Qian (Tsien) Jian (1939–2018), a Chinese theoretical physicist and fluid dynamicist, devoted the second part of his scientific life to the physical understanding of small-scale turbulence to the exclusion of all else. To place Qian's contribution in an appropriate position in the field of small-scale turbulence, a historical overview and a state-of-the art review are attempted. Qian developed his own

Cryogenic fluids are used in a myriad of different applications not limited to green fuels, medical devices, spacecraft, and cryoelectronics. In this review, we elaborate on these applications and synthesize recent lattice Boltzmann methods (LBMs) including collision operators, boundary conditions, grid-refinement techniques, and multiphase models that have enabled the simulation of turbulence, thermodynamic

Leidenfrost drops often exhibit symmetric shape-oscillations when placed onto curved substrates. We give some brief remarks on vibrations found in liquids floating over highly heated surfaces for the overlooked period from the late 1700s up to the work of Holter and Glasscock in 1952 [J. Acoust. Soc. Am. 26, 253 (1952)]; the latter pair were incorrectly credited as the first to observe such behavior

During the pandemic of COVID-19, the public is encouraged to take stairs or escalators instead of elevators. However, the dispersion of respiratory droplets in these places, featured by slopes and human motion, is not well understood yet. It is consequently unclear whether the commonly recommended social-distancing guidelines are still appropriate in these scenarios. In this work, we analyze the dispersion

Linear and nonlinear viscoelastic properties of hydrogels significantly contribute to functionality, long-term performance, and stability of the hydrogels. With respect to the nonlinear viscoelastic response of chemically crosslinked hydrogels, the vast majority of publications have reported the type III response (weak strain overshoot). Herein, to measure the true mechanical response of hydrogels

Algebraic explicit wall models covering the entire inner region of the turbulent boundary layer are proposed to reduce the computational effort for large eddy simulation of wall-bounded turbulent flows. The proposed formulas are given in closed forms with either logarithmic- or power-function-based laws of the wall, allowing straightforward evaluation of the friction velocity on near wall grids independent

The circle of Willis (CoW) is a set of arteries located in the basis of the brain. Prediction of perfusion rates and hemodynamics in the CoW is necessary to understand the relevant vascular diseases and to prescribe effective treatments. In this paper, the effect of ischemic stroke in the CoW is studied, taking into consideration the anatomical variations of the CoW. Moreover, an analysis on the effect

Fractional calculus has shown good adaptability in describing the mechanical properties of viscoelastic materials. Based on fractional order theory, the characteristics of blood flow in the fractal network of blood vessels are analyzed, and experiments are carried out by using a mixed solution to prove the results of simulation analysis that contains glycerol, gelatin, sodium chloride, etc. Under different

Viral immune evasion by sequence variation is a significant barrier to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine design and coronavirus disease-2019 diffusion under lockdown are unpredictable with subsequent waves. Our group has developed a computational model rooted in physics to address this challenge, aiming to predict the fitness landscape of SARS-CoV-2 diffusion using

We propose a novel integral model describing the motion of both flexible and rigid slender fibers in viscous flow and develop a numerical method for simulating dynamics of curved rigid fibers. The model is derived from nonlocal slender body theory (SBT), which approximates flow near the fiber using singular solutions of the Stokes equations integrated along the fiber centerline. In contrast to other

Wave propagation models in the time domain have been extensively used in the available literature to study the flow characteristics in blood vessels. Most of the wave propagation models have considered flat or parabolic velocity profile functions to estimate the nonlinear convection and diffusion terms present in the conservation of momentum equation. There are only a few works available on the wave

Complex fluids flow in complex ways in complex structures. Transport of water and various organic and inorganic molecules in the central nervous system (CNS) are important in a wide range of biological and medical processes [C. Nicholson and S. Hrabětová, “Brain extracellular space: The final frontier of neuroscience,” Biophys. J. 113(10), 2133 (2017)]. However, the exact driving mechanisms are often

