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  • Investigating channel flow using wall shear stress signals at transitional Reynolds numbers
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-22
    Rishav Agrawal; Henry C.-H. Ng; David J.C. Dennis; Robert J. Poole

    Time-resolved wall shear stress measurements are conducted to investigate channel flow at transitional Reynolds numbers. Constant temperature anemometry (CTA) is employed to measure the instantaneous wall shear stress using glue-on hot films as the sensing probes. Pressure-drop measurements are conducted to calibrate the mean hot-film voltage signals and to ensure that the pressure drop is measured in the so-called “fully-developed” region of the channel, a study of effect of entrance length on the pressure-drop measurements is carried out. Time history and higher order statistics of wall shear stress fluctuations reveal that the flow remains laminar until Reτ(=uτh/ν)≈43 in our channel flow facility, where uτ, h and ν are the friction velocity, channel half-height and kinematic viscosity, respectively. Third and fourth order moments of wall shear stress jump at the onset of transition and increase significantly until they reach maxima at about Reτ ≈ 48. After this Reynolds number, these two higher order moments start to decrease gradually with increasing Reynolds number and after Reτ≈73−79, any significant dependence of these two moments on Reynolds number disappears. Multiple hot-film measurements, which are located at different spatial locations, are conducted to characterize the large-scale turbulent structures. It is observed that there are structures, at least 7h wide, for Reτ between 46.8 and 53.9. Two-point spatial correlations reveal that on average these large structures are angled at approximately 17o for Reτ=46.8 and roughly between 32o and 37o for 48.7 < Reτ < 53.9 relative to the streamwise direction.

    更新日期:2020-01-23
  • Prediction of drag reduction effect by streamwise traveling wave-like wall deformation in turbulent channel flow at practically high Reynolds numbers
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-21
    Yusuke Nabae; Ken Kawai; Koji Fukagata

    A fully developed turbulent channel flow controlled by traveling wave-like wall deformation under a constant pressure gradient condition is studied numerically and theoretically. First, direct numerical simulation (DNS) at three different friction Reynolds numbers, Reτ=90, 180, and 360, are performed to investigate the modification in turbulence statistics and their scaling. Unlike the previous study assuming a constant flow rate condition, suppression of the quasi-streamwise vortices is not observed in either drag decrease cases or drag increase cases. It is found in the drag reduction case, however, that the periodic component of the Reynolds shear stress (periodic RSS) is largely negative in the viscous sublayer and the buffer layer. For the maximum drag reduction case, the set of control parameters is found to be identical in wall units regardless of the Reynolds number, and the resulting mean velocity profiles are also observed to be approximately similar even with an additional case of Reτ=720. Based on this scaling, we propose a semi-empirical formula for the mean velocity profile modified by the present control. With this formula, about 20%−25% drag reduction effect is predicted even at practically high Reynolds numbers, Reτ∼105−106.

    更新日期:2020-01-22
  • Turbulence characteristics over k-type rib roughened porous walls
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-21
    Yuki Okazaki; Ayumi Shimizu; Yusuke Kuwata; Kazuhiko Suga

    To understand the permeability effects on turbulent rib-roughened porous channel flows, particle image velocimetry measurements are performed at the bulk Reynolds number of 5000–20000. Solid impermeable and porous ribs are considered for the rib-roughness whose geometry is categorised in the k-type roughness whose pitch/rib-height is 10. Three isotropic porous media with nearly the same porosity: 0.8, and different permeabilities (0.004, 0.020, 0.033 mm2) are applied. It is observed that the recirculation between the ribs becomes weak and the recirculation vortex submerges into the porous wall as the wall permeability and Reynolds number increase for both solid and porous rib cases while the recirculation vanishes in high permeable cases. These phenomena result in characteristic difference in turbulence quantities. By fitting the mean velocity profiles to the log-law form, the permeability effects of both rib and bottom wall on the log-law parameters and the equivalent sand-grain roughness are discussed. It is concluded that the zero-plane displacement increases while the von Kármán constant and the equivalent sand-grain roughness decrease as the wall and rib permeability increases.

    更新日期:2020-01-22
  • Vortex dynamics mechanisms of separated boundary layer in a highly loaded low pressure turbine cascade
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-21
    Seyed Morteza Sajadmanesh; Arman Mohseni; Mohammad Mojaddam

    This paper investigates the vortex dynamics in the suction-side boundary layer on an aero-engine low pressure turbine blade at two different Reynolds numbers at which short and long laminar separation bubbles occur. Different vortical patterns are observed and investigated through large eddy simulation (LES). The results show that at the higher Reynolds number, streamwise streaks exist upstream of separation line. These streaks initiate spanwise undulation in the form of vortex tubes, which roll-up and shed from the shear layer due to the Kelvin–Helmholtz instability. The vortex tubes alternately pair together and eventually distort and break down to small-scale turbulence structures near the mean reattachment location and convect into a fully turbulent boundary layer. At the lower Reynolds number, streamwise streaks are strong and the separated flow is unable to reattach to the blade surface immediately after transition to turbulence. Therefore, bursting of short bubbles into long bubbles can occur, and vortex tubes have larger diameters and cover a part of the blade span. In this case vortex pairing does not occur and vortex shedding process is promoted mainly by flapping phenomenon. Moreover, the results of dynamic mode decomposition (DMD) analysis show a breathing motion as a source of unsteadiness in the separation location, which is accompanied by the flapping phenomenon.

    更新日期:2020-01-22
  • Flow and mixing characteristics of a forward-inclined stack-issued jet in crossflow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-20
    Y.A. Altaharwah; R.F. Huang; C.M. Hsu
    更新日期:2020-01-21
  • A phase averaged PIV study of circular and non-circular synthetic turbulent jets issuing from sharp edge orifices
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-15
    A. Hashiehbaf; G.P. Romano

    An extensive experimental study using Particle Image Velocimetry (PIV) on synthetic jets issuing from different orifice shapes is reported. All data are phase and time averaged to derive mean velocity, half-velocity width and rms velocity profiles in the near field of the jet (0 < X/D < 7), at a Reynolds number around 10,000. Different non-circular orifice shapes as rectangular, square, elliptic and triangular are considered and results are compared to those of the circular orifice in order to investigate the effect of asymmetry on the turbulent flow field in view of mixing enhancement. The measurements are carried out on two orthogonal planes to capture three dimensional features of non-circular jets. Results show highest velocity decay rate for elongated orifices, especially the rectangular one, in comparison to the circular one, both in phase and time-averaged plots. Time averaged results show higher velocity decay rate of synthetic jets in comparison to those of continuous ones. It is also observed that, for X/D > 5, only profiles of circular and square jets become partially self-similar. For synthetic jets, higher turbulence content is measured for all orifice shapes at the centerline and close to the orifice exit in comparison to continuous jets.

    更新日期:2020-01-15
  • 更新日期:2020-01-13
  • Comparison of the flow-field characteristics of a slot synthetic jet with and without sidewalls
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-09
    Gopal Krishan; Kean C Aw; Rajnish N. Sharma

    In this paper, experimental investigation of the flow-field of a slot synthetic jet (SJ) with and without sidewalls, issuing into a quiescent environment is systematically reported. Two parallel sidewalls were mounted along the shorter side of the slot extending in the streamwise direction to constrain the flow along the slot span. Hot-wire anemometry was used to explore the flow-field characteristics of both configurations at a Reynolds number of 4000 based on the slot-width and slot average exit velocity during the ejection phase. The present work is a first step towards the investigation of the SJ flow-field characteristics in a bounded region. In a number of generic situations, this work is of high importance as the SJ is deployed in constrained environments (e.g., in cooling applications) where sidewalls may be present. The relative difference in the magnitude of the distinct peaks in the near-field spanwise velocity profiles for both configurations reveals that the vortex does not get curled up towards the centerline in the case of the synthetic jet with sidewalls due to the presence of the no-slip walls. Spectral analysis in the near-wall region further confirms the absence of the phenomenon of axis-switching in the case of the synthetic jet with sidewalls. This behavior is also demonstrated with the help of the relative spreading rate in both configurations, where the unbounded synthetic jet spreads rapidly compared to the bounded one, due to greater entrainment of the surrounding fluid. The statistically two-dimensional region for a synthetic jet with sidewalls is found to extend over a longer axial distance in the downstream. The other jet properties such as turbulence intensity, skewness, and flatness factors further reveal the differences in the flow-field of the two configurations. The results show that the presence of the sidewalls strongly influences the SJ flow-field and hence, it would significantly impact the heat transfer capacity of the SJ.

    更新日期:2020-01-09
  • Influence of jet exit conditions on mixing and statistics of flow fine structures
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-09
    Valery Zhdanov; Nikolai Kornev; Egon Hassel

    Mixing in a coaxial jet mixer was investigated at variable initial conditions simultaneously using PIV and PLIF methods. Velocity and concentration fields were measured at two spatial resolutions: across the mixer and in a small area at its axis resolving Kolmogorov's scale. Jet exit conditions were modeled by varying flow rate (3⋅103 ≤ Red ≤ 1.78⋅104) and by applying vortex generators (tabs) at the jet exit. The study was executed within a transition flow region. Nevertheless, it is possible to claim that in the self-similarity flow region mean flow parameters do not depend on the initial conditions but turbulence does. The influence of the initial conditions is manifested in turbulent characteristics measured at a high spatial resolution and in the size of fine structures of the studied flows.

