of the contact line. Larger viscous drag force at higher viscosity of the displaced fluid resists the interface motion to travel along the capillary. ACET forces are the direct consequences of the thermal and electric fields. For low thermal conductivity of the displaced fluid, the temperature gradient becomes much stronger leading to higher ACET forces and contact line velocity. Below a threshold limit of thermal conductivity, stronger electrothermal forces cause misbalance between contact line velocity and bulk fluid velocity, which, in turn, trigger an interesting phenomena of interface breaking. Mismatch in electrical conductivity generates electrical stresses across the interface that deforms the interface profile and causes boosting impact across the interface leading to an increase in contact line velocity. The net force across the interface changes the direction depending on the deviation of electrical conductivity ratio from unity. Finally, we observe that larger channel height and wider electrode spacing decrease the net force on the bulk fluid and contact line velocity.
Self-propelled Leidenfrost drops on a thermal gradient: A theoretical study Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-02 B. Sobac, A. Rednikov, S. Dorbolo, P. Colinet
We theoretically investigate the behavior of Leidenfrost drops on a flat substrate submitted to a horizontal thermal gradient and highlight that they are able to self-propel in a preferential direction. Namely, they are found to travel towards the colder parts of the substrate, as if they were trying to maximize their lifetime. In particular, a centimetric water drop can reach velocities of the order of cm/s for thermal gradients of the order of a few K/mm. In general, the presented model, based upon the lubrication approximation in the vapor cushion as in the work of Sobac et al. [“Leidenfrost effect: Accurate drop shape modeling and new scaling laws,” Phys. Rev. E 90, 053011 (2014)] and here formulated for simplicity for a 2D drop, enables predicting the values of these velocities as a function of the thermal gradient, drop size, superheat, and fluid properties. Surprisingly, the variability of vapor properties with temperature turns out to be instrumental for the drop to move, even if the vapor film profile is always asymmetric anyway. Finally, this asymmetry being typically weak, its effect also proved to be well captured by linearization around the corresponding symmetric Leidenfrost state.
Numerical study on the stick-slip motion of contact line moving on heterogeneous surfaces Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-02 Ming Liu, Xiao-Peng Chen
We present a numerical study of a moving contact line (CL) crossing the intersecting region of hydrophilic and hydrophobic patterns on a solid wall using lattice Boltzmann methods (LBMs). To capture the interface between the two phases properly, we applied a phase field model coupled with the LBM. The evolutions of the CL velocity, dynamic contact angle, and apparent contact angle are analyzed for the so-called “stick” and “slip” processes. In the two processes, the evolution of the quantities follows different rules shortly after the initial quick transition, which is probably caused by finite interfacial thickness or non-equilibrium effects. For the stick process, the CL is almost fixed and energy is extracted from the main flow to rebuild the meniscus’ profile. The evolution of the meniscus is mainly governed by mass conservation. The CL is depinned after the apparent contact angle surpasses the dynamic one, which implies that the interfacial segment in the vicinity of contact line is bended. For the slip process, the quantities evolve with features of relaxation. In the microscopic scale, the velocity of the CL depends on the balance between unbalanced Young’s capillary force and viscous drag. To predict the apparent contact angle evolution, a model following the dynamics of an overdamped spring-mass system is proposed. Our results also show that the capillary flows in a channel with heterogeneous wall can be described generally with the Poiseuille flow superimposed by the above transient one.
Investigation of the impact of high liquid viscosity on jet atomization in crossflow via high-fidelity simulations Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 Xiaoyi Li, Hui Gao, Marios C. Soteriou
Atomization of extremely high viscosity liquid can be of interest for many applications in aerospace, automotive, pharmaceutical, and food industries. While detailed atomization measurements usually face grand challenges, high-fidelity numerical simulations offer the advantage to comprehensively explore the atomization details. In this work, a previously validated high-fidelity first-principle simulation code HiMIST is utilized to simulate high-viscosity liquid jet atomization in crossflow. The code is used to perform a parametric study of the atomization process in a wide range of Ohnesorge numbers (Oh = 0.004–2) and Weber numbers (We = 10–160). Direct comparisons between the present study and previously published low-viscosity jet in crossflow results are performed. The effects of viscous damping and slowing on jet penetration, liquid surface instabilities, ligament formation/breakup, and subsequent droplet formation are investigated. Complex variations in near-field and far-field jet penetrations with increasing Oh at different We are observed and linked with the underlying jet deformation and breakup physics. Transition in breakup regimes and increase in droplet size with increasing Oh are observed, mostly consistent with the literature reports. The detailed simulations elucidate a distinctive edge-ligament-breakup dominated process with long surviving ligaments for the higher Oh cases, as opposed to a two-stage edge-stripping/column-fracture process for the lower Oh counterparts. The trend of decreasing column deflection with increasing We is reversed as Oh increases. A predominantly unimodal droplet size distribution is predicted at higher Oh, in contrast to the bimodal distribution at lower Oh. It has been found that both Rayleigh-Taylor and Kelvin-Helmholtz linear stability theories cannot be easily applied to interpret the distinct edge breakup process and further study of the underlying physics is needed.
Morphology of drop impact on a superhydrophobic surface with macro-structures Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 Kartik Regulagadda, Shamit Bakshi, Sarit Kumar Das
Drop-surface interaction is predominant in nature as well as in many industrial applications. Superhydrophobic surfaces show potential for various applications as they show complete drop rebound. In a recent work, it has been reported that the drop lift-off time on a superhydrophobic substrate could be further reduced by introducing a macro-ridge. The macro-ridge introduces asymmetry on the morphology of drop spreading and retraction on the surface. This changes the hydrodynamics of drop retraction and reduces the lift-off time. Keeping practical applications in view, we decorate the surface with multiple ridges. The morphology of the hydrodynamic asymmetry is completely different for the drops impacting onto the tip of the ridges from those impacting onto the middle of the valley between the ridges. We show that the morphology forms the key to the lift-off time. We also show that the outward flow from the ridge triggers a Laplace pressure driven de-wetting on the tip of the ridge, thus aiding the lift-off time. At the end of this work, we propose a ridge to ridge separation that effectively reduces the lift-off times for impacts both at the tip of the ridge and offset from it.
A simple hydrodynamic model of a laminar free-surface jet in horizontal or vertical flight Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-14 Herman D. Haustein, Ron S. Harnik, Wilko Rohlfs
A useable model for laminar free-surface jet evolution during flight, for both horizontal and vertical jets, is developed through joint analytical, experimental, and simulation methods. The jet’s impingement centerline velocity, recently shown to dictate stagnation zone heat transfer, encompasses the entire flow history: from pipe-flow velocity profile development to profile relaxation and jet contraction during flight. While pipe-flow is well-known, an alternative analytic solution is presented for the centerline velocity’s viscous-driven decay. Jet-contraction is subject to influences of surface tension (We), pipe-flow profile development, in-flight viscous dissipation (Re), and gravity (Nj = Re/Fr). The effects of surface tension and emergence momentum flux (jet thrust) are incorporated analytically through a global momentum balance. Though emergence momentum is related to pipe flow development, and empirically linked to nominal pipe flow-length, it can be modified to incorporate low-Re downstream dissipation as well. Jet contraction’s gravity dependence is extended beyond existing uniform-velocity theory to cases of partially and fully developed profiles. The final jet-evolution model relies on three empirical parameters and compares well to present and previous experiments and simulations. Hence, micro-jet flight experiments were conducted to fill-in gaps in the literature: jet contraction under mild gravity-effects, and intermediate Reynolds and Weber numbers (Nj = 5–8, Re = 350–520, We = 2.8–6.2). Furthermore, two-phase direct numerical simulations provided insight beyond the experimental range: Re = 200–1800, short pipes (Z = L/d · Re ≥ 0.01), variable nozzle wettability, and cases of no surface tension and/or gravity.