The phenomenon of droplets impacting fiber has important applications in the recovery of waste liquid, separation of solid and liquid phases, gas and liquid phases, and glass wool manufacturing. This study explored the impact of droplets on fiber based on the many-body dissipative particle dynamics (MDPD) method. First, the impact of droplets on fiber at different angles was simulated, and the results

In this study, the nonlinear effect of contactless bubble–bubble interactions in inertial micropumps is characterized via reduced parameter one-dimensional and three-dimensional computational fluid dynamics (3D CFD) modeling. A one-dimensional pump model is developed to account for contactless bubble-bubble interactions, and the accuracy of the developed one-dimensional model is assessed via the commercial

In this article, we construct a novel one-dimensional model of microfluidic laminar flows in tapered circular and rectangular channels assuming the flow in channels fully developed. In the model, we take into account the inertance and dynamic pressure terms. The model can be used for a wide range of flows: from the pure capillary flow regime, where the capillary forces are the main driver of the liquid

In the present study, the magnetic field induced self-assembly processes of magnetic microparticles in an aqueous liquid (the pure magnetic fluid) and nonmagnetic microparticles in ferrofluid (the inverse magnetic fluid) are experimentally investigated. The microparticles are formed into chain-like microstructures in both the pure magnetic fluid and the inverse magnetic fluid by applying the external

Microscale gas flow attracts significant research interest in recent years since it is concerned with a wide range of engineering applications. It is noted that the Navier–Stokes equations-based scheme and the standard lattice Boltzmann method both encounter a great challenge in the simulation of such flows. The newly developed discrete unified gas kinetic scheme (DUGKS) has been demonstrated to be

Electrokinetic flow in a microchannel driven by charged surface heterogeneity in the presence of an external electric field is investigated by three-dimensional simulations. A computational framework is developed coupling a two-relaxation-time lattice Boltzmann solver for the transport equations of fluids, charged species, and passive tracing scalars and a fast Poisson solver for the electric potential

In this study, molecular dynamics simulations are performed to estimate the equilibrium pressure of liquid confined in nanopores. The simulations show that pressure is highly sensitive to the pore size and can significantly change from absolute positive to absolute negative values for a very small (0.1 nm) change in the pore size. The contribution from the solid–liquid interaction always dominates

Droplet impact hydrodynamics on “V”-shaped valleys or grooves of variant wettability and geometric dimensions have been studied experimentally and probed theoretically. The groove geometry makes the hydrodynamics three-dimensional, as in addition to the droplet dynamics in the lateral direction, liquid jets are generated from the post-impact droplet along the axial direction of the groove. The effect

We investigate the gas transport enhancement through nanotubes, relative to the prediction by the prevailing century-old Knudsen diffusion model. This enhancement is usually attributed to the partly specular molecular reflections at the smooth nanotube surface, which break the model assumption of completely diffusive reflections. However, an oversighted cause of the discrepancy between the measurement

We show that the permeability of periodic simply connected porous media can be reliably predicted from the Minkowski tensors (MTs) describing the pore microstructure geometry. To this end, we consider a large number of two-dimensional simulations of flow through periodic unit cells containing complex-shaped obstacles. The prediction is achieved by training a deep neural network using the simulation

The bouncing dynamics of microdroplets with various viscosities on a superhydrophobic surface is numerically investigated. An axisymmetric lattice Boltzmann method is developed on the basis of Zheng et al. capable of handling multiphase flows with a large density ratio, which is implemented to simulate the impact. It is shown that in the low-viscosity regime, the contact time tc remains constant over

A combined ab initio modeling and experimental study of water adsorption on a dry hydrophobic dielectric surface is presented. This is an important phenomenon for controlled droplet deposition in various technological applications. The ab initio density functional theory calculations are performed to reveal the dominant water adsorption sites, energetics, and the electron density profile on Teflon