    更新日期:2020-01-09
  • Interaction between separation bubble and impinging vortices over a finite blunt plate
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-09
    Yifan Deng; Peng Wang; Zaomin Cao; Yingzheng Liu

    In this work, the interactive process between separation bubble and impinging leading-edge vortices, which is superimposed in unsteady separated and reattaching flows over a finite blunt plate, was experimentally determined. We used an effective recognition strategy based on an offline data analysis by dynamic mode decomposition (DMD) and online DMD computation in a field-programmable gate array (FPGA). The approach was established for phase-locking particle image velocimetry (PIV) measurements of the underlying vortex dynamics. The DMD mode coefficients of wall-pressure fluctuations following the temporal evolution of the corresponding unsteady events clearly reflected the interactive process, in which three consecutive impinging vortices appeared in one period of the separation bubble. The global interactive process between the separation bubble and the impinging vortices was classified into three consecutive processes: an amplification process, a transition process and a preparation process. The phase-averaged PIV measurements revealed the following: (a) During the amplification process, the separation bubble and impinging vortices are in a synchronous enlargement process beginning from the shortest position of the separation bubble atx/D=3.00. (b) In both the transition and preparation processes, the interaction was characterized by a flapping separation bubble (which enlarged in the first half of the period and then shrank in the second half of the period) and by the impinging vortices constantly shedding to the downstream region. Finally, the underlying shear layer instabilities and vortex movements of these three interactive processes were analyzed.

    更新日期:2020-01-09
  • Mixing characteristics of a film-exciting flapping jet
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-09
    M. Wu; M. Xu; J. Mi; R.C. Deo

    We have recently discovered a new type of self-excited flapping jets due to a flexible film whose leading edge is fixed at the nozzle exit [Exp Ther Fluid Sci, 106, 226-233]. This paper is to report the experimental investigation on mixing characteristics of the jet induced by a rectangular FEP film. Hot wire anemometry and flow visualization are used to examine the flapping jet flow versus the non-flapping counterpart. Experiments are conducted under the following conditions: i.e., L/D = 1.0 (fixed), W/D = 0.03 ~ 1.0 (varying) and Re = 10000 ~ 45000 (varying); where W and L are the film's width and length, D is the nozzle-exit diameter, and Re is the Reynolds number defined by Re UoD/ν with Uo and ν being the jet-exit velocity and fluid viscosity. It is found that the jet-flapping frequency fF varies with W in a complex fashion while it grows roughly linearly with increasing Uo for W/D ≥ 0.5. The flapping Strouhal number StF fFD/Uo ranges in 0.13 ≤ StF ≤ 0.23 for Re = 15,000 ~ 45,000. These Strouhal numbers are substantially lower than that (≈ 0.45 ~ 0.7) for the primary vortex generation in the free jet, but one to two orders of magnitude higher than those from the conventional self-exciting fluidic devices. In general, the flapping jet decays and spreads more rapidly than does the free jet. As W increases, the decaying and spreading rates both grow. Of significance, the centerline evolutions of Taylor and Kolmogorov scales versus the integral scale are examined to characterize the small scales of turbulence against the large-scale motion.

    更新日期:2020-01-09
  • 更新日期:2020-01-07
  • Diffusion layer thickness in turbulent flow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-05
    A.A. Burluka

    Average thickness of diffusive layers in a turbulent flow is described using an idea of Lagrangian meso-scale element convected by mean flow and large scale turbulence. This idea enables a formulation of a simple model for the diffusive layer thickness assuming that its evolution is determined by the diffusive growth and two components, compressive normal and tangential, of the turbulent strain rate tensor. Analysis of the possible effects of the folding action of the turbulence leads to the conclusion that the folding becomes significant only at the scales far superior to the considered dimensions of the meso-scale elements, thus it may be neglected in the present formulation. The evolution equation for the meso-scale element thickness is derived and put to test against experiments conducted in plane and round jets. The model proved capable of producing, using the same values of two model constants, values of the diffusive layer thickness in good qualitative agreement with the measurements. While the present numerical simulations of the turbulent jets are made using very simple, perhaps simplistic, flow and turbulence description, they nonetheless allow a fairly accurate estimation of turbulence microscales at different locations in a jet. It turns out that neither Kolmogorov nor Taylor scale provides a good universal reference scale for the diffusive layer thickness and it is local turbulence conditions and history of the meso-scale element determining the latter.

    更新日期:2020-01-06
  • Vortex dynamics and scalar transport in the wake of a flat-plate controlled by a vibrating trailing-edge flap
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-02
    R.H. Hernández; L. Tapia

    We investigate the onset and development of vortical flow disturbances introduced into the wake of a horizontally fixed flat-plate by means of the controlled motion of a trailing edge flap. The vibrating mechanics of the flap allows for the introduction of both impulsive and harmonic weak amplitude velocity disturbances which are propagated downstream into the wake flow of the flat-plate. Quantitative experimental and numerical predictions of both steady and unsteady wake flow velocity resulting from different flapping frequencies are made at low Reynolds numbers (Re < 104). Frequency response tests of the wake confirmed the existence of two dominant frequencies where the wake flow organises with a particular arrangement of downstream moving vortex structures. Numerical predictions of steady (unforced) and forced wake velocity profiles and kinetic energy profiles are in good agreement with the experimental results. In order to understand practical implications of the dominant vortex structures in scalar transport, we have extended the numerical part of the study solving for the concentration equation of a passive scalar being injected in particular regions of the physical domain. A spatial correlation between the trajectory of vortex structures and the scalar concentration downstream the wake is observed. Moreover, the onset of tip vortex structures produced during the forcing cycle seems to be responsible of a local increase of scalar concentration near the span wise flap ends.

    更新日期:2020-01-02
  • A comprehensive assessment of the Reynolds Analogy in predicting heat transfer in turbulent wall-bounded shear flows
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2020-01-02
    Matthias Ziefuß; Amirfarhang Mehdizadeh

    Heat transfer modeling plays a major role in design and optimization of modern and efficient thermal-fluid systems. However, currently available models suffer from a fundamental shortcoming: their development is based on the general notion that accurate prediction of the flow field will guarantee an appropriate prediction of the thermal field, known as the Reynolds Analogy. This investigation presents a comprehensive assessment of the capability of the Reynolds Analogy in predicting turbulent heat transfer when applied to turbulent shear flows of fluids with different Prandtl numbers. It turns out that the Reynolds Analogy is able to provide acceptable results for first order statistics only when fluids with Prandtl number close to unity are considered. Further, it is shown that unsteady simulations could provide acceptable results on second order statistics concerning fluids with different Prandtl numbers, if appropriate grid design/resolution is provided that allows to resolve essential dynamics of the thermal field. However, accurate prediction of higher order statistics close to solid surface requires more advanced heat transfer models that can provide accurate information on thermal time scales, in case the grid is too coarse to support accurate resolving of the essential thermal dynamics in these regions.

    更新日期:2020-01-02
  • Secondary flow and heat transfer in turbulent flow over streamwise ridges
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-18
    A. Stroh; K. Schäfer; P. Forooghi; B. Frohnapfel

    Surface structuring in form of streamwise-aligned triangular ridges is investigated in the framework of a fully developed turbulent channel flow with constant wall temperatures of different values prescribed on the upper and lower walls at Reb=18000. Two arrangements of the ridges on both channel walls are considered – a symmetrical arrangement and a staggered arrangement with a spanwise shift of the upper wall structure by a half ridge separation. The ridges generate a strong large-scale secondary motion and hence enhance momentum and heat transfer in the channel by approximately 30% relatively to the smooth channel. In spite of the fact that both arrangements translate into very similar global flow properties, the composition of skin friction coefficient and Stanton number significantly differs. The componental split-up of the friction coefficient reveals that the enhancement of momentum transfer mainly originates from the dispersive component linked to the secondary flows. For the Stanton number, however, the enhancement arises not only from the dispersive component, but also from a strong modification of the turbulent flow properties.

    更新日期:2019-12-18
  • Investigation of thermal dispersion and intra-pore turbulent heat flux in porous media
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-16
    Nima Fallah Jouybari; T. Staffan Lundström; J.Gunnar I. Hellström

    In the present study, the importance of the thermal dispersion and the turbulent heat flux in porous media and their effects on the macroscopic distribution of thermal energy are investigated. To this end, turbulent flow and heat transfer within five unit-cells mimicking porous media are solved using large eddy simulation. It is shown that the thermal dispersion and the turbulent heat flux are negligible as compared to the convection term in the macroscopic energy equation. When further scrutinizing this equation, it is revealed that except for the longitudinal components of the thermal dispersion, the other components of thermal dispersion and turbulent heat flux may be neglected away from the boundaries as compared to the interfacial heat transfer. Visualizations of vortices show that the size of the turbulence structures within the cells is of the same order as the size of the pores; therefore, the turbulent heat flux is limited to the intra-pore level. Finally, a discussion is provided on the accuracy of the gradient type diffusion model commonly used for turbulent heat flux in porous media in the absence of macroscopic turbulence. It is shown that the intra-pore turbulence does not affect the macroscopic transport of thermal energy within the porous media studied.