Drop impact on spherical soft surfaces Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-15 Simeng Chen, Volfango Bertola
The impact of water drops on spherical soft surfaces is investigated experimentally through high-speed imaging. The effect of a convex compliant surface on the dynamics of impacting drops is relevant to various applications, such as 3D ink-jet printing, where drops of fresh material impact on partially cured soft substrates with arbitrary shape. Several quantities which characterize the morphology of impacting drops are measured through image-processing, including the maximum and minimum spreading angles, length of the wetted curve, and dynamic contact angle. In particular, the dynamic contact angle is measured using a novel digital image-processing scheme based on a goniometric mask, which does not require edge fitting. It is shown that the surface with a higher curvature enhances the retraction of the spreading drop; this effect may be due to the difference of energy dissipation induced by the curvature of the surface. In addition, the impact parameters (elastic modulus, diameter ratio, and Weber number) are observed to significantly affect the dynamic contact angle during impact. A quantitative estimation of the deformation energy shows that it is significantly smaller than viscous dissipation.
Secondary cavitation in a rigid tube Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-15 Chen Ji, Bo Li, Jun Zou
The oscillation of a single spark-generated bubble in a rigid tube is studied experimentally, with the help of a high-speed camera and a hydrophone. The non-dimensional collapse position XF is divided into three regimes, according to the various phenomena after the first oscillation period T1. In an asymmetric regime, secondary cavitation is observed. Both the axial position and the oscillation period of the secondary cavitation show a linear correlation with the first bubble. The period ratio k is independent of the relative bubble size L1* but is affected by the tube diameter Dt. In a symmetric regime, the secondary cavitation is much weaker and unrepeatable. The rebound bubble is strengthened in this regime, and the rebound ratio kr is independent of both L1* and Dt. A mechanism of reflected rarefaction wave is proposed to explain the position relation between the first and secondary cavities, and the energy partition in different regimes is discussed.
High-speed oblique drop impact on thin liquid films Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-17 Yisen Guo, Yongsheng Lian
We numerically investigate high-speed drop impact on thin liquid films with a focus on oblique impact. The flow behavior is described by solving the incompressible Navier-Stokes equations using the variable density pressure projection method. The phase interfaces are captured using the moment-of-fluid method. The numerical method is validated against experiments and theoretical predictions. Our study on high-speed oblique impact reveals that the tangential velocity can significantly alter impact phenomena: a higher tangential velocity leads to a lower lamella height and radius on the side behind the advancing drop, and the higher tangential velocity also leads to stronger vortices at the drop and film interface due to Kelvin-Helmholtz instability. Our investigation on the effect of liquid film thickness shows that a thinner liquid film leads to an earlier crown breakup. Last, our study shows that lowering the film density can prompt earlier splashing.
Instability of eccentric compound threads Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-23 Hanyu Ye, Jie Peng, Lijun Yang
This paper investigates the temporal instability of an eccentric compound liquid thread. Results of linear stability are obtained for a typical case in the context of compound threads in microencapsulation. It is found that the disturbance growth rate of an eccentric compound liquid thread is close to that of the corresponding concentric one, in terms of both the maximum growth rate and the dominant wavenumber. Furthermore, linear stability results over a wide parameter range are obtained and the conclusion is basically unchanged. Energy balance of the destabilization process is analyzed to explain the mechanism of instability, and it is found that although the disturbance growth rate of an eccentric compound thread is close to that of the corresponding concentric thread, their energy balances are distinctively different. The disturbance interface shape and disturbance velocity distributions are plotted. It is found that the behavior of the disturbance velocity in the cross section plane is different from that of the axial disturbance velocity. The disturbance velocity distributions in the cross section plane explain the trend in the disturbance interface shape. A fully nonlinear simulation of the destabilization process is performed by the Gerris flow solver and the results agree well with those obtained by linear stability analysis.
Nonlinear interaction between underwater explosion bubble and structure based on fully coupled model Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-24 A. M. Zhang, W. B. Wu, Y. L. Liu, Q. X. Wang
The interaction between an underwater explosion bubble and an elastic-plastic structure is a complex transient process, accompanying violent bubble collapsing, jet impact, penetration through the bubble, and large structural deformation. In the present study, the bubble dynamics are modeled using the boundary element method and the nonlinear transient structural response is modeled using the explicit finite element method. A new fully coupled 3D model is established through coupling the equations for the state variables of the fluid and structure and solving them as a set of coupled linear algebra equations. Based on the acceleration potential theory, the mutual dependence between the hydrodynamic load and the structural motion is decoupled. The pressure distribution in the flow field is calculated with the Bernoulli equation, where the partial derivative of the velocity potential in time is calculated using the boundary integral method to avoid numerical instabilities. To validate the present fully coupled model, the experiments of small-scale underwater explosion near a stiffened plate are carried out. High-speed imaging is used to capture the bubble behaviors and strain gauges are used to measure the strain response. The numerical results correspond well with the experimental data, in terms of bubble shapes and structural strain response. By both the loosely coupled model and the fully coupled model, the interaction between a bubble and a hollow spherical shell is studied. The bubble patterns vary with different parameters. When the fully coupled model and the loosely coupled model are advanced with the same time step, the error caused by the loosely coupled model becomes larger with the coupling effect becoming stronger. The fully coupled model is more stable than the loosely coupled model. Besides, the influences of the internal fluid on the dynamic response of the spherical shell are studied. At last, the case that the bubble interacts with an air-backed stiffened plate is simulated. The associated interesting physical phenomenon is obtained and expounded.
Viscous decay of nonlinear oscillations of a spherical bubble at large Reynolds number Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-30 W. R. Smith, Q. X. Wang
The long-time viscous decay of large-amplitude bubble oscillations is considered in an incompressible Newtonian fluid, based on the Rayleigh–Plesset equation. At large Reynolds numbers, this is a multi-scaled problem with a short time scale associated with inertial oscillation and a long time scale associated with viscous damping. A multi-scaled perturbation method is thus employed to solve the problem. The leading-order analytical solution of the bubble radius history is obtained to the Rayleigh–Plesset equation in a closed form including both viscous and surface tension effects. Some important formulae are derived including the following: the average energy loss rate of the bubble system during each cycle of oscillation, an explicit formula for the dependence of the oscillation frequency on the energy, and an implicit formula for the amplitude envelope of the bubble radius as a function of the energy. Our theory shows that the energy of the bubble system and the frequency of oscillation do not change on the inertial time scale at leading order, the energy loss rate on the long viscous time scale being inversely proportional to the Reynolds number. These asymptotic predictions remain valid during each cycle of oscillation whether or not compressibility effects are significant. A systematic parametric analysis is carried out using the above formula for the energy of the bubble system, frequency of oscillation, and minimum/maximum bubble radii in terms of the Reynolds number, the dimensionless initial pressure of the bubble gases, and the Weber number. Our results show that the frequency and the decay rate have substantial variations over the lifetime of a decaying oscillation. The results also reveal that large-amplitude bubble oscillations are very sensitive to small changes in the initial conditions through large changes in the phase shift.
A deformable plate interacting with a non-Newtonian fluid in three dimensions Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-01 Luoding Zhu, Xijun Yu, Nansheng Liu, Yongguang Cheng, Xiyun Lu
We consider a deformable plate interacting with a non-Newtonian fluid flow in three dimensions as a simple model problem for fluid-structure-interaction phenomena in life sciences (e.g., red blood cell interacting with blood flow). A power-law function is used for the constitutive equation of the non-Newtonian fluid. The lattice Boltzmann equation (the D3Q19 model) is used for modeling the fluid flow. The immersed boundary (IB) method is used for modeling the flexible plate and handling the fluid-plate interaction. The plate drag and its scaling are studied; the influences of three dimensionless parameters (power-law exponent, bending modulus, and generalized Reynolds number) are investigated.