This paper exhibits the oscillatory characteristics of a free convective flow of nanofluids in horizontal concentric annuli of pilot dimensions to provide a mechanical solution against their particles settling which occurs by aggregation. These nanofluids are generated according to each class of particles that may exist with four types of industrial base liquids. Koo–Kleinstreuer semi-empirical models

The phenomenon of droplet impact is commonly found in industrial and agricultural processes. The basic characteristics and theories of a droplet impacting solid walls have been extensively studied, but the regulation of the droplet impact phenomenon has not been adequately examined. This study investigates the regulation of droplet impact on a hydrophobic surface based on alternating current elect

The microscopic Rayleigh–Taylor instability (RTI) is studied via molecular dynamics (MD) simulation for single- and dual-mode interfaces under a strong acceleration. The growth behavior of microscopic RTI as well as the underlying regime exhibits considerable differences from the macroscopic counterpart. At a microscopic scale, the flow Reynolds number is very low and thus viscosity effect plays an

We introduce a mesoscale approach for the simulation of multicomponent flows to model the direct-writing printing process, along with the early stage of ink deposition. As an application scenario, alginate solutions at different concentrations are numerically investigated alongside processing parameters, such as apparent viscosity, extrusion rate, and print head velocity. The present approach offers

The impact dynamics of a water droplet on a flexible substrate is useful for designing pesticide sprays and understanding insects flying in rainfall. We experimentally analyze the impact dynamics of a microliter water droplet on a superhydrophobic cantilever beam for Weber number in the range of 30–76. A thin copper sheet was coated with a commercial coating to render it superhydrophobic and high-speed

Enhancing drop deposition on solid surfaces has received significant attention in various fields. Breaking the circular symmetry in typical impact dynamics has opportunities for altering the mass and momentum distributions significantly and improving the deposition. Here, we study the impact dynamics of ellipsoidal drops on nonwetted solid surfaces to reduce the bounce magnitude as a function of the

A set of large eddy simulations for the free surface backward facing step (FSBFS) are carried out to study wave formation behind the step. The volume-of-fluid ghost fluid method is employed to capture the free surface. Previous studies have indicated that the wave physics depend on the step draught-based Froude number (Fr). For small Fr, the rear face of the step (transom) becomes wet, while for large

An experimental study of the antibubble formation by a single drop impact on an identical liquid bath is presented. With the increase in the impact velocity, different phenomena are observed and classified into four regimes: No droplet, Single droplet, Double droplets, and Antibubble formation. In fact, the Antibubble formation is part of the Double droplets regime. A high-speed drop impact leads to

The propagation of elevation internal solitary waves (ISWs) over a large triangular ridge was investigated experimentally in a stratified fluid flume. The propagation characteristics of the elevation ISWs over the ridge were measured by using the real-time wave measuring instrument and particle image velocimetry, respectively. At the front slope of the ridge, the obvious fluid accumulation appeared

A surfactant-covered droplet subject to both electric field and shear flow is studied using a lattice Boltzmann and finite difference hybrid method, which breaks the limitation of asymptotic approaches that allow only small droplet deformation. It is found that in the electric system where electric field induces circulating flows directed from equator to poles, the presence of surfactants promotes

Molecular dynamics is used to investigate the thermocapillary motion of a water nanodroplet suspended in benzene subjected to a constant temperature gradient. This framework lets us identify the average behavior of the fluid particles by revealing their mean evolution. We connect such statistics to the behavior of the temporally evolving nanodroplet, thereby providing a microphysical foundation to

The stability of convective flows in a non-homogeneous temperature field is affected by the shape of the container hosting the fluid. We present a nonlinear two-phase computational study of convection in a liquid bridge that develops under the action of buoyancy and Marangoni forces. The hydrothermal instability is examined for three shapes of disks supporting liquid bridge: both disks flat, the upper