    更新日期:2019-12-17
  • Wall jet similarity of impinging planar underexpanded jets
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-09
    Patrick Fillingham, Igor V. Novosselov

    Velocity profiles and wall shear stress values in the wall jet region of planar underexpanded impinging jets are parameterized based on nozzle parameters (stand-off height, jet hydraulic diameter, and nozzle pressure ratio). Computational fluid dynamics is used to calculate the velocity fields of impinging jets with height-to-diameter ratios in the range of 15–30 and nozzle pressure ratio in the range of 1.2–3.0. The wall jet has an incomplete self-similar profile with a typical triple-layer structure as in traditional wall jets. The effects of compressibility are found to be insignificant for wall jets with Ma < 0.8. Wall jet analysis yielded power-law relationships with source dependent coefficients describing maximum velocity, friction velocity, and wall distances for maximum and half-maximum velocities. Source dependency is determined using the conjugate gradient method. These power-law relationships can be used for mapping wall shear stress as a function of nozzle parameters.

    更新日期:2019-12-11
  • A study on thrombus influenced red blood cell flow in microvasculature using moving particle semi-implicit method
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-10
    Ze-Xiao Wang, Jian Guan, Lei Chen, Wen-Quan Tao

    Microvasculature plays a decisive role on the normal operation of the human body. Previous studies have shown that the causes of microvascular hemolytic anemia and other diseases are closely related to the interaction between micro-thrombi and RBCs. The movement and deformation of Red Blood Cells (RBCs) in microvasculature with hemicyclic micro-thrombi of different sizes on the wall are simulated based on the Moving Particle Semi-implicit method (MPS) and the spring network model of RBCs membrane. Simulation of a single RBC passing the straight blood vessel indicates the strong squeeze of the RBC caused by the thrombus, which leads to a 38.5% increasing of the RBC velocity and a greater deformation, and such squeeze effect is positively related with the size of the thrombus. When two RBCs pass through the straight blood vessel with two thrombi on the both sidewalls, the deformation of the RBCs first increases and then decreases. Results show that when the axial position between the two thrombi is 10 × d0 different, the deformation of RBCs reaches the maximum of 3.10 (upper) and 2.79 (lower), respectively. When two side-by-side RBCs pass through a bifurcated blood vessel with a sidewall thrombus, the velocity and deformation of RBCs are greatly affected by the thrombus. When the thrombus radius changes from 0 × d0 to 20 × d0, the peak velocities of the two cells increase by 51.6% (upper) and 67.9% (lower), respectively.

    更新日期:2019-12-11
  • Predicting crossflow induced transition with laminar kinetic energy transition model
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-06
    Yupei Qin, Xin Yu

    The hypersonic laminar kinetic energy transition model is developed to be appropriate for crossflow induced boundary layer transition prediction. A crossflow timescale is constructed and incorporated in the kT-kL-ω transition model to reflect crossflow effect during three-dimensional boundary layer transition. The stream-wise vorticity is selected as the indicator of crossflow strength. Regarding the inviscid unstable characteristic of crossflow instability, the crossflow timescale is constructed by reference to the second mode timescale. To eliminate inappropriate development of the crossflow timescale where the effective length scale is large enough while the crossflow strength remains at a quite low level, a crossflow velocity limit function is proposed. The revised kT-kL-ω transition model has been applied to HIFiRE-5 and blunt cone with 1°angle of attack test cases. Results show good correspondence with the experimental data and DNS data, which demonstrates that the constructed crossflow timescale makes the revised transition model capable of reproducing crossflow induced transition behavior with a reasonable degree of accuracy.

    更新日期:2019-12-07
  • Direct numerical simulation of turbulent heat transfer over fully resolved anisotropic porous structures
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-05
    Yudai Nishiyama, Yusuke Kuwata, Kazuhiko Suga

    Lattice Boltzmann direct numerical simulations of turbulent heat transfer over and inside anisotropic porous media are performed. This study considers turbulent plane channel flows whose bottom walls are made from the porous media at the bulk Reynolds number of 2900 with isothermal and conjugate heat transfer wall conditions. Four different porous walls are considered. They are walls with only the wall-normal permeability, with the wall-normal and spanwise permeabilities, with the wall-normal and streamwise permeabilities, and with the isotropic wall-normal, spanwise and streamwise permeabilities. The porosity of the porous walls ranges from 0.6 to 0.8. Discussions on the effects of the anisotropic permeability on turbulent thermal fields are carried out by the instantaneous flow visualizations and the statistical quantities. In particular, temperature fluctuations, turbulent and dispersion heat fluxes are examined both inside and outside the porous walls. Finally, the heat transfer performance is discussed considering the effects of the anisotropic permeability.

    更新日期:2019-12-05
  • Structural inclination angle of near-field scalar fluctuations in a turbulent boundary layer
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-12-04
    Krishna M. Talluru, Kapil A. Chauhan

    Two-point concentration measurements are obtained in a meandering passive-scalar plume released at five different heights within the fully-turbulent region of a high-Reynolds number turbulent boundary layer (TBL). Mean statistics of two-point concentration measurements are found to agree very well with the single-point measurements previously reported in Talluru et al. (2017a). The two-point correlation results of concentration indicate strong coherence in the scalar field similar to the large-scale coherence observed in the streamwise velocity fluctuations in a TBL (Marusic and Heuer, 2007). Particularly, the isocontours in the two-dimensional correlation map of concentration fluctuations illustrate that the scalar structures are inclined at 30∘ to the direction of the flow; such a trend is consistently observed for all the elevated plumes below z/δ ≤ 0.33. This observation of steeper inclination angle of scalar structures relative to the inclination angle of large-scale velocity fluctuations in a TBL is explained using the physical model put forth by Talluru et al. (2018). Most importantly, these results provide insights on the differences in the structural organisation of a passive scalar plume in the near- and the far-field regions.

    更新日期:2019-12-04
  • Numerical study of mechanisms of air-core vortex evolution in an intake flow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-28
    Dan Zi, Anqing Xuan, Fujun Wang, Lian Shen

    The mechanism of the generation and evolution of the air-core vortex formed between a free surface and a horizontal flow intake is investigated using large-eddy simulation of two-phase flows, with the air-water interface captured by a coupled level-set and volume-of-fluid method. The simulation provides a detailed description of the flow features of the air-core vortex formation. Based on the simulation data, the flow patterns and the underlying vorticity dynamics at different stages of the vortex formation are analyzed. It is found that the development of the air-core vortex is associated with the evolution of the vertical component of the vorticity. Analyses of the vorticity evolution dynamics reveal the fundamental mechanism of the air-core vortex generation, which is the amplification of the free-surface vertical vortices due to the vertical stretching effect above the intake associated with the pipe suction. Initially, weak vertical vortices caused by the spanwise disturbance of streamwise flows are randomly distributed near the surface and are enhanced by the stretching effect. Then, these randomly distributed vortices interact with each other and evolve into a dominant vortex, which eventually draws air into the intake and creates a fully-developed air-core vortex. The mechanism responsible for maintaining the fully-developed air-core vortex is still the vertical stretching effect in the core. The tilting from streamwise and spanwise vortices to vertical vortices, albeit large in the presence of the fully-developed vortex, nearly cancel each other and has negligible influence on sustaining the vortex.

    更新日期:2019-11-29
  • Energy redistribution dynamics in coupled Couette–Poiseuille flows using large-Eddy simulation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-28
    Santiago López Castaño, Bernard J. Geurts, Vincenzo Armenio

    The problem of turbulent Couette flow driven by a statistically steady external wind is studied in the framework of spatially filtered Navier–Stokes equations. The phenomenon of wind-driven flow of water is represented by a layer of air modeled as Poiseuille flow (air sub-domain), coupled to a layer of water modeled as Couette flow (water sub-domain). We focus on changes in the statistics in either the air or the water sub-domain, due to the coupling with the other sub-domain. We also highlight dynamic flow structures forming near the air-water interface. Simulations based on different Reynolds numbers in the air and the water sub-domains are compared to computationally less demanding simulations with equal Reynolds numbers. Results of these simulations indicate strong similarities, i.e., the flow is well approximated by simulating air and water at the same Reynolds numbers. Further analysis shows that the flow in the water domain shares important features with classical Couette flows. The horizontal turbulent mixing renders a thinner boundary layer in the water sub-domain. Moreover, an increased intermittency in the flow velocities is observed, which may be linked to so-called splat events near the air-water interface. These splats characterize the interaction of coherent structures across the interface, being stronger in the water phase. An analysis of the pressure-strain correlation near the air-water interface on the water side shows that such splats are responsible for redistributing energy from the streamwise and spanwise directions, to the vertical direction. This behavior, although qualitatively similar to wall-bounded flows, differ mainly on the fact that most of the energy drained comes from the streamwise direction: in wall-bounded the main contributor is the spanwise direction. The boundary layers near the air-water interface show inclined vortical structures. Unlike in coupled Couette–Couette flow, the peak in the Reynolds stress is displaced from the channel’s center into the buffer region of the water sub-domain.