Characterisation of elastic turbulence in a serpentine micro-channel Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-02 Antoine Souliès, Julien Aubril, Cathy Castelain, Teodor Burghelea
A systematic experimental investigation of the onset, development, and statistical and scaling properties of elastic turbulence in a curvilinear micro-channel of a dilute solution of a high molecular weight polymer is presented. By measurements of time series of high spatial resolution flow fields performed over a time 320 times longer than the average relaxation, we show that the transition to elastic turbulence occurs via an imperfect bifurcation. Slightly above the onset of the primary elastic instability, rare events manifested through a local deceleration of the flow are observed. By measurements of the spatial distributions and statistics of the second invariant of the rate of strain tensor, we show that the main prediction of the theory regarding the saturation of root mean square of fluctuations of the velocity gradients is qualitatively verified though a quantitative agreement could not be found. A systematic analysis of the statistics of the fluctuations of flow fields in terms of spatial and temporal correlations, power spectra, and probability distributions is presented. The scaling properties of structure functions of the increments of the velocity gradients are discussed. Our experimental findings call for further developments of the theory of elastic turbulence in bounded flow channels.
Unsteady flow of a thixotropic fluid in a slowly varying pipe Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-21 Andrew I. Croudace, David Pritchard, Stephen K. Wilson
We analyse the unsteady axisymmetric flow of a thixotropic or antithixotropic fluid in a slowly varying cylindrical pipe. We derive general perturbation solutions in regimes of small Deborah numbers, in which thixotropic or antithixotropic effects enter as perturbations to generalised Newtonian flow. We present results for the viscous Moore–Mewis–Wagner model and the viscoplastic Houška model, and we use these results to elucidate what can be predicted in general about the behaviour of thixotropic and antithixotropic fluids in lubrication flow. The range of behaviour we identify casts doubt on the efficacy of model reduction approaches that postulate a generic cross-pipe flow structure.
Simulation of deterministic energy-balance particle agglomeration in turbulent liquid-solid flows Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-04 Derrick O. Njobuenwu, Michael Fairweather
An efficient technique to simulate turbulent particle-laden flow at high mass loadings within the four-way coupled simulation regime is presented. The technique implements large-eddy simulation, discrete particle simulation, a deterministic treatment of inter-particle collisions, and an energy-balanced particle agglomeration model. The algorithm to detect inter-particle collisions is such that the computational costs scale linearly with the number of particles present in the computational domain. On detection of a collision, particle agglomeration is tested based on the pre-collision kinetic energy, restitution coefficient, and van der Waals’ interactions. The performance of the technique developed is tested by performing parametric studies on the influence of the restitution coefficient (en = 0.2, 0.4, 0.6, and 0.8), particle size (dp = 60, 120, 200, and 316 μm), Reynolds number (Reτ = 150, 300, and 590), and particle concentration (αp = 5.0 × 10−4, 1.0 × 10−3, and 5.0 × 10−3) on particle-particle interaction events (collision and agglomeration). The results demonstrate that the collision frequency shows a linear dependency on the restitution coefficient, while the agglomeration rate shows an inverse dependence. Collisions among smaller particles are more frequent and efficient in forming agglomerates than those of coarser particles. The particle-particle interaction events show a strong dependency on the shear Reynolds number Reτ, while increasing the particle concentration effectively enhances particle collision and agglomeration whilst having only a minor influence on the agglomeration rate. Overall, the sensitivity of the particle-particle interaction events to the selected simulation parameters is found to influence the population and distribution of the primary particles and agglomerates formed.
Homogeneous cooling state of dilute granular gases of charged particles Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-21 Satoshi Takada, Dan Serero, Thorsten Pöschel
We describe the velocity distribution function of a granular gas of electrically charged particles by means of a Sonine polynomial expansion and study the decay of its granular temperature. We find a dependence of the first non-trivial Sonine coefficient, a2, on time through the value of temperature. In particular, we find a sudden drop of a2 when temperature approaches a characteristic value, T * , describing the electrostatic interaction. For lower values of T, the velocity distribution function becomes Maxwellian. The theoretical calculations agree well with numerical direct simulation Monte Carlo to validate our theory.
Three-dimensional flow with elevated helicity in driven cavity by parallel walls moving in perpendicular directions Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-01 Alex Povitsky
The proposed flow in a 3-D cubic cavity is driven by its parallel walls moving in perpendicular directions to create a genuinely three-dimensional highly separated vortical flow, yet having simple single-block cubical geometry of computational domain. The elevated level of helicity is caused by motion of a wall in the direction of axis of primary vortex created by a parallel wall. The velocity vector field is obtained numerically by using second-order upwind scheme and 2003 grid. Helicity, magnitude of normalized helicity, and kinematic vorticity number are evaluated for Reynolds numbers ranging from 100 to 1000. Formation of two primary vortices with their axis oriented perpendicularly and patterns of secondary vortices are discussed. Computational results are compared to the well-known 3-D recirculating cavity flow case where the lid moves in the direction parallel to the cavity side walls. Also results are compared to the diagonally top-driven cavity and to the cavity flow driven by moving top and side walls. The streamlines for the proposed flow show that the particles emerging from top and bottom of the cavity do mix well. Quantitative evaluation of mixing of two fluids in the proposed cavity flow confirms that mixing occurs faster than in the benchmark case.
Control of vortex-induced vibration using a pair of synthetic jets: Influence of active lock-on Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-01 Chenglei Wang, Hui Tang, Simon C. M. Yu, Fei Duan
While conventional vortex-induced vibration (VIV) of bluff bodies is suppressed through reducing the strength of asymmetric vortex shedding, it can also be mitigated by shifting the vortex shedding frequency away from the natural frequency of the body structures using active lock-on. Recently Du and Sun [“Suppression of vortex-induced vibration using the rotary oscillation of a cylinder,” Phys. Fluids 27, 023603 (2015)] utilized periodical rotation to induce the lock-on of the frequency of vortex shedding from a transversely vibrating cylinder to the rotation frequency and demonstrated successful VIV suppression. However, questions were raised from this investigation: Does the occurrence of active lock-on always suppress VIV? If not, how to ensure the appropriate usage of active lock-on for VIV suppression? To address these research questions, a numerical investigation is conducted on the active VIV control of a circular cylinder using a pair of synthetic jets (SJs) at a low Reynolds number of 100. The SJ pair operates with various phase differences over a wide frequency range so that the influence of various lock-on can be investigated. It is found that the VIV control can be affected not only by the occurrence of the primary lock-on but also by the occurrence of other lock-on such as secondary and tertiary lock-on. The occurrence of lock-on does not always result in successful VIV suppression. Sometimes it even causes the augmentation of VIV. Compared to the VIV suppression using the conventional vortex-strength-reduction method, the control by the means of active lock-on seems less effective.
Vortex motion around a circular cylinder above a plane Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-01 G. L. Vasconcelos, M. Moura
The study of vortex flows around solid obstacles is of considerable interest from both a theoretical and practical perspective. One geometry that has attracted renewed attention recently is that of vortex flows past a circular cylinder placed above a plane wall, where a stationary recirculating eddy can form in front of the cylinder, in contradistinction to the usual case (without the plane boundary) for which a vortex pair appears behind the cylinder. Here we analyze the problem of vortex flows past a cylinder near a wall through the lenses of the point-vortex model. By conformally mapping the fluid domain onto an annular region in an auxiliary complex plane, we compute the vortex Hamiltonian analytically in terms of certain special functions related to elliptic theta functions. A detailed analysis of the equilibria of the model is then presented. The location of the equilibrium in front of the cylinder is shown to be in qualitative agreement with recent experimental findings. We also show that a topological transition occurs in phase space as the parameters of the systems are varied.