Both surface and internal freak waves can be regarded as special interfacial waves. Using a two-layer model, we investigated the influence of linear shear flow (LSF) in the upper layer on interfacial waves. Specially, the model was designed to study the effects of wind shear on surface freak waves and LSF on internal freak waves. Based on the model, a nonlinear Schrödinger equation was derived to describe

A mathematical model is established to investigate a vertical gravity-driven drainage flow containing a soluble surfactant when considering the effect of wall slip. The lubrication theory is employed to obtain the evolution equations describing film thickness, surface velocity, surfactant concentrations at the air–liquid, solid–liquid interface, and in the bulk. The influence of constant slip length

In order to study the mechanism of ice formation after water droplets produced by splashing waves attach to ship superstructure in cold ocean regions, a numerical framework that considers the effect of supercooling degree on the meso-scale water droplet freezing is developed to explore the freezing mechanism of water droplets after impacting. This model can track the solid–liquid and air–liquid interface

There are several numerical approaches to define a permanent magnet in terms of mathematical equations, and each approach has progressed since its inception, but still endures some limitations on specific numerical phenomena. This study seeks to propose a novel numerical representation of a permanent magnet without incorporating its effect through boundary conditions, which overcomes the limitations

In a powder bed fusion additive manufacturing process, the balling effect has a significant impact on the surface quality of the printing parts. Surface wetting helps the bonding between powder and substrate and the inter-particle fusion, whereas the balling effect forms large spheroidal beads around the laser beam and causes voids, discontinuities, and poor surface roughness during the printing process

The ring-sheared drop is a containerless system where shear is imparted by two contact rings, one rotating and the other stationary. In microgravity, aqueous drops can be studied in the air at the centimeter scale. Drops of this scale can also be studied experimentally on Earth, but the effects of gravity need to be mitigated by density matching the drop liquid and its surrounding fluid. The use of

The initial spreading of glycerol and silicon oil droplets on smooth, corrugated, and orthogonal surfaces is numerically investigated by an effective, sharp-interface modeling method. In this study, the temporal evolution of spreading radius during the initial phase is scaled by R/R0 = C(t/τi)α for inertial regime and R/R0 = C(t/τμ)α for the viscous regime. We focus on exploring how wettability, liquid

This paper presents three volume of fluid (VoF)-based methods for large eddy simulations of atomizing sprays with different treatments of the unresolved interface. The turbulent filtered VoF model uses conventional turbulent viscosity models to close the combined interfacial and turbulent sub-grid fluctuations. The hybrid turbulence filtering and artificial compression model includes an additional

The present article aims to decipher the effect of preheating a segment of the pipe on the pressure propagation mechanisms and flow restart operation in a gelled pipeline. During the restart operation, shear-thinning thixotropic rheology governs the gel properties, where the viscosity is a function of shear strain and thermal history. A finite volume method is employed to solve the governing equations

Although the rheology of cellulose nanocrystalline (CNC) suspensions has been widely studied, less attention has been paid to the modified cellulose nanocrystals such as cationic and anionic cellulose hydrogels. In this work, the rheological behavior of cellulose nanocrystals (CNCs), anionic CNCs (pCNCs), and cationic CNCs (nCNCs), was comparatively studied. The rheological behavior demonstrated that

Due to both liquid properties and ferromagnetic material characteristics, ferrofluids are developed as the favored component solvents. The art of investigating the lubrication mechanism of ferrofluids in spur gear drives is significant for improving the anti-wear of gear pairs and prolonging the service life. This paper proposes a mathematical model of oil-based ferrofluids lubrication and derives

The shear thinning behavior of non-colloidal suspensions is investigated experimentally with emphasis on the effect of surface roughness of suspending particles. The first shear thinning at a low shear-rate is observed, which originates from particle–particle interaction, and the second shear thinning at a high shear rate is also reported as the polymeric solvent shear thins. Due to the decrease of