    更新日期:2019-11-28
  • Large-eddy simulation of counter-rotating Taylor–Couette flow: The effects of angular velocity and eccentricity
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-15
    Dhaval Paghdar, Sourabh Jogee, Kameswararao Anupindi

    In the present work, turbulent flow in the annulus of a counter-rotating Taylor-Couette (CRTC) system is studied using large-eddy simulation. The numerical methodology employed is validated, for both the mean and second-order statistics, with the direct numerical simulation (DNS) data available in the literature, for a range of Reynolds numbers from 500 to 4000. Thereafter, turbulent flow occurring in this system at Reynolds numbers of 8000 and 16000 are studied, and the results obtained are analyzed using mean and second-order statistics, vortical structures, velocity vector plots and power energy spectra. Further, the spatio-temporal variation of azimuthal velocity, extracted near the inner cylinder, shows the existence of herringbone like patterns similar to that observed in the previous studies. The effect of eccentricity of the inner cylinder with respect to the outer cylinder is studied, on the turbulent flow in the CRTC system, for two different eccentricity ratios of 0.2 and 0.5 and for two different Reynolds numbers of 1500 and 4000. The results of the eccentric CRTC are analyzed using contours of pressure, mean and second-order statistics, velocity vectors, vortical structures, and turbulence anisotropy maps. It is observed from the eccentric CRTC simulations that the smaller-gap region seems to contain higher amplitude fluctuations and more vortical structures when compared with the larger-gap region. The mean turbulent kinetic energy contours do not change qualitatively with the Reynolds number, however, quantitatively a higher turbulent kinetic energy is observed in the higher Reynolds number case of 4000.

    更新日期:2019-11-18
  • Regularized, parameter free scale similarity type models for Large Eddy Simulation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-10
    Markus Klein, S. Ketterl, L. Engelmann, A. Kempf, H. Kobayashi

    The fidelity of Large Eddy Simulations (LES) depends strongly on the closures of the sub-grid scale (SGS) stress tensor. Although it is well known that the SGS stresses in LES are not aligned with the strain rate tensor, the most widely used models are still of eddy viscosity type, due to their robust behavior in LES and reasonable performance in a posteriori testing. The unstable behavior of more advanced anisotropic models, that is typically found in LES, has been attributed to either the fact that these models provide backscatter or to the fact that they do not provide a sufficient amount of dissipation. Based on recent advances in the field, an alternative modeling strategy is suggested, which can be used to regularize an arbitrary anisotropic (e.g. scale similarity type) model. The resulting model is easy to implement, can be written in compact form and is free of model parameters. The model has been tested a-posteriori and results are presented for a Taylor-Green-Vortex, a free plane jet and a turbulent channel flow of friction Reynolds numbers 395, 590 and 934. The results are compared to well-known eddy viscosity models and when applicable, to simulations without explicit LES model. The new model exhibits good performance for a variety of mesh resolutions and for all configurations. Furthermore, a-priori analysis results in the context of liquid atomization indicate that the model might be suitable as well in more complex physical scenarios. The a-priori analysis performance of the model is found to be nearly equivalent to the underlying structural anisotropic model in terms of its correlation coefficient, but the model is free of backscatter and provides good stability in LES.

    更新日期:2019-11-11
  • Numerical modeling of electrostatic spray painting transfer processes in rotary bell cup for automotive painting
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-08
    Mohammad-Reza Pendar, José Carlos Páscoa

    Here we implemented a 3D comprehensive Eulerian-Lagrangian model in order to investigate the electrostatic spray transfer processes in the high-speed rotary bell sprayer. This efficient algorithm contains spray dynamics, airflow, paint droplets tracking and an electrostatic effect to simulate atomization. The algorithm is implemented using the OpenFOAM package. A solver for the particle trajectory was used to illustrate the process of spray transport and also the interaction of the airflow and the particle that is solved by momentum coupling. Creating an initial condition of the particle approach has been proposed that is matched with practical applications. The fluid-dynamics is simulated by solving the unsteady 3D compressible Navier-Stokes equations. Unsteady flow is computed by using a Large eddy simulation (LES) turbulence approach, while the motion of the particles is simulated by tracking the droplet size distribution approach. The model correctly predicts that the bell cup spin forces the paint particles to fall off from the bell surface towards the high-velocity airflow. The present work illustrates a tentative benchmark and contains a systematic analysis of the recirculation zone length, the toroidal vortex, the overspray phenomena and the flowfield characteristics like mean velocity, pressure, turbulent kinetic energy and velocity fluctuation. The results indicate as dominant operating parameter the air-paint flow rate with voltage level deeply affecting the spray shape. A more uniform distribution of the coating is obtained by growing this high-velocity shaping airflow, although the values of the transfer efficiency (TE) are reduced. The distribution of the particle size is very sensitive to changes in the rotational speed. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable insights to design modern ERBS. The paint spray distribution obtained in the present work is validated against coating experimental results with suitable accuracy.

    更新日期:2019-11-08
  • Experimental and numerical characterization of the vortex zones along a labyrinth milli-channel used in drip irrigation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-07
    Jafar Al-Muhammad, Séverine Tomas, Nassim Ait-Mouheb, Muriel Amielh, Fabien Anselmet

    The labyrinth-channel is largely used in dripper systems. The baffles play an important role to generate the head losses and induce the flow regulation on the drip irrigation network. But they also develop vorticity regions where the velocity is low or zero. These vorticity regions promote the deposition of particles or other biochemical development causing dripper clogging. The flow in the dripper labyrinth-channel must be described to analyze dripper clogging sensibility which drastically reduces its performance. This characterization is performed experimentally using the micro-particle-image-velocimetry (Micro - PIV) method, and numerically using the RSM Simulation. In this study, Micro-PIV experiments allow to analyze the flow in ten-pattern repeating baffles which reproduce the micro-irrigation dripper. The cross section is equal to 1 mm2 and the inlet Reynolds number varies from 345 to 690. The present study first introduces a global analysis of the flow through the mean velocity modulus, the Reynolds stresses u′2¯, v′2¯ and u′v′¯ and the turbulence Reynolds number. Then, results for the mean strain rate and the mean spanwise vorticity are presented and discussed. Next, advanced methods of vortex detection are introduced and analyzed to better distinguish the vortex zones and to determine the vortex sizes. Furthermore, the numerical model is used to validate and analyze in a more detailed way the experimental results obtained by Micro-PIV.

    更新日期:2019-11-08
  • Mechanism of compressor airfoil boundary layer flow control using nanosecond plasma actuation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-06
    Haideng Zhang, Yun Wu, Yinghong Li

    The flow control effects of nanosecond plasma actuation on the boundary layer flow of a typical compressor controlled diffusion airfoil are investigated using large eddy simulation method. Three types of plasma actuation are designed to control the boundary layer flow, and two mechanisms of compressor airfoil boundary layer flow control using nanosecond plasma actuation have been found. The plasma actuations located within the laminar boundary layer flow can induce a small vortex structure through influencing on the density and pressure of the flow field. As the small vortex structure moves downstream along the blade surface with the main flow, it can suppress the turbulent flow mixing and reduce the total pressure loss. The flow control effect of the small vortex structure is summarized as wall jet effect. Differently, the plasma actuation located within the turbulent boundary layer flow can act on the shear layer flow and induce a large vortex structure. While moving downstream, this large vortex structure can suppress the turbulent flow mixing too.

    更新日期:2019-11-07
  • Direct numerical simulation of turbulent heat transfer in a wall-normal rotating channel flow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-06
    Cale Bergmann, Bing-Chen Wang

    Investigations into the characteristics of turbulent heat transfer and coherent flow structures in a plane-channel subjected to wall-normal system rotation are conducted using direct numerical simulation (DNS). In order to investigate the influence of system rotation on the temperature field, a wide range of rotation numbers are tested, with the flow pattern transitioning from being fully turbulent to being quasilaminar, and eventually, fully laminar. In response to the Coriolis force, secondary flows appear as large vortical structures, which interact intensely with the wall shear layers and have a significant impact on the distribution of turbulence kinetic energy (TKE), turbulence scalar energy (TSE), temperature statistics, and turbulent heat fluxes. The characteristic length scales of turbulence structures responsible for the transport of TSE are the largest at the quasilaminar state, which demands a very large computational domain in order to capture the two-dimensional spectra of temperature fluctuations. The effects of the Coriolis force on the turbulent transport processes of the temperature variance and turbulent heat fluxes are thoroughly examined in terms of their respective budget balances.

    更新日期:2019-11-06
  • Fluid dynamics of a bifurcation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-06
    Kaustav Pradhan, Abhijit Guha

    The objectives of the present paper are to accurately determine the modifications to a three-dimensional flow field caused by a bifurcation module, to study the downstream evolution of the generated flow field, and to enhance understanding by establishing the individual and combined roles of five factors (viz. curvature of flow path, flow division at the bifurcation ridge, possible change in flow area from mother to daughter branches, complex shape changes in the bifurcation and inertia of the flow) in giving rise to such a flow field in the bifurcation module. The effects of the aforementioned five factors on the loss production in a bifurcation module and on the potential of further loss in downstream units are also studied, and new correlations are developed. The detailed analysis is systematized here by establishing two novel methods of construction of a bifurcation, viz. “co-joining of two bent pipes” and “splitter in a pipe”, and by formally deriving the equivalence condition for the flow in a bifurcation and its constituent elements. Through this systematization an attempt is made to understand comprehensively the complexity of the fluid dynamics occurring in a single bifurcation, which is often masked in the usual studies of flow in large bifurcating networks. Several bifurcation geometries are studied, and about 500 separate three-dimensional computations are performed to achieve a degree of generalization. Use of fine grid (with up to 20 million computational elements in some simulations), double-precision arithmetic and stringent convergence criteria (10−8 for each scaled residual) ensures high accuracy of the computed solutions. Both primary and secondary flow fields are investigated. Flow path curvature is responsible for the development of Dean-type secondary motion while flow division at the bifurcation ridge generates secondary motion opposite to that induced by curvature. An increase of flow area from inlet to outlet results in an increase of asymmetry in cross-sectional velocity distribution. Although the loss across a bifurcation may sometimes be smaller than that across its constituent elements, it is shown here through the introduction of two parameters that a greater potential for incurring losses in a following straight section is generated in the bifurcation.