Numerical study of shear rate effect on unsteady flow separation from the surface of the square cylinder using structural bifurcation analysis Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-03 Rajendra K. Ray, Atendra Kumar
In this paper, an incompressible two-dimensional shear flow past a square cylinder problem is investigated numerically using a higher order compact finite difference scheme. Simulations are presented for three sets of Reynolds numbers, 100, 200, and 500, with various shear parameter (K) values ranging from 0.0 to 0.4. The purpose of the present study is to elaborate the influence of shear rate on the vortex shedding phenomenon behind the square cylinder. The results presented here show that the vortex shedding phenomenon strongly depends on Re as well as K. The strength and size of vortices shed behind the cylinder vary as a function of Re and K. When K is larger than a critical value, the vortex shedding phenomenon has completely disappeared depending on the Reynolds number. Apart from the numerical study, a thorough theoretical investigation has been done by using a topology based structural bifurcation analysis for unsteady flow separations from the walls of the cylinder. Through this analysis, we study the exact locations of the bifurcation points associated with secondary and tertiary vortices with appropriate non-dimensional time of occurrence. To the best of our knowledge, this is the first time, a topological aspect based structural bifurcation analysis has been done to understand the vortex shedding phenomenon and flow separation for this problem.
An immersed boundary-simplified sphere function-based gas kinetic scheme for simulation of 3D incompressible flows Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 L. M. Yang, C. Shu, W. M. Yang, Y. Wang, J. Wu
In this work, an immersed boundary-simplified sphere function-based gas kinetic scheme (SGKS) is presented for the simulation of 3D incompressible flows with curved and moving boundaries. At first, the SGKS [Yang et al., “A three-dimensional explicit sphere function-based gas-kinetic flux solver for simulation of inviscid compressible flows,” J. Comput. Phys. 295, 322 (2015) and Yang et al., “Development of discrete gas kinetic scheme for simulation of 3D viscous incompressible and compressible flows,” J. Comput. Phys. 319, 129 (2016)], which is often applied for the simulation of compressible flows, is simplified to improve the computational efficiency for the simulation of incompressible flows. In the original SGKS, the integral domain along the spherical surface for computing conservative variables and numerical fluxes is usually not symmetric at the cell interface. This leads the expression of numerical fluxes at the cell interface to be relatively complicated. For incompressible flows, the sphere at the cell interface can be approximately considered to be symmetric as shown in this work. Besides that, the energy equation is usually not needed for the simulation of incompressible isothermal flows. With all these simplifications, the simple and explicit formulations for the conservative variables and numerical fluxes at the cell interface can be obtained. Second, to effectively implement the no-slip boundary condition for fluid flow problems with complex geometry as well as moving boundary, the implicit boundary condition-enforced immersed boundary method [Wu and Shu, “Implicit velocity correction-based immersed boundary-lattice Boltzmann method and its applications,” J. Comput. Phys. 228, 1963 (2009)] is introduced into the simplified SGKS. That is, the flow field is solved by the simplified SGKS without considering the presence of an immersed body and the no-slip boundary condition is implemented by the immersed boundary method. The accuracy and efficiency of the present scheme are validated by simulating the decaying vortex flow, flow past a stationary and rotating sphere, flow past a stationary torus, and flows over dragonfly flight.
Vortex-induced vibrations of three staggered circular cylinders at low Reynolds numbers Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-16 Suresh Behara, B. Ravikanth, Venu Chandra
Vortex-induced vibrations of three staggered circular cylinders are investigated via two-dimensional finite element computations. All the cylinders are of equal diameter (D) and are mounted on elastic supports in both streamwise ( x − ) and transverse ( y − ) directions. The two downstream cylinders are placed symmetrically on either side of the upstream body at a streamwise gap of 5D, with the vertical distance between them being 3D. Flow simulations are carried out for Reynolds numbers (Re) in the range of Re = 60-160. Reduced mass ( m * ) of 10 is considered and the damping is set to zero value. The present investigations show that the upstream cylinder exhibits initial and lower synchronization response modes like an isolated cylinder does at low Re. Whereas for both the downstream cylinders, the upper lock-in branch also appears. The initial and the upper modes are characterized by periodic oscillations, while the lower lock-in branch is associated with nonperiodic vibrations. The 2S mode of vortex shedding is observed in the near wake of all the cylinders for all Re, except for the upper branch corresponding to the downstream bodies. In the upper branch, both the downstream cylinders shed the primary vortices of the P+S mode. For the upstream cylinder, the phase between lift and the transverse displacement exhibits a 18 0 ° jump at certain Re in the lower branch. On the other hand, the downstream bodies undergo transverse oscillations in phase with lift in all lock-in modes, while the phase jumps by 18 0 ° as the oscillation response reaches the desynchronization regime.
Evaluation of flamelet/progress variable model for laminar pulverized coal combustion Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-29 Xu Wen, Haiou Wang, Yujuan Luo, Kun Luo, Jianren Fan
In the present work, the flamelet/progress variable (FPV) approach based on two mixture fractions is formulated for pulverized coal combustion and then evaluated in laminar counterflow coal flames under different operating conditions through both a priori and a posteriori analyses. Two mixture fractions, Zvol and Zchar, are defined to characterize the mixing between the oxidizer and the volatile matter/char reaction products. A coordinate transformation is conducted to map the flamelet solutions from a unit triangle space (Zvol, Zchar) to a unit square space (Z, X) so that a more stable solution can be achieved. To consider the heat transfers between the coal particle phase and the gas phase, the total enthalpy is introduced as an additional manifold. As a result, the thermo-chemical quantities are parameterized as a function of the mixture fraction Z, the mixing parameter X, the normalized total enthalpy Hnorm, and the reaction progress variable YPV. The validity of the flamelet chemtable and the selected trajectory variables is first evaluated in a priori tests by comparing the tabulated quantities with the results obtained from numerical simulations with detailed chemistry. The comparisons show that the major species mass fractions can be predicted by the FPV approach in all combustion regions for all operating conditions, while the CO and H2 mass fractions are over-predicted in the premixed flame reaction zone. The a posteriori study shows that overall good agreement between the FPV results and those obtained from detailed chemistry simulations can be achieved, although the coal particle ignition is predicted to be slightly earlier. Overall, the validity of the FPV approach for laminar pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.
Convective mixing in vertically-layered porous media: The linear regime and the onset of convection Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-02 Zohreh Ghorbani, Amir Riaz, Don Daniel
We study the effect of permeability heterogeneity on the stability of gravitationally unstable, transient, diffusive boundary layers in porous media. Permeability is taken to vary periodically in the horizontal plane normal to the direction of gravity. In contrast to the situation for vertical permeability variation, the horizontal perturbation structures are multimodal. We therefore use a two-dimensional quasi-steady eigenvalue analysis as well as a complementary initial value problem to investigate the stability behavior in the linear regime, until the onset of convection. We find that thick permeability layers enhance instability compared with thin layers when heterogeneity is increased. On the contrary, for thin layers the instability is weakened progressively with increasing heterogeneity to the extent that the corresponding homogeneous case is more unstable. For high levels of heterogeneity, we find that a small change in the permeability field results in large variations in the onset time of convection, similar to the instability event in the linear regime. However, this trend does not persist unconditionally because of the reorientation of vorticity pairs due to the interaction of evolving perturbation structures with heterogeneity. Consequently, an earlier onset of instability does not necessarily imply an earlier onset of convection. A resonant amplification of instability is observed within the linear regime when the dominant perturbation mode is equal to half the wavenumber of permeability variation. On the other hand, a substantial damping occurs when the perturbation mode is equal to the harmonic and sub-harmonic components of the permeability wavenumber. The phenomenon of such harmonic interactions influences both the onset of instability as well as the onset of convection.