The outbreak of the coronavirus disease has drawn public attention to the transmission of infectious pathogens, and as major carriers of those pathogens, respiratory droplets play an important role in the process of transmission. This Review describes respiratory droplets from a physical and mechanical perspective, especially their correlation with the transmission of infectious pathogens. It covers

Gas and vapor explosions have been involved in industrial accidents since the beginnings of industry. A century ago, at 11:55 am on Friday September 24, 1920, the petroleum barge Warwick exploded in London's docklands and seven men were killed. Understanding what happened when it blew up as it was being refurbished, and how to prevent similar explosions, involves fluid mechanics and thermodynamics

This Letter reports the first large eddy simulation of a turbulent flame using a lattice-Boltzmann model. To that end, simulation of a bluff-body stabilized propane–air flame is carried out, showing an agreement similar to those available in the literature. Computational costs are also reported, indicating that lattice-Boltzmann modeling of reactive flows is competitive, with around 1000cpuh required

Developing data-driven subgrid-scale (SGS) models for large eddy simulations (LESs) has received substantial attention recently. Despite some success, particularly in a priori (offline) tests, challenges have been identified that include numerical instabilities in a posteriori (online) tests and generalization (i.e., extrapolation) of trained data-driven SGS models, for example, to higher Reynolds

The dominant view in the theory of fluid turbulence assumes that, once the effect of the Reynolds number is negligible, moments of order n of the longitudinal velocity increment, ( δ u), can be described by a simple power-law r ζ n, where the scaling exponent ζn depends on n and, except for ζ 3 ( = 1 ), needs to be determined. In this Letter, we show that applying Hölder's inequality to the power-law

In this work, we estimate the probability of an infected person infecting another person in the vicinity by coughing in the context of COVID-19. The analysis relies on the experimental data of Simha and Rao [“Universal trends in human cough airflows at large distances,” Phys. Fluids 32, 081905 (2020)] and similarity analysis of Agrawal and Bhardwaj [“Reducing chances of COVID-19 infection by a cough

This paper examines baroclinic vortices embedded in a large-scale vertical shear. We describe a new class of steady propagating vortices that radiate Rossby waves but yet do not decay. This is possible since they can extract available potential energy from a large-scale vertically sheared flow, even though this flow is linearly stable. The vortices generate Rossby waves, which induce a meridional vortex

In this Letter, it is revealed that the convection velocity of a localized wave packet in two-dimensional plane-Poiseuille flow is determined by the solitary wave at the centerline of a vortex dipole, which is evinced by subtracting the base flow from the mean flow. The fluctuation component propagates obeying the local dispersion relation of the mean flow and oscillates with a global frequency selected

Small solid particles, droplets, and bubbles can form clusters in turbulent flows by the action of coherent vortices. This phenomenon, sometimes called the preferential concentration, was often thought to be most conspicuous when the velocity relaxation time τp of particles is comparable with the Kolmogorov time τ η. However, since high-Reynolds number turbulence consists of coherent eddies with different

Here, we point out on an intriguing mapping between the quantum harmonic oscillator ground state and the zero absolute vorticity plane Couette flow of a Boltzmann-like density distributed ideal gas in thermal equilibrium. The mapping is obtained when the gas is embedded in a rotating frame whose rotation rate is equal to half of the frequency of the quantum harmonic oscillator.

The standard two-phase lattice Boltzmann equation (LBE) method usually fails to capture the physical equilibrium state (zero velocity and constant chemical potential). Consequently, spurious velocities and inconsistent thermodynamic density properties are frequently encountered in LBE simulations. In this work, based on a rigorous analysis of the discrete balance equation of LBE, the structure of force

In this work, we consider the question of whether a simple diffusive model can explain the scent tracking behaviors found in nature. For such tracking to occur, both the concentration of a scent and its gradient must be above some threshold. Applying these conditions to the solutions of various diffusion equations, we find that the steady state of a purely diffusive model cannot simultaneously satisfy