    更新日期:2019-11-06
  • Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-06
    Aniket P. Kulkarni, Vasudev D. Chaudhari, Shubham R. Bhadange, D. Deshmukh

    This paper presents planar measurements of Sauter mean diameter (SMD) and liquid volume fraction distributions of airblast spray injected into cross-flow. The experiments are conducted using a combination of structured laser illumination planar imaging with laser sheet drop-sizing (SLIPI-LSD) and particle/droplet imaging analysis (PDIA) techniques. Effect of gas to liquid mass ratio (GLR) and cross-flow velocity (Ucross) is studied. Planar SMD distribution at low GLR improved with increase in Ucross due to secondary atomization of large droplets. Uniform SMD distribution in a range of 10–20 µm is observed for GLR more than 3. The distribution of liquid volume fraction at low GLR condition shows poor dispersion with most of the liquid concentrated near the injector. The liquid volume fraction distribution improves with increase in GLR and better dispersion is observed for GLR more than 3 and two-phase momentum ratio (q2) greater than 13.06. Spatial bifurcation in liquid fraction is found for high GLR conditions. The SMD in the range of 10–20 µm and uniform distribution of liquid are observed for GLR more than 3 and q2 > 25.6.

    更新日期:2019-11-06
  • Using a Gaussian process regression inspired method to measure agreement between the experiment and CFD simulations
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-05
    Yu Duan, Christopher Cooling, Ji Soo Ahn, Christopher Jackson, Adam Flint, Matthew D. Eaton, Michael J. Bluck

    This paper presents a Gaussian process regression inspired method to measure the agreement between experiment and computational fluid dynamics (CFD) simulation. Because of misalignments between experimental and numerical outputs in spatial or parameter space, experimental data are not always suitable for quantitative assessing the numerical models. In this proposed method, the cross-validated Gaussian process regression (GPR) model, trained based on experimental measurements, is used to mimic the measurements at positions where there are no experimental data. The agreement between an experiment and the simulation is mimicked by the agreement between the simulation and GPR models. The statistically weighted square error is used to provide tangible information for the local agreement. The standardised Euclidean distance is used for assessing the overall agreement. The method is then used to assess the performance of four scale-resolving CFD methods, such as URANS k-ω-SST, SAS-SST, SAS-KE, and IDDES-SST, in simulating a prism bluff-body flow. The local statistically weighted square error together with standardised Euclidean distance provide additional insight, over and above the qualitative graphical comparisons. In this example scenario, the SAS-SST model marginally outperformed the IDDES-SST and better than the other two other, according to the distance to the validated GPR models.

    更新日期:2019-11-06
  • Modelling of thermal wall boundary conditions with temperature-dependent material properties for use in RANS
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-11-02
    H. Steiner, C. Irrenfried

    The present work extends a recently proposed P-function based model for describing the near-wall variation of temperature in forced convective turbulent flow to the case with temperature-dependent material properties. The extension essentially modifies the model formulations for describing the local variation of the turbulent mixing length and the turbulent Prandtl number. Direct Numerical Simulations (DNS) and experimental measurements are carried to provide comprehensive validation data for a wide range of Reynolds numbers, considering molecular Prandtl numbers well beyond unity. The observed good agreement of the predictions with the DNS data and experiments proves the present extended model as a well-suited approach for prescribing reliable thermal boundary conditions in Reynolds Averaged Navier-Stokes (RANS) simulations, assuming temperature-dependent material properties.

    更新日期:2019-11-04
  • Effect of heat flux on boundary layer flow under rotating conditions
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-31
    Ruquan You, Shengjun Zhou, Haiwang Li, Zhi Tao

    The boundary layer flow behaviour in a smooth rotating channel with heated walls is measured by particle image velocimetry (PIV). To simulate the real operation environment of an internal coolant channel in a turbine blade, airflow is analysed in a rotating channel, whose four walls are uniformly heated by Indium Tin Oxide (ITO) glass. The flow is measured in the middle plane of the rotating channel with a Reynolds number equal to 10000 and rotation numbers ranging from 0 to 0.52. The results are presented for the boundary layer flow behaviour with and without heated thermal boundary conditions. The buoyancy force generated by the heated walls influences the flow behaviour under rotating conditions. Separated flow occurs, which substantially influences the turbulent flow behaviours. Sometimes, this buoyancy force can determine the flow behaviours. The results also showed that the displacement thickness and the momentum loss thickness present new changes at different radius positions due to the heated thermal boundary conditions. The displacement thicknesses of both the leading and trailing sides with heated walls are both thicker than those of the leading and trailing sides without heated walls. Then, the difference of the boundary layer thickness between these two cases increases with the increase of rotation number. For momentum loss thickness, a sharp drop happens when the rotation number increases to a certain value. At the large radius position, the drop in momentum loss thickness is much greater than that in the small radius position.

    更新日期:2019-11-01
  • Nature of turbulence inside a cubical lid-driven cavity: Effect of Reynolds number
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-31
    Debabrat Samantaray, Manab Kumar Das

    Large eddy simulation of incompressible flow inside a cubic lid driven cavity for a range of Reynolds number (Re=1000,3200,5000,10000,15000) is carried out using dynamic Smagorinsky model (DSM). Centerline average velocity profiles are compared with the existing experimental and numerical results. Except for Re = 1000, the boundary layer slope at wall remains almost same up to the point of inflexion on the mid-bottom wall and on the mid-upstream wall. With increase in Re, the point of inflexion adjusts in such a way so as to make larger and larger volume of fluid a sluggishly-rotating central core. The turbulent quantities such as turbulent kinetic energy and turbulent dissipation are found to be increasing with increase in Re. The auto and cross-correlation coefficient of velocity components are studied and it is found that low Re (= 3200) flow shows periodic nature and as Re increases it becomes aperiodic and turbulent. The cross-correlation decreases with increase in Re. The Fourier spectra of auto-correlation coefficient follows Kolmogorov’s −53 slope in the inertial subrange irrespective of the Re, but the length of inertial subrange increases as Re increases. Kolmogorov length scale (η) and Taylor micro scale (λ) are calculated and it is found that both decrease towards the wall as dissipation is high at the walls. Anisotropic invariant map (AIM) has been plotted; nature of turbulence is found to be non-homogeneous and anisotropic even at low Re (= 3200) and non-homogeneity increases as Re increases. With increasing Re, the turbulence at the core region of the cavity shifts closer to isotropic.

    更新日期:2019-11-01
  • Validation of a closing procedure for fourth-order RANS turbulence models with DNS data in an incompressible zero-pressure-gradient turbulent boundary layer.
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2015-12-01
    S V Poroseva,B E Kaiser,J A Sillero,S M Murman

    Among factors affecting the accuracy of flow simulations with Reynolds-Averaged Navier-Stokes turbulence models is modeling turbulent diffusion processes. With the use of the Gram-Charlier series expansions, the turbulent diffusion in fourth-order one-point statistical closures of the Reynolds-Averaged Navier-Stokes equations can be modeled without introducing unknown model coefficients and without assuming turbulence being Gaussian. Terms representing turbulent diffusion processes in transport equations for second- and third-order velocity correlations do not require any modeling in such closures. In this regard, fourth-order closures are a more accurate alternative to lower-order closures where turbulent diffusion is modeled on semi-empirical or Gaussian turbulence assumptions. In the current paper, the accuracy of the closing procedure based on the Gram-Charlier series expansions is evaluated using data of direct numerical simulations in an incompressible zero-pressure-gradient turbulent boundary layer over a flat plate. One-point third-, fourth-, and fifth-order velocity moments were extracted for this purpose from the dataset collected by the Fluid Dynamics Group at the Universidad Politécnica de Madrid at two streamwise locations Reθ= 4101 and 5200 that correspond to channels and pipes at δ+= 1331 and 1626. Results demonstrate that the truncated Gram-Charlier series expansions are an accurate approximation of the fifth-order velocity moments in the considered flow.

    更新日期:2019-11-01
  • Reynolds number dependence of Reynolds and dispersive stresses in turbulent channel flow past irregular near-Gaussian roughness
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-28
    Thomas O. Jelly, Angela Busse

    Direct numerical simulations of fully-developed turbulent channel flow with irregular rough walls have been performed at four friction Reynolds numbers, namely, 180, 240, 360 and 540, yielding data in both the transitionally- and fully-rough regime. The same roughness topography, which was synthesised with an irregular, isotropic and near-Gaussian height distribution, is used in each simulation. Particular attention is directed towards the wall-normal variation of flow statistics in the near-roughness region and the fluid-occupied region beneath the crests, i.e. within the roughness canopy itself. The goal of this study is twofold. (i) Provide a detailed account of first- and second-order double-averaged velocity statistics (including profiles of mean velocity, dispersive stresses, Reynolds stresses, shear stress gradients and an analysis of the mean force balance) with the overall aim of understanding the relative importance of “form-induced” and “turbulence-induced” quantities as a function of the friction Reynolds number. (ii) Investigate the possibility of predicting the levels of streamwise dispersive stress using a phenomenological closure model. Such an approach has been applied successfully in the context of idealised vegetation canopies (Moltchanov & Shavit, 2013, Water Resour. Res., vol. 49, pp. 8222-8233) and is extended here, for the first time, to an irregular rough surface. Overall, the results reveal that strong levels of dispersive stress occur beneath the roughness crests and, for the highest friction Reynolds number considered in this study, show that the magnitude (and gradient) of these “form-induced” stresses exceed their Reynolds stress counterparts. In addition, this study emphasises that the dominant source of spatial heterogeneity within the irregular roughness canopy are “wake-occupied” regions and that a suitable parameterisation of the wake-occupied area is required to obtain an accurate prediction of streamwise dispersive stress.