Higher order dynamic mode decomposition to identify and extrapolate flow patterns Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-09 Soledad Le Clainche, José M. Vega
This article shows the capability of using a higher order dynamic mode decomposition (HODMD) algorithm both to identify flow patterns and to extrapolate a transient solution to the attractor region. Numerical simulations are carried out for the three-dimensional flow around a circular cylinder, and both standard dynamic mode decomposition (DMD) and higher order DMD are applied to the non-converged solution. The good performance of HODMD is proved, showing that this method guesses the converged flow patterns from numerical simulations in the transitional region. The solution obtained can be extrapolated to the attractor region. This fact sheds light on the capability of finding real flow patterns in complex flows and, simultaneously, reducing the computational cost of the numerical simulations or the required quantity of data collected in experiments.
Hopf/steady-state mode interaction in a vertical slot—Effect of 1:4 resonance Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-10 Kaoru Fujimura, Shuhei Tsunoda
Hopf/steady-state mode interaction in a vertical slot with side-wall heating is revisited. Linear stability analysis reveals that a pair of Hopf modes and a steady mode share the critical Grashof number at a crossover Prandtl number P * = 12.454 256 44 . About a quarter century ago, the non-resonant mode interaction between the critical modes was investigated for a Prandtl number close to P * . In the present paper, we claim that the critical Hopf modes and the steady mode have wavenumbers approximately in a ratio of 1:4. We then examine the effect of 1:4 resonance on the Hopf/steady mode interaction although the 1:4 resonance arises at the quartic-order approximation. This implies that the resonance is so weak that its effect of phase coupling is expected to be negligibly small compared with the effect of coupling acting through the moduli of amplitudes arising at the cubic order. We find, however, that the resonance is indispensable for the bifurcation of symmetric mixed modes with phase differences of 0 and π , from which an asymmetric mixed mode bifurcates.
Instability in a channel with grooves parallel to the flow Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-16 Nikesh Yadav, S. W. Gepner, J. Szumbarski
Flow in a channel with corrugated walls has been studied, with the primary goal of establishing channel geometries that enhance achievable mixing at possibly low drag increase. The wall corrugation has the form of a sinusoidal wave oriented transversely, i.e., the lines of constant elevation (or phase) are parallel to the direction of the flow. The analysis is performed up to the Reynolds numbers resulting in the formation of secondary states. The first part of the analysis is focused on the properties of the two-dimensional, base flow. Mainly, the dependence of the drag on the channel’s geometry is characterized. The second part of the analysis discusses the onset of the three-dimensional traveling wave instability. Linear stability is investigated by the Direct Numerical Simulation of the Navier-Stokes equations. Critical conditions for the onset of instabilities at a range of geometric parameters are determined. Finally, nonlinear saturation of the unstable modes and the resulting secondary flows is examined. It is shown that the drag reduction property of the base flow can be maintained in the state resulting from non-linear saturation of the disturbance.
Hybrid POD-FFT analysis of nonlinear evolving coherent structures of DNS wavepacket in laminar-turbulent transition Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-24 Kean Lee Kang, K. S. Yeo
This paper concerns the study of direct numerical simulation data of a wavepacket in laminar turbulent transition in a Blasius boundary layer. The decomposition of this wavepacket into a set of “modes” (a basis that spans an approximate solution space) can be achieved in a wide variety of ways. Two well-known tools are the fast Fourier transform (FFT) and the proper orthogonal decomposition (POD). To synergize the strengths of both methods, a hybrid POD-FFT is pioneered, using the FFT as a tool for interpreting the POD modes. The POD-FFT automatically identifies well-known fundamental, subharmonic, and Klebanoff modes in the flow, even though it is blind to the underlying physics. Moreover, the POD-FFT further separates the subharmonic content of the wavepacket into three fairly distinct parts: a positively detuned mode resembling a Lambda-vortex, a Craik-type tuned mode, and a Herbert-type positive-negative detuned mode pair, in decreasing order of energy. This distinction is less widely recognized, but it provides a possible explanation for the slightly positively detuned subharmonic mode often observed in previous experiments and simulations.
Effects of location of excitation on the spiral vortices in the transitional region of a rotating-disk flow Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-31 Keunseob Lee, Yu Nishio, Seiichiro Izawa, Yu Fukunishi
The laminar-turbulent transition of a rotating-disk flow dominated by global instability is studied by solving the full Navier-Stokes equations in direct numerical simulations. A flow field in the 2 π /32 region is computed using a periodic boundary condition. The flow field is disturbed in two ways. In the first case, a disturbance is introduced at the Reynolds number, R e ≈ 600, while in the second case, a disturbance is introduced at R e ≈ 650. In both cases, wall-normal short-duration suction and blowing are used to disturb the flow field. When a disturbance is added upstream at R e ≈ 600, the wavenumber 64 component becomes dominant when the flow reaches a steady state, whereas when a disturbance is added downstream at R e ≈ 650, the wavenumber 96 component becomes prominent. The transition points are different between the two cases. In addition, in both cases, the distances between neighboring spiral vortices are quite the same when measured at the locations where the turbulence begins.
Effects of Karlovitz number on turbulent kinetic energy transport in turbulent lean premixed methane/air flames Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-02 Zhiyan Wang, John Abraham
Direct numerical simulations of lean methane/air flames are carried out to study the effects of premixed combustion on turbulence. The equivalence ratio of the flame is 0.5 and non-dimensional turbulence intensities (urms/SL) are between 2 and 25. The mixture pressure is 20 bars and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. The Karlovitz number (Ka) varies from 1.1 to 49.4, and the Damköhler number (Da) varies from 0.26 to 3.2 corresponding to turbulent premixed combustion in the thin reaction zone (TRZ) regime. It is found that turbulence kinetic energy (TKE) and its dissipation rate decrease monotonically across the flame brush while the integral length scale increases monotonically for flames in the TRZ regime. The transport equation of TKE is then examined, and the scaling of the terms in the equation is discussed. It is found that the sink term which represents molecular diffusion and viscous dissipation is the dominant term in the TKE balance and it scales with the square of Ka. The relative importance of the other terms with respect to the dissipation term is studied. With increasing Ka, the other terms in the TKE balance become less important compared to the dissipation term.
Dissipation element analysis of a turbulent non-premixed jet flame Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-07 Michael Gauding, Felix Dietzsch, Jens Henrik Goebbert, Dominique Thévenin, Abouelmagd Abdelsamie, Christian Hasse
The objective of the present work is to examine the interaction between turbulent mixing and chemistry by employing the method of dissipation elements in a non-premixed turbulent jet flame. The method of dissipation elements [L. Wang and N. Peters, J. Fluid Mech. 554, 457–475 (2006)] is used to perform a space-filling decomposition of the turbulent jet flow into different regimes conditioned on their location with respect to the reaction zone. Based on the non-local structure of dissipation elements, this decomposition allows us to discern whether points away from stoichiometry are connected through a diffusive layer with the reaction zone. In a next step, a regime based statistical analysis of dissipation elements is carried out by means of data obtained from a direct numerical simulation. Turbulent mixing and chemical reactions depend strongly on the mixture fraction gradient. From a budget between strain and dissipation, the mechanism for the formation and destruction of mean gradients along dissipation elements is inspected. This budget reveals that large gradients in the mixture fraction field occur at a small but finite length scale. Finally, the inner structure of dissipation elements is examined by computing statistics along gradient trajectories of the mixture fraction field. Thereby, the method of dissipation elements provides a statistical characterization of flamelets and novel insight into the interaction between chemistry and turbulence.
Identification of coherent structures in the flow past a NACA0012 airfoil via proper orthogonal decomposition Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-08 Jean Hélder Marques Ribeiro, William Roberto Wolf
Proper orthogonal decomposition, POD, is employed to identify coherent structures in the turbulent flow past a NACA0012 airfoil. An analysis of different POD techniques is presented including the standard snapshot method, the spectral POD (SPOD) method, and a Fourier-POD implementation combined with the SPOD. The latter technique is viable once the present turbulent flow has a homogeneous direction along the airfoil span. An assessment of different vector norms and filter functions employed in the POD reconstruction is presented. The evaluation of the several POD techniques allows the investigation of the physics of the current turbulent flow including the observation of coherent structures and their relation to the airfoil noise generation mechanisms. The SPOD technique is applied assuming periodic and non-periodic temporal signals in the construction of the covariance matrix. Although it slightly alters the spectrum of the POD singular values, the periodic SPOD considerably reduces the noise observed in the singular vectors. The application of the Gaussian filtering allows an enhanced control in the response of the SPOD when compared with a square-box filter. The present POD reconstructions employ norms based on kinetic energy and pressure. For both norms, two-dimensional coherent structures are observed along the turbulent boundary layer and wake regions for the first pair of modes. These structures have a spectral content at the same frequency of the tonal noise radiation by the airfoil. The POD analysis also allows the identification of further coherent structures in the present flow. The kinetic energy norm is able to reconstruct low-frequency structures in the flow field for higher POD modes, while the pressure norm reconstructs high-frequency structures. This behavior is related to the physics captured by each POD norm in the present turbulent flow.
Geometrical aspects of turbulent/non-turbulent interfaces with and without mean shear Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-09 Tomoaki Watanabe, Carlos B. da Silva, Koji Nagata, Yasuhiko Sakai
The geometry of turbulent/non-turbulent interfaces (TNTIs) arising from flows with and without mean shear is investigated using direct numerical simulations of turbulent planar jets (PJET) and shear free turbulence (SFT), respectively, with Taylor Reynolds number of about R e λ ≈ 100 . In both flows, the TNTI is preferentially aligned with the tangent to the TNTI displaying convex, where the turbulent fluid nearby tends to have a stronger enstrophy, more frequently than concave shapes. The different flow configurations are reflected in different orientations of the TNTI with respect to the flow direction (and its normal). While the interface orientation with respect to the mean flow direction in PJET has an influence on the velocity field near the TNTI and the enstrophy production in the turbulent sublayer, there is no particular discernible dependence on the interface orientation in SFT. Finally, the intense vorticity structures or “worms,” which are possibly associated with “nibbling” entrainment mechanism, “feel” the local geometry of the TNTI, and it is shown that in PJET, a smaller local radius of these structures arises in regions near the TNTI where the local TNTI faces the mean flow direction.
The behaviour of the scalar gradient across the turbulent/non-turbulent interface in jets Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-18 Tiago S. Silva, Carlos B. da Silva
The dynamics of a passive scalar field near a turbulent/non-turbulent interface is analysed through direct numerical simulations of turbulent planar jets, with Reynolds numbers ranging from 142 ≤ R e λ ≤ 246 , and Schmidt numbers from 0.07 ≤ S c ≤ 7.0 . A scalar-gradient turbulent/non-turbulent interface (SG-TNTI) forms at the outer edge of the jet, which does not coincide with the vorticity turbulent/non-turbulent interface (VO-TNTI) for the lower Schmidt number cases (Sc = 0.07 and 0.7). Specifically, for Sc = 0.07 and 0.7, the scalar gradient maxima, and thus the bulk of the mixing takes place in the irrotational region, between 10 and 30 Kolmogorov micro-scale distances from the start of the VO-TNTI. For these moderate Schmidt number cases, the SG-TNTI exhibits an irrotational-diffusive superlayer, where the scalar gradient diffusion dominates, while the production is negligible, followed by an irrotational-straining sublayer where the scalar gradient production dominates. In contrast for Sc = 7.0, the SG-TNTI consists of a v iscous-con v ective superlayer that closely matches the viscous superlayer from the VO-TNTI and an inertial-convective sublayer, where scalar gradient production dominates, which is much smaller than the turbulent sublayer of the VO-TNTI. The scaling laws and mean thicknesses of each one of these (sub)layers are briefly discussed. This work presents a systematic study of the effects of the Schmidt number on the scalar gradient evolution and of the SG-TNTI characteristics.
Scalar transport and the validity of Damköhler’s hypotheses for flame propagation in intense turbulence Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-21 Girish V. Nivarti, R. Stewart Cant
The turbulent burning velocity of premixed flames is sensitive to the turbulence intensity of the unburned mixture. Premixed flame propagation models that incorporate these effects of turbulence rest on either of the two hypotheses proposed by Damköhler. The first hypothesis applies to low-intensity turbulence that acts mainly to increase the turbulent burning velocity by increasing the flame surface area. The second hypothesis states that, at sufficiently high intensities of turbulence, the turbulent burning velocity is governed mainly by enhanced diffusivity. Most studies to date have examined the validity of the first hypothesis under increasingly high intensities of turbulence. In the present study, the validity of Damköhler’s second hypothesis is investigated. A range of turbulence intensities is addressed by means of direct numerical simulations spanning the “flamelet” and “broken reaction zones” regimes. The validity of Damköhler’s second hypothesis is found to be strongly linked to the behaviour of turbulent transport within the flame.
Aerodynamics of a translating comb-like plate inspired by a fairyfly wing Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-18 Seung Hun Lee, Daegyoum Kim
Unlike the smooth wings of common insects or birds, micro-scale insects such as the fairyfly have a distinctive wing geometry, comprising a frame with several bristles. Motivated by this peculiar wing geometry, we experimentally investigated the flow structure of a translating comb-like wing for a wide range of gap size, angle of attack, and Reynolds number, Re = O(10) − O(103), and the correlation of these parameters with aerodynamic performance. The flow structures of a smooth plate without a gap and a comb-like plate are significantly different at high Reynolds number, while little difference was observed at the low Reynolds number of O(10). At low Reynolds number, shear layers that were generated at the edges of the tooth of the comb-like plate strongly diffuse and eventually block a gap. This gap blockage increases the effective surface area of the plate and alters the formation of leading-edge and trailing-edge vortices. As a result, the comb-like plate generates larger aerodynamic force per unit area than the smooth plate. In addition to a quasi-steady phase after the comb-like plate travels several chords, we also studied a starting phase of the shear layer development when the comb-like plate begins to translate from rest. While a plate with small gap size can generate aerodynamic force at the starting phase as effectively as at the quasi-steady phase, the aerodynamic force drops noticeably for a plate with a large gap because the diffusion of the developing shear layers is not enough to block the gap.