    更新日期:2019-10-28
  • Large Eddy Simulation of boundary layer transition over an isolated ramp-type micro roughness element
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-24
    Hatim Belkhou, Serge Russeil, Talib Dbouk, Mohammed Mobtil, Daniel Bougeard, Nicolas-Yoan Francois

    Boundary layer transition over an isolated surface roughness element is investigated by means of numerical simulation. Large Eddy Simulation (LES) flow-modeling approach is employed to study flow characteristics and transition phenomenon past a roughness element immersed within an incoming developing boundary layer, at a height-based Reynolds number of 1170. LES numerical results are compared to experimental data from literature showing the time-averaged velocity distribution, the velocity fluctuation statistics and the instantaneous flow topology. Despite slight difference in the intensity of streamwise velocity fluctuations, the present LES results and experimental data show very good agreement. The mean flow visualization shows streamwise counter-rotating vortices pairs formation downstream of the obstacle. The primary pair induces an upwash motion and a momentum deficit that creates a Kelvin-Helmholtz type flow instability. The instantaneous flow topology reveals the formation of coherent K-H vortices downstream that produce turbulent fluctuations in the wake of the roughness element. These vortices are streched and lifted up when moving downstream. The velocity fluctuations results show that the onset of the turbulence is dominated by the energy transfer of large-scale vortices.

    更新日期:2019-10-25
  • Large eddy simulations on the effect of the irregular roughness shape on turbulent channel flows
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-19
    M. De Marchis, B. Milici, E. Napoli

    Large Eddy Simulations (LES) are carried out to investigate on the mean flow in turbulent channel flows over irregular rough surfaces. Here the attention is focused to selectively investigate on the effect induced by crests or cavities of the roughness. The irregular shape is generated through the super-imposition of sinusoidal functions having random amplitude and four different wave-lengths. The irregular roughness profile is reproduced along the spanwise direction in order to obtain a 2D rough shape. The analysis of the mean velocity profiles shows that roughness crests induce higher effect in the outer-region whereas roughness cavities cause the highest effects in the inner-region with a reduced effect in the external region. The numerical simulations have been carried out at friction Reynolds number Reτ=395. Similar results have been found for the higher order statistics: turbulence intensities or shear stresses. The analysis of the Reynolds stress anisotropy tensor confirms the existence of specific roles of cavities and crests in the turbulence modulation.

    更新日期:2019-10-19
  • On the accuracy of partially averaged Navier–Stokes resolution estimates
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-17
    M. Klapwijk, T. Lloyd, G. Vaz

    Partially Averaged Navier–Stokes computations can employ three different approaches for specifying the ratio of modelled-to-total turbulence kinetic energy fk. Use can be made of either a constant, a spatially- or a spatially- and temporally-varying value. This work compares different estimates for fk found in literature and evaluates them for two test-cases: a circular cylinder at Re=3900 and a turbulent channel flow at Reτ=395. Additionally, the estimates are compared to the a posteriori computed ratio of modelled-to-total turbulence kinetic energy()hfk˜ obtained from the PANS flow solution. The trends observed for the estimates are similar, although the magnitude varies significantly. All spatially varying fk approaches reduce the PANS model to a DES-like model, thereby entangling modelling and discretisation errors. At the same time, fk˜ shows that the behaviour of these estimates is incorrect: fk becomes too large near the wall of the object and in the far field. It is observed that fk˜ is always lower than the set value, when using fk fixed in space and time. Finally, it is clear that the estimates, applied to internal, boundary layer, flows yield too high values for fk. In order to minimise errors and increase the reliability of industrial CFD results, the approach with a constant fk is still preferable, assuming suitably fine grids are used.

    更新日期:2019-10-17
  • Controlling spatio-temporal evolution of natural and excited square jets via inlet conditions
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-17
    Artur Tyliszczak, Bernard J. Geurts

    The paper presents numerical investigations of square jets in a wide range of Reynolds numbers with varying inlet turbulence characteristics. The research focuses on flow characteristics depending on inflow turbulent length/time scales and excitation frequencies in case of excited jets. It is found that the parameters of inlet turbulence affect the solutions qualitatively when the Reynolds number is sufficiently low. In these cases the impact of varying the turbulent time scale is considerably larger than changing the turbulent length scale. It was also observed that at sufficiently high Reynolds numbers the jets become quite independent of the inlet turbulence characteristics. This confirms findings of Xu et al. (Phys. Fluids, 2013) concerning weak/strong dependence of the jet evolution on inflow conditions. In case of excited jets the excitation frequencies play an important role and influence the jet behaviour most strongly at lower values of the Reynolds number. For some forcing frequencies a bifurcation occurs at sufficiently large forcing amplitudes. This phenomenon turned out to be independent of the assumed length and time scales of the turbulent fluctuations, both in terms of robustness as well as amplitude.

    更新日期:2019-10-17
  • Understanding migratory flow caused by helicoid wire spacers in rod bundles: An experimental and theoretical study
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-14
    F. Bertocchi, M. Rohde, J.L. Kloosterman

    The core of a Liquid Metal Fast Breeder Reactor (LMFBR) consists of cylindrical fuel rods that are wrapped by a helicoidally-wound wire spacer to enhance mixing and to prevent damage by fretting. It is known that the liquid metal close to the rod is forced to follow the wires, and that liquid metal further away from the rod crosses the wires (called: migratory flow). This work aims at gaining more insight into the physics behind migratory flow and to provide a model for its bending angle. To this purpose, the flow field in a 7-rods, wire-wrapped, hexagonal bundle with water is studied within the Reynolds number range of 4990–16330 by using Particle Image Velocimetry (PIV). Refraction of the light is minimized by using Fluorinated Ethylene Propylene (FEP), which is a refractive index-matching (RIM) material. These measurements confirm that liquid near the rod follows the helicoid path and bends cross-wise with respect to the wire further away from the rod. A theoretical model for the bending angle of the flow is derived from the Euler equations and shows that the bending is primarily caused by the pressure gradient field induced by the wire. The model shows a very good correspondence with the experimentally obtained PIV data. These findings improve our understanding of the physics at play in rod bundle flows with wrapped wires and can be of assistance in developing practical correlations for frictional pressure losses and heat transfer in such bundles.

    更新日期:2019-10-14
  • Control of two-degree-of-freedom vortex-induced vibrations of a circular cylinder using a pair of synthetic jets at low Reynolds number: Influence of position angle and momentum coefficient
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-12
    Haibo Wang, Lin Ding, Li Zhang, Qunfeng Zou, Chunmei Wu

    In order to understand the effects of synthetic jets on the active control of two-degree-of-freedom (2DOF) vortex-induced vibrations (VIVs) of a circular cylinder, a series of numerical simulation were carried out at Reynolds number of 150. The synthetic jet excitation frequency was fixed at five times of the natural frequency of the cylinder in still water. The influence of two key parameters of synthetic jets, the position angle (α) and the momentum coefficient (Cu), on 2DOF VIVs was analyzed. Results indicated that both in-flow oscillation and cross-flow oscillation can be suppressed when the synthetic jets with sufficient momentum coefficient were positioned at the circular cylinder's leeward side (0° ≤ α ≤ 75°). When Cu = 4, 15° ≤ α ≤ 60°, the reductions of cross-flow and in-flow oscillation amplitudes were all larger than 99% and 70%, respectively. Besides, the in-flow oscillation frequency was locked-in to the excitation frequency of synthetic jets when the in-flow oscillation was effectively suppressed. A symmetric wake can be observed when the cross-flow oscillation was completely suppressed, and the 2P+2S vortex pattern can be observed when Cu = 4, α =165° and 180°.

    更新日期:2019-10-12
  • Simulation of the free jet using the vortex particle intensified LES (VπLES)
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-10
    S. Samarbakhsh, N. Kornev

    This paper presents validation and verification study of a novel VπLES method which is based on the decomposition of the flow structures in large scale ones, resolved on the grid, and small scale ones, represented by vortex particles for the turbulent circular free jet at Re=104. Comparison with experiments for mean velocities and fluctuations shows that the VπLES simulation with 1.5 · 105 cells attains the similar accuracy as the LES with the Dynamic Smagorinsky model on the Grid with 6.04 · 106 cells. The model is capable of reproducing the backward energy transfer, anisotropy of fine scales velocities and is automatically switched off when the flow is laminar. Numerical scheme parameters and the model reduction are studied and discussed.

    更新日期:2019-10-10
  • Spectral and modal analysis of the flow in a helical coil steam generator experiment with large eddy simulation
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-05
    Mustafa Alper Yildiz, Elia Merzari, Yassin A Hassan

    This paper presents an analysis of flow in the shell side of a 24-degree cut helical coil steam generator (HCSG) with large eddy simulation (LES). In the study, the open source, high-order spectral element CFD code Nek5000 was employed for numerical simulation. LES was performed at a Reynolds number of 9000 based on the inlet velocity and tube diameter. Spectral analysis was carried out along the tube bundle to investigate vortex shedding and to reveal shedding frequencies in HCSGs. Wavelet analysis was carried out to determine the time-frequency information of the fluctuating velocities in the domain. Flow was decomposed into modes with proper orthogonal decomposition, which revealed coherent structures in the flow and evolution of coherent structures in time. For the validation of Nek5000 in complex geometries, results from the numerical simulation were compared with experiments conducted at Texas A&M University. Overall, the LES results showed good agreement with experimental data.