Characterization of interfacial waves and pressure drop in horizontal oil-water core-annular flows Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-18 Sumit Tripathi, Rico F. Tabor, Ramesh Singh, Amitabh Bhattacharya
We study the transportation of highly viscous furnace-oil in a horizontal pipe as core-annular flow (CAF) using experiments. Pressure drop and high-speed images of the fully developed CAF are recorded for a wide range of flow rate combinations. The height profiles (with respect to the centerline of the pipe) of the upper and lower interfaces of the core are obtained using a high-speed camera and image analysis. Time series of the interface height are used to calculate the average holdup of the oil phase, speed of the interface, and the power spectra of the interface profile. We find that the ratio of the effective velocity of the annular fluid to the core velocity, α, shows a large scatter. Using the average value of this ratio (α=0.74) yields a good estimate of the measured holdup for the whole range of flow rate ratios, mainly due to the low sensitivity of the holdup ratio to the velocity ratio. Dimensional analysis implies that, if the thickness of the annular fluid is much smaller than the pipe radius, then, for the given range of parameters in our experiments, the non-dimensional interface shape, as well as the non-dimensional wall shear stress, can depend only on the shear Reynolds number and the velocity ratio. Our experimental data show that, for both lower and upper interfaces, the normalized power spectrum of the interface height has a strong dependence on the shear Reynolds number. Specifically, for low shear Reynolds numbers, interfacial modes with large wavelengths dominate, while, for large shear Reynolds numbers, interfacial modes with small wavelengths dominate. Normalized variance of the interface height is higher at lower shear Reynolds numbers and tends to a constant with increasing shear Reynolds number. Surprisingly, our experimental data also show that the effective wall shear stress is, to a large extent, proportional to the square of the core velocity. Using the implied scalings for the holdup ratio and wall shear stress, we can derive an expression for the pressure drop across the pipe in terms of the flow rates, which agrees well with our experimental measurements.
A smoothed particle hydrodynamics (SPH) study of sediment dispersion on the seafloor Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-08-18 Thien Tran-Duc, Nhan Phan-Thien, Boo Cheong Khoo
Ocean-scale sediment dispersion and sedimentation problems are studied using the Smoothed Particle Hydrodynamics (SPH). A SPH formulation based on a mixture model for two-phase flows is developed to investigate the problem. The sediment mass transport via the settling advection and the turbulent diffusion of the suspended sediment are fully accounted for in the current SPH model. The simulations are carried out in an opened boundary domain with a unidirectional underlined current, with relevant deposition/re-suspension boundary conditions on the seafloor. The factors influencing the sedimentation process, such as the hindering and the bottom shear stress effects, are also considered. The simulation results reveal that the sediment convection near the sediment source location is caused by both the ocean current and secondary density driven flows that are created by the concurrent settling motion of suspended sediment particles, while the downstream sediment transport in the far field is only driven by the ocean current. The peak sediment concentration in the ambient ocean water is found to correlate with the sediment release rate, and the settlement rate is inversely proportional to the initial height of the disturbed sediment.
Physics considerations in targeted anticancer drug delivery by magnetoelectric nanoparticles Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-04-06 Emmanuel Stimphil, Abhignyan Nagesetti, Rakesh Guduru, Tiffanie Stewart, Alexandra Rodzinski, Ping Liang, Sakhrat Khizroev
In regard to cancer therapy, magnetoelectric nanoparticles (MENs) have proven to be in a class of its own when compared to any other nanoparticle type. Like conventional magnetic nanoparticles, they can be used for externally controlled drug delivery via application of a magnetic field gradient and image-guided delivery. However, unlike conventional nanoparticles, due to the presence of a non-zero magnetoelectric effect, MENs provide a unique mix of important properties to address key challenges in modern cancer therapy: (i) a targeting mechanism driven by a physical force rather than antibody matching, (ii) a high-specificity delivery to enhance the cellular uptake of therapeutic drugs across the cancer cell membranes only, while sparing normal cells, (iii) an externally controlled mechanism to release drugs on demand, and (iv) a capability for image guided precision medicine. These properties separate MEN-based targeted delivery from traditional biotechnology approaches and lay a foundation for the complementary approach of technobiology. The biotechnology approach stems from the underlying biology and exploits bioinformatics to find the right therapy. In contrast, the technobiology approach is geared towards using the physics of molecular-level interactions between cells and nanoparticles to treat cancer at the most fundamental level and thus can be extended to all the cancers. This paper gives an overview of the current state of the art and presents an ab initio model to describe the underlying mechanisms of cancer treatment with MENs from the perspective of basic physics.
Wireless power transfer inspired by the modern trends in electromagnetics Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-04-28 Mingzhao Song, Pavel Belov, Polina Kapitanova
Since the beginning of the 20th century, researchers have been looking for an effective way to transfer power without wired connections, but the wireless power transfer technology started to attract extensive interest from the industry side only in 2007 when the first smartphone was released and a consumer electronics revolution was triggered. Currently, the modern technology of wireless power transfer already has a rich research and development history as well as outstanding advances in commercialization. This review is focused on the description of distinctive implementations of this technology inspired by the modern trends in electrodynamics. We compare the performances of the power transfer systems based on three kinds of resonators, i.e., metallic coil resonators, dielectric resonators, and cavity mode resonators. We argue that metamaterials and meta-atoms are powerful tools to improve the functionalities and to obtain novel properties of the systems. We review different approaches to enhance the functionality of the wireless power transfer systems including control of the power transfer path and increase of the operation range and efficiency. Various applications of wireless power transfer are discussed and currently available standards are reviewed.
Progress on bioinspired, biomimetic, and bioreplication routes to harvest solar energy Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-04-28 Raúl J. Martín-Palma, Akhlesh Lakhtakia
Although humans have long been imitating biological structures to serve their particular purposes, only a few decades ago engineered biomimicry began to be considered a technoscientific discipline with a great problem-solving potential. The three methodologies of engineered biomimicry––viz., bioinspiration, biomimetic, and bioreplication––employ and impact numerous technoscientific fields. For producing fuels and electricity by artificial photosynthesis, both processes and porous surfaces inspired by plants and certain marine animals are under active investigation. Biomimetically textured surfaces on the subwavelength scale have been shown to reduce the reflectance of photovoltaic solar cells over the visible and the near-infrared regimes. Lenticular compound lenses bioreplicated from insect eyes by an industrially scalable technique offer a similar promise.
Charge transfer plasmons: Recent theoretical and experimental developments Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-05-12 Alemayehu Nana Koya, Jingquan Lin
The unique property of a charge transfer plasmon (CTP) that emerges in conductively bridged plasmonic nanoparticles makes linked nanosystems suitable candidates for building artificial molecules, nanomotors, sensors, and other optoelectronic devices. In this focused review, we present recent theoretical and experimental developments in fundamentals and applications of CTPs in conductively coupled metallic nanoparticles of various sizes and shapes. The underlying physics of charge transfer in linked nanoparticles with nanometer- and atomic-scale inter-particle gap is described from both classical and quantum mechanical perspectives. In addition, we present a detailed discussion of mechanisms of controlling charge transfer and tuning the corresponding CTP spectra in bridged nanoparticles as functions of junction conductance and nanoparticle parameters. Furthermore, the active control of reversible switching between capacitive and conductive coupling in plasmonic nanoshell particles and dynamic evolution of related plasmon modes are emphasized. Finally, after highlighting the implication of the CTP resonance shift for surface-based sensing applications, we end up with the current challenges and future outlooks of the topic that need to be addressed.
Energy band offsets of dielectrics on InGaZnO4 Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-04-18 David C. Hays, B. P. Gila, S. J. Pearton, F. Ren
Thin-film transistors (TFTs) with channels made of hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) are used extensively in the display industry. Amorphous silicon continues to dominate large-format display technology, but a-Si:H has a low electron mobility, μ ∼ 1 cm2/V s. Transparent, conducting metal-oxide materials such as Indium-Gallium-Zinc Oxide (IGZO) have demonstrated electron mobilities of 10–50 cm2/V s and are candidates to replace a-Si:H for TFT backplane technologies. The device performance depends strongly on the type of band alignment of the gate dielectric with the semiconductor channel material and on the band offsets. The factors that determine the conduction and valence band offsets for a given material system are not well understood. Predictions based on various models have historically been unreliable and band offset values must be determined experimentally. This paper provides experimental band offset values for a number of gate dielectrics on IGZO for next generation TFTs. The relationship between band offset and interface quality, as demonstrated experimentally and by previously reported results, is also explained. The literature shows significant variations in reported band offsets and the reasons for these differences are evaluated. The biggest contributor to conduction band offsets is the variation in the bandgap of the dielectrics due to differences in measurement protocols and stoichiometry resulting from different deposition methods, chemistry, and contamination. We have investigated the influence of valence band offset values of strain, defects/vacancies, stoichiometry, chemical bonding, and contamination on IGZO/dielectric heterojunctions. These measurements provide data needed to further develop a predictive theory of band offsets.
Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-05-01 Rama K. Vasudevan, Nina Balke, Peter Maksymovych, Stephen Jesse, Sergei V. Kalinin
Ferroelectric materials have remained one of the major focal points of condensed matter physics and materials science for over 50 years. In the last 20 years, the development of voltage-modulated scanning probe microscopy techniques, exemplified by Piezoresponse force microscopy (PFM) and associated time- and voltage spectroscopies, opened a pathway to explore these materials on a single-digit nanometer level. Consequently, domain structures and walls and polarization dynamics can now be imaged in real space. More generally, PFM has allowed studying electromechanical coupling in a broad variety of materials ranging from ionics to biological systems. It can also be anticipated that the recent Nobel prize [“The Nobel Prize in Chemistry 2016,” http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2016/ (Nobel Media, 2016)] in molecular electromechanical machines will result in rapid growth in interest in PFM as a method to probe their behavior on single device and device assembly levels. However, the broad introduction of PFM also resulted in a growing number of reports on the nearly ubiquitous presence of ferroelectric-like phenomena including remnant polar states and electromechanical hysteresis loops in materials which are non-ferroelectric in the bulk or in cases where size effects are expected to suppress ferroelectricity. While in certain cases plausible physical mechanisms can be suggested, there is remarkable similarity in observed behaviors, irrespective of the materials system. In this review, we summarize the basic principles of PFM, briefly discuss the features of ferroelectric surfaces salient to PFM imaging and spectroscopy, and summarize existing reports on ferroelectric-like responses in non-classical ferroelectric materials. We further discuss possible mechanisms behind observed behaviors and possible experimental strategies for their identification.
How to measure the pyroelectric coefficient? Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-05-16 S. Jachalke, E. Mehner, H. Stöcker, J. Hanzig, M. Sonntag, T. Weigel, T. Leisegang, D. C. Meyer
The precise quantification of the pyroelectric coefficient p is indispensable for the characterization of pyroelectric materials and the development of pyroelectric-based devices, such as radiation sensors or energy harvesters. A summary of the variety of techniques to measure p is given in the present review. It provides a classification after the thermal excitation and an outline of capabilities and drawbacks of the individual techniques. The main selection criteria are: the possibility to separate different contributions to the pyroelectric coefficient, to exclude thermally stimulated currents, the capability to measure p locally, and the requirement for metallic electrodes. This overview should enable the reader to choose the technique best suited for specific samples.
Gas sensing in 2D materials Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-05-16 Shengxue Yang, Chengbao Jiang, Su-huai Wei
Two-dimensional (2D) layered inorganic nanomaterials have attracted huge attention due to their unique electronic structures, as well as extraordinary physical and chemical properties for use in electronics, optoelectronics, spintronics, catalysts, energy generation and storage, and chemical sensors. Graphene and related layered inorganic analogues have shown great potential for gas-sensing applications because of their large specific surface areas and strong surface activities. This review aims to discuss the latest advancements in the 2D layered inorganic materials for gas sensors. We first elaborate the gas-sensing mechanisms and introduce various types of gas-sensing devices. Then, we describe the basic parameters and influence factors of the gas sensors to further enhance their performance. Moreover, we systematically present the current gas-sensing applications based on graphene, graphene oxide (GO), reduced graphene oxide (rGO), functionalized GO or rGO, transition metal dichalcogenides, layered III-VI semiconductors, layered metal oxides, phosphorene, hexagonal boron nitride, etc. Finally, we conclude the future prospects of these layered inorganic materials in gas-sensing applications.
Advanced materials for magnetic cooling: Fundamentals and practical aspects Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-05-24 M. Balli, S. Jandl, P. Fournier, A. Kedous-Lebouc
Over the last two decades, the research activities on magnetocalorics have been exponentially increased, leading to the discovery of a wide category of materials including intermetallics and oxides. Even though the reported materials were found to show excellent magnetocaloric properties on a laboratory scale, only a restricted family among them could be upscaled toward industrial levels and implemented as refrigerants in magnetic cooling devices. On the other hand, in the most of the reported reviews, the magnetocaloric materials are usually discussed in terms of their adiabatic temperature and entropy changes (ΔTad and ΔS), which is not enough to get more insight about their large scale applicability. In this review, not only the fundamental properties of the recently reported magnetocaloric materials but also their thermodynamic performance in functional devices are discussed. The reviewed families particularly include Gd1-xRx alloys, LaFe13-xSix, MnFeP1-xAsx, and R1-xAxMnO3 (R = lanthanide and A = divalent alkaline earth)–based compounds. Other relevant practical aspects such as mechanical stability, synthesis, and corrosion issues are discussed. In addition, the intrinsic and extrinsic parameters that play a crucial role in the control of magnetic and magnetocaloric properties are regarded. In order to reproduce the needed magnetocaloric parameters, some practical models are proposed. Finally, the concepts of the rotating magnetocaloric effect and multilayered magnetocalorics are introduced.
Graphene and related two-dimensional materials: Structure-property relationships for electronics and optoelectronics Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-06-06 Xinming Li, Li Tao, Zefeng Chen, Hui Fang, Xuesong Li, Xinran Wang, Jian-Bin Xu, Hongwei Zhu
The exfoliation and identification of the two-dimensional (2D) single atomic layer of carbon have opened the opportunity to explore graphene and related 2D materials due to their unique properties. 2D materials are regarded as one of the most exciting solutions for next generation electronics and optoelectronics in the technological evolution of semiconductor technology. In this review, we focus on the core concept of “structure-property relationships” to explain the state-of-the-art of 2D materials and summarize the unique electrical and light-matter interaction properties in 2D materials. Based on this, we discuss and analyze the structural properties of 2D materials, such as defects and dopants, the number of layers, composition, phase, strain, and other structural characteristics, which could significantly alter the properties of 2D materials and hence affect the performance of semiconductor devices. In particular, the building blocks principles and potential electronic and optoelectronic applications based on 2D materials are explained and illustrated. Indeed, 2D materials and related heterostructures offer the promise for challenging the existing technologies and providing the chance to have social impact. More efforts are expected to propel this exciting field forward.
Functionality and versatility of aggregation-induced emission luminogens Appl. Phys. Rev. (IF 13.667) Pub Date : 2017-06-08 Guangxue Feng, Ryan T. K. Kwok, Ben Zhong Tang, Bin Liu
Breakthrough innovations in light-emitting materials have opened new exciting avenues for science and technology over the last few decades. Aggregation-induced emission (AIE) represents one of such innovations. It refers to a unique light-emitting phenomenon, in which luminescent materials that are non-emissive in molecular state can be induced to emit efficiently in aggregated state. The design and development of AIE luminogens (AIEgens) have overcome technical and fundamental limitations that exist in conventional light-emitting materials, and thus generate great opportunities for various applications. In this review, we aim to introduce the wonderful world of AIE to scientists from different disciplines by summarizing the recent progress made in this exciting research field. The mechanistic analyses and the working principles of the AIE processes are first elaborated, which reveal the restriction of intramolecular motions as the main cause for the AIE effect. The different molecular engineering strategies for the design of new AIEgens are subsequently discussed with examples of various AIEgen systems. The recent high-tech applications of AIEgens as optoelectronic materials, chemical sensors, and biomedical probes are presented and discussed. We hope that this review will stimulate more research interest from physics, chemistry, life science, and biomedical fields to this wonderland of AIE.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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