    更新日期:2019-10-07
  • Detached eddy simulation of blade trailing-edge cutback cooling performance at various ejection slot angles
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-05
    M. Effendy, Y.F. Yao, J. Yao, D.R. Marchant

    Detached eddy simulation (DES) has been carried out to study a three-dimensional trailing-edge (TE) cutback turbine blade model with five rows of staggered circular pin-fin arrays inside the cooling passage, in order to evaluate the cooling performance in relation to coolant ejection slot angle. Simulations were performed by adopting a shear-stress transport k-ω turbulence model, and the effects of three different ejection slot angles 5°, 10° and 15° were investigated in terms of the characteristics of adiabatic film-cooling effectiveness, coefficient of discharge, and vortex shedding frequencies, respectively. The results obtained have shown that the TE cutback blade cooling with a 5° coolant ejection slot angle produced a better heat transfer coefficient than the other two ejection slot angles tested. The distributions of adiabatic film-cooling effectiveness along the cutback walls were found to be sensitive to the coolant ejection slot angle, e.g. the increase of ejection slot angle to 15° yielded near unity of cooling effectiveness along the entire breakout walls, whereas the decrease of ejection slot angle caused a drastic decay of cooling effectiveness after the maximum effectiveness has been reached. Of the three angles studied, a TE cutback blade model with a 15° ejection slot angle produced an optimum film-cooling effectiveness. In the breakout region, vortex shedding was observed along the shear layer between the hot gas and the coolant airflow. The shedding frequencies were evaluated to be 2.93, 2.21, and 2.18 kHz for the ejection slot angles of 5°, 10° and 15°, respectively. The findings from this study could be useful to improve existing TE cutback turbine blade design to achieve optimum film-cooling performance.

    更新日期:2019-10-05
  • One-dimensional turbulence investigation of variable density effects due to heat transfer in a low Mach number internal air flow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-10-03
    Juan A. Medina Méndez, Marten Klein, Heiko Schmidt

    A novel spatial formulation of the One-Dimensional Turbulence (ODT) model is applied to a vertical pipe-flow with heat transfer, analogous to the Direct Numerical Simulation (DNS) performed by Bae et al. [Phys. Fluids 18, (075102) (2006)]. The framework presented here is an extension for radially confined domains of the cylindrical ODT spatial formulation for low Mach number flows with variable density. The variable density simulations for air (Prandtl number Pr=0.71) are performed at an initial bulk Reynolds number Reb,0,DNS=6000 and Grashof number Gr0,DNS=6.78×106. ODT results are presented for both the spatial formulation introduced in this work and the standard temporal formulation for cylindrical flows introduced by Lignell et al. [Theor. Comput. Fluid Dyn. 32, 4 (2018), pp. 495–20]. Streamwise bulk profiles and radial profiles at specific streamwise positions for the temporal and spatial formulations are in good agreement with the DNS results from Bae et al. For the present application, the spatial formulation yields physically better results in comparison to the temporal formulation. Overall, the findings in the original work of Bae et al. were corroborated with ODT. Although the framework proposed in this work is not a compressible framework and has some clear limitations regarding conservation properties, we suggest its use for future studies in the low Mach number variable density regime.

    更新日期:2019-10-03
  • 更新日期:2019-09-26
  • Enriching MRI mean flow data of inclined jets in crossflow with Large Eddy Simulations
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-26
    Pedro M. Milani, Ian E. Gunady, David S. Ching, Andrew J. Banko, Christopher J. Elkins, John K. Eaton

    Measurement techniques such as Magnetic Resonance Velocimety (MRV) and Magnetic Resonance Concentration (MRC) are useful for obtaining 3D time-averaged flow quantities in complex turbulent flows, but cannot measure turbulent correlations or near-wall data. In this work, we use highly resolved Large Eddy Simulations (LES) to complement the experiments and bypass those limitations. Coupling LES and magnetic resonance experimental techniques is especially advantageous in complex non-homogeneous flows because the 3D data allow for extensive validation, creating confidence that the simulation results portray a physically realistic flow. As such we can treat the simulation as data, which “enrich” the original MRI mean flow results. This approach is demonstrated using a cylindrical and inclined jet in crossflow with three distinct velocity ratios, r=1, r=1.5, and r=2. The numerical mesh is highly refined in order for the subgrid scale models to have negligible contribution, and a systematic, iterative procedure is described to set inlet conditions. The validation of the mean flow data shows excellent agreement between simulation and experiments, which creates confidence that the LES data can be used to enrich the experiments with near-wall results and turbulent statistics. We also discuss some mean flow features and how they vary with velocity ratio, including wall concentration, the counter rotating vortex pair, and the in-hole velocity.

    更新日期:2019-09-26
  • Effect of an axial throughflow on buoyancy-induced flow in a rotating cavity
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-26
    Diogo B. Pitz, John W. Chew, Olaf Marxen

    In this paper large-eddy simulation is used to study buoyancy-induced flow in a rotating cavity with an axial throughflow of cooling air. This configuration is relevant in the context of secondary air systems of modern gas turbines, where cooling air is used to extract heat from compressor disks. Although global flow features of these flows are well understood, other aspects such as flow statistics, especially in terms of the disk and shroud boundary layers, have not been studied. Here, previous work for a sealed rotating cavity is extended to investigate the effect of an axial throughflow on flow statistics and heat transfer. Time- and circumferentially-averaged results reveal that the thickness of the boundary layers forming near the upstream and downstream disks is consistent with that of a laminar Ekman layer, although it is shown that the boundary layer thickness distribution along the radial direction presents greater variations than in the sealed cavity case. Instantaneous profiles of the radial and azimuthal velocities near the disks show good qualitative agreement with an Ekman-type analytical solution, especially in terms of the boundary layer thickness. The shroud heat transfer is shown to be governed by the local centrifugal acceleration and by a core temperature, which has a weak dependence on the value of the axial Reynolds number. Spectral analyses of time signals obtained at selected locations indicate that, even though the disk boundary layers behave as unsteady laminar Ekman layers, the flow inside the cavity is turbulent and highly intermittent. In comparison with a sealed cavity, cases with an axial throughflow are characterised by a broader range of frequencies, which arise from the interaction between the laminar jet and the buoyant flow inside the cavity.

    更新日期:2019-09-26
  • Large eddy simulation and self-similarity analysis of the momentum spreading in the near field region of turbulent submerged round jets
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-24
    Andrea Boghi, Ivan Di Venuta, Matteo Angelino, Fabio Gori

    Self-Similarity in turbulent round jets has been the object of investigation from several decades. The evolution of turbulent submerged jets is characterized by the presence of two regions: the region of flow establishment, or near field region (NFR) and the fully developed region (FDR), or far-field region (FFR). The momentum spreading in the FDR is known to be self-similar and few mathematical models have been presented in the past to describe it. The flow evolution in the NFR has been rarely studied since there is a certain consensus on the idea that the flow in the NFR is not self-similar. In this work, we study the flow evolution of a turbulent submerged round jet by means of large eddy simulation (LES) at several Reynolds numbers ranging from 2492 to 19,988. Three new self-similar laws are proposed to describe the flow evolution in the NFR, one for the initial region, called Undisturbed Region of Flow, (URF), and two for the final region, the potential core region (PCR). The numerical results presented in this work are also validated with the self-similar laws for the FDR proposed by Tollmien (1926) and Görtler (1942), and the experimental data of Hussein et al. (1994), and Panchapakesan and Lumley (1993), in the FDR; those of Davies et al. (1963), in the PCR; and van Hout et al. (2018), in the URF. The conclusion is that previous inability to find the self-similarity law in the NFR is due to the attempt to find a unique self-similar variable to describe the momentum spreading in both the URF and the PCR.

    更新日期:2019-09-25
  • Direct numerical simulation of Taylor–Couette turbulent flow controlled by a traveling wave-like blowing and suction
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-23
    K. Ogino, H. Mamori, N. Fukushima, K. Fukudome, M. Yamamoto

    In wall turbulence, a traveling wave-like control is known to decrease the skin-friction drag and induce the relaminarization phenomenon. Because it is noteworthy to investigate the control effect in other canonical flows, direct numerical simulations of fully developed turbulent Taylor–Couette flows are performed. The Reynolds number, based on the wall velocity of a rotating inner cylinder and the radius of a centerline between cylinders, is set to 84,000. The traveling wave-like blowing and suction is imposed on the inner or outer cylinder wall, and the control effect is parametrically investigated. In the inner cylinder control, the torque reduction is obtained when the wave travels in the co-rotating direction with the inner cylinder, and its wavespeed is faster than the rotation. In the outer cylinder control, in contrast, the torque reduction is obtained when the wave propagates in the opposite direction. While the control is imposed on one side wall (i.e., inner or outer cylinder), the control affects the entire flow region. The Taylor vortex remains, while the traveling wave affects its strength. The three-component decomposition analysis shows that the traveling wave creates the coherent contribution on the torque, while the random contribution on it is reduced. Accordingly, a major factor of the torque reduction in the Taylor–Couette flow is the reduction of the random contribution. In addition, for the faster wavespeed cases with the small wavenumber (i.e., the long wavelength), the drag reduction larger than 60% is obtained and the relaminarization occurs in these cases.

    更新日期:2019-09-23
  • A zonal noise prediction method for trailing-edge noise with a porous model
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-23
    P. Bernicke, R.A.D. Akkermans, V.B. Ananthan, R. Ewert, J. Dierke, L. Rossian

    In this contribution, a hybrid zonal simulation tool with volumetric inflow turbulence forcing is applied to trailing-edge noise of a NACA0012 airfoil with and without a porous insert at representative Mach and Reynolds number of 0.1118 and 1.0 × 106, respectively. The governing equations constitute the non-linear perturbation equations with viscous terms (i.e., the full Navier–Stokes equations), in which the porous material is modelled by a volume-averaged approach. Generic simulations with a single vortex passing the trailing edge revealed the expected noise reduction as well as an additional noise source. This new source originates from the turbulent flow passing the transition from solid to porous surface and was shown to increase significantly with increasing permeability. 3D simulations with a solid and porous trailing-edge showed good agreement with experimental aerodynamic and aeroacoustic validation data. The application of porous material to trailing-edge noise confirms the results reported in literature and underlines the validity of the porous model as well as it illustrates possible applications.

    更新日期:2019-09-23
  • Numerical analysis of wind velocity effects on fire-wind enhancement
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-23
    Esmaeel Eftekharian, Maryam Ghodrat, Yaping He, Robert H. Ong, Kenny C.S. Kwok, Ming Zhao

    Variation in flow characteristics triggered through the fire-wind interface can potentially damage the buildings during bushfires. Fire-wind enhancement which is referred to as the increase of wind velocity, caused by the fire-wind interaction, is one of the destructive phenomena in this regard. In spite of the significance, the underlying mechanism contributing to this phenomenon is still not well understood. This study employs computational fluid dynamic (CFD) simulation to fundamentally investigate the effects of free-stream wind velocity on fire-wind enhancement through analyzing the momentum and buoyancy of fluid. Fire-wind interaction is shown to cause the generation of fire-induced longitudinal negative pressure gradient which results in fire-induced pressure and viscous forces in longitudinal direction. These forces are further found as the prime reason for the distortion of the wind velocity profile. A module is implemented to the FireFOAM solver to calculate and extract these forces quantitatively. The results reveal that under a constant fire intensity, the level of distortion and/or enhancement in the wind velocity profile comparatively reduces with the increase of free-stream wind velocity. A new non-dimensional group (modified Euler number) is introduced to take into account dominant fire-induced forces causing fire-wind enhancement. Richardson number and the modified Euler number are employed to determine the influence of free-stream wind velocity and longitudinal distance from the fire source on wind velocity enhancement. Large-eddy simulation (LES) results indicate that while the level of enhancement generally depends on both Richardson and the modified Euler number, the location of the maximum level of enhancement along the plume centreline coincides with the maximum value of modified Euler number under a constant free-stream wind velocity scenario.

    更新日期:2019-09-23
  • Turbulence statistics in rotating channel flows with rough walls
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-20
    Wen Wu, Ugo Piomelli, Junlin Yuan

    We performed direct simulations of channel flow subjected to rotation about a spanwise axis, comparing cases with smooth and rough walls. The destabilizing effect of roughness counteracts the stabilizing effect of rotation on the cyclonic (stable) side. When the surface is rough the Reynolds stresses remain significant at all rotation rates considered, even those that results in a quasi-laminar state when the wall is smooth. The wake fluctuations result in significant dispersive stresses, which give an important contribution to the generation of turbulence on the stable side, mainly through added production of shear stresses. The dispersive stresses are mostly associated with the channeling of the flow between roughness elements.

    更新日期:2019-09-21
  • Extensive investigation of the influence of wall permeability on turbulence
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-20
    Y. Kuwata, K. Suga

    A series of direct numerical simulations of turbulent porous-walled channel flows is performed to extensively investigate the influence of wall permeability on turbulence modification. The bulk mean Reynolds number is fixed at 3000, and porous media consisting of perforated plates are considered in the lower side of the channel. The mean-permeability Reynolds number is varied from 14−118 by varying the hole size of the perforated plates. A spectral analysis reveals the presence of two characteristic perturbation modes, namely, the streamwise perturbation mode originating from the Kelvin–Helmholtz (K–H) type of instability and the spanwise perturbation mode. When the mean permeability Reynolds number is relatively low, the streamwise perturbation model by the K–H instability is dominant, and this increases the coherence of the wall-ward turbulence motion, thus resulting in considerable turbulence enhancement. However, as the mean permeability Reynolds number increases further, the streamwise perturbations tend to decrease in strength, and the streamwise elongated high- and low-speed streaky structure, the mean spacing of which is much longer than that over a smooth wall, is developed owing to the spanwise perturbation mode. In this regime, the turbulence enhancement effect is weakened because of an increased slippage velocity at the porous interface.

    更新日期:2019-09-21
  • Analysis of tornado and near-ground turbulence using a hybrid meteorological model/engineering LES method
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-18
    Masaharu Kawaguchi, Tetsuro Tamura, Hidenori Kawai

    A new hybrid meteorological model/engineering LES method was used to analyze a tornado and near-ground turbulence under realistic conditions. The hybrid method estimates high frequency turbulence regeneration in an actual severe storm that has complex thermal conditions in order to alleviate the problem of high frequency component dissipation in meteorological models and generate appropriate field data to investigate near-ground flow and wind loading on buildings. The high frequency regeneration method was validated in the convective boundary layer and the energy spectrum of the velocity field was successfully extended to higher frequency region in good agreement with −5/3 Kolmogorov law. Multi-scale simulations of a tornado were conducted based on meteorological model outputs with three different terrain conditions, including an actual urban building geometry and terrain, and the changes in the main tornado vortex and the interactions with near-ground turbulent field were examined. On roughened surfaces with uniform blocks and buildings, the main tornado vortex was connected to near-ground vertical vortices arising due to the urban geometry. The structure was significantly deformed, splitting into numerous finer vortices below several times the building height. These near-ground vortices create localized sharp pressure drop patterns and also affect the movement of the tornado. These effects of low pressure on buildings were quantitatively analyzed.

    更新日期:2019-09-19
  • Parametric study of the collision modes of compound droplets in simple shear flow
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-13
    Truong V. Vu

    Compound droplet collision has been found in various industrial and academic applications. Such colliding phenomena of two droplets in simple shear flow are numerically resolved by a front-tracking technique. Initially, each compound droplet, assumed cylindrical, contains one concentric inner droplet. They are separated at the lateral and vertical intervals denoted by Δx0 and Δy0. Because of the shear flow, the compound droplets interact with each other and exhibit three collision modes: passing-over, merging (i.e. coalescence) and reversing. These modes and their transition are affected by many parameters including the Reynolds Re and Capillary Ca numbers (based on the properties of the outer fluid), the viscosity ratios μ13 and μ23, the interfacial tension ratio σ12 of the inner to outer interfaces, the ratio of the radii of the inner to outer droplets R12 and the initial distance between them (in terms of Δx0 and Δy0). It is found that from a merging mode, decreasing Re from 2.51 to a value less than or equal to 1.0 induces a transition to a reversing mode, whereas, increasing Ca from 0.005 to a value greater than or equal to 0.04 leads to a transition to a passing-over mode. A transition from the merging to passing-over modes also appears when varying μ23 in the range of 0.1–10.0 or varying R12 in the range of 0.2–0.8. A transition from a passing-over mode to a reversing one is available when increasing Δx0 or decreasing Δy0. Three modes of collision all occur when μ13 is varied in the range of 0.1–10.0. However, the variation of σ12 does not induce any transition between different modes. Several phase diagrams in terms of Re versus Ca, or Δx0 versus Δy0 are also proposed to show the transitions between these modes.

    更新日期:2019-09-14
  • Multiphase Eulerian–Lagrangian LES of particulate fouling on structured heat transfer surfaces
    Int. J. Heat Fluid Flow (IF 2.000) Pub Date : 2019-09-03
    Robert Kasper, Johann Turnow, Nikolai Kornev

    In this study, multiphase Eulerian–Lagrangian large-eddy simulation (LES) is used to analyze the interaction between local flow structures, convective heat transfer and particle depositions (fouling) for a turbulent channel flow with a dimpled surface. Thus, eddy-resolving LES is applied for calculation of the turbulent working fluid (carrier flow), combined with an efficient Lagrangian particle tracking (LPT) algorithm, suitable to predict the trajectories of the suspended foulant particles precisely in time and space. In order to decrease the computational effort and to enhance the applicability of the proposed approach, deposited particles are deactivated and converted into a second continuous phase (fouling layer), which is modeled as a porous medium and accounts for the hydraulic losses as well as for the additional thermal resistance due to the fouling deposits. Prior to the fouling simulations, the envisaged method is validated for a turbulent particle-laden channel flow at Reτ=150, based on DNS results available in the literature, which shows the capability of the Eulerian–Lagriangian LES of capturing the flow physics within turbulent particle-laden, wall-bounded flows. Based on this observations, fouling simulations were carried out for a turbulent particle-laden channel flow over a staggered arrangement of sharp-edged spherical dimples, whereby two different dimple depth-to-diameter ratios are considered (d/D=0.26 and 0.35). The thermo-hydraulic performance is analyzed for the clean and contaminated dimpled surfaces.

    更新日期:2019-09-04
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