# Latest papers in fluid mechanics

### Large-scale clustering of light small particles in developed turbulence

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 [math]. However, since high-Reynolds number turbulence consists of coherent eddies with different timescales, particles can form clusters even when [math]. We demonstrate, by direct numerical simulations, that light particles with different τp values form clusters around axes of coherent vortices with different sizes in developed turbulence.

### Minimizing deformation of a thin fluid film driven by fluxes of momentum and heat

Author(s): Davin Lunz

We consider a thin fluid film flowing down an inclined substrate subjected to localized external sources of momentum and heat flux that induce deformations of the fluid's free surface. This scenario is encountered in several industrial processes and of particular interest is the case where these def...

[Phys. Rev. E 103, 033105] Published Tue Mar 16, 2021

### Wall-attached structures over a traveling wavy boundary: Turbulent velocity fluctuations

Author(s): Li-Hao Wang, Chun-Xiao Xu, Hyung Jin Sung, and Wei-Xi Huang

The attached-eddy framework is extended to turbulent coherent structures over a traveling wavy boundary. The results provide evidence for the presence of the hierarchically distributed self-similar wall-attached structures of streamwise velocity fluctuations in the presence of a wavy boundary. This is helpful for the modeling and prediction of flow properties in wind-wave interactions.

[Phys. Rev. Fluids 6, 034611] Published Tue Mar 16, 2021

### Direct measurements of the thermal dissipation rate in turbulent Rayleigh–Bénard convection

We report measurements of the thermal dissipation rate in turbulent Rayleigh-Bénard convection using a four-thermistor temperature gradient probe. The measurements have been undertaken in a Rayleigh-Bénard cell filled with air (Prandtl number [math]. The focus of this work is on large aspect ratios [math] (ratio between the horizontal and vertical extension of the cell), for which reason four datasets in the range of Rayleigh number [math] to [math] were taken at [math]. In order to extend the range toward higher Rayleigh numbers, two smaller aspect ratios were also investigated ([math] with [math] and [math] with [math]). We present highly resolved, vertical profiles of the thermal dissipation rate in the central vertical axis and discuss how these profiles change with the Rayleigh number. With its maximum near the wall and at the highest Rayleigh number, the thermal dissipation rate decreases monotonically with the distance from the plate. Moreover, the normalized, volume-averaged thermal dissipation rate, which effectively results in the Nusselt number [math], scales with an exponent of about [math] with the Rayleigh number. In the Rayleigh number range investigated here, the dissipation is always higher in the boundary layer than in the bulk region. However, by means of an extrapolation of the considered Rayleigh number range to larger Rayleigh numbers, the intersection point between the dissipation in the boundary layer and the bulk region can be estimated as [math].

### Jet propulsion of a squid-inspired swimmer in the presence of background flow

Inspired by recent studies of a squid-like swimmer, we propose a three-dimensional jet propulsion system composed of an empty chamber enclosed within a deformable body with an opening. By prescribing the body deformation and jet velocity profile, we numerically investigate the jet flow field and propulsion performance under the influence of background flow during a single deflation procedure. Three jet velocity profiles, i.e., constant, cosine and half cosine, are considered. We find that the maximum circulation of the vortex ring is reduced at a higher background flow velocity. This is because stronger interaction between the jet flow and background flow makes it harder to feed the leading vortex ring. Regarding thrust production, our analysis based on conservation of momentum indicates that with the constant profile the peak thrust is dominated by the time derivative of the fluid momentum inside the body, while momentum flux related thrust accounts for the quasi-steady thrust. For the cosine profile, its peak is mainly sourced from momentum flux associated with the unsteady vortex ring formation. No prominent thrust peak exists with the half cosine profile whose thrust continuously increases during the jetting. For all the three jet velocity profiles, added-mass related thrust attributed to body deformation enhances the overall thrust generation non-negligibly. Under the present tethered mode, the background flow has negligible influence on the thrust attributed to momentum flux and momentum change of the fluid inside the body. However, it indeed affects the over pressure-related thrust but its effect is relatively small. The overall thrust declines due to the significantly increased drag force at large incoming flow speed despite the rise of added-mass related thrust. Unsteady thrust involving vortex ring formation becomes more important in the overall thrust generation with an increased background flow velocity, reflected by larger ratios of the unsteady impulse to jet thrust impulse.

### Inner structures of Görtler streaks

The hypersonic boundary layer transition over a concave wall is investigated in a Mach 6.5 quiet wind tunnel using temperature sensitive paint (TSP), CO2-enhanced filtered Rayleigh scattering flow visualization, PCB fast-response pressure sensors, and a high-frequency schlieren technique. The TSP shows that low- and high-temperature streaks are distributed in the spanwise direction. The wavelengths of naturally developing Görtler streaks are randomly distributed, with an average of approximately 7 mm, and change little as the unit Reynolds number increases. More importantly, three-dimensional waves are clearly visualized and quantitatively measured inside the Görtler streaks. This is the first time that the entire evolution of the Görtler instability has been visualized using the Rayleigh-scattering flow visualization in hypersonic flow. The results demonstrate that three-dimensional waves are amplified as a result of the Görtler instability, resulting in a localized high-shear layer around the interface of the three-dimensional waves, which contributes to the formation of hairpin vortices and mushroom-like structures. The three-dimensional waves grow and play an important role in Görtler instability-induced boundary layer transitions.

### Sloshing in a rotating liquid inside a closed sea cage for fish farming

Sloshing in a sea cage with a slowly rotating liquid is investigated. The cage is axisymmetric, and the liquid is subjected to a nearly uniform angular velocity about the vertical axis of the cage. Both experimental and theoretical investigations are presented. It is shown that rotation modifies the sloshing regimes of a non-rotating liquid by splitting the natural frequencies. Therefore, resonant sloshing regimes can be manipulated by varying the rotation rate of the liquid.

### A priori assessment of convolutional neural network and algebraic models for flame surface density of high Karlovitz premixed flames

Accurate modeling of the unresolved flame surface area is critical for the closure of reaction source terms in the flame surface density (FSD) method. Some algebraic models have been proposed for the unresolved flame surface area for premixed flames in the flamelet or thin reaction zones (TRZ) regimes where the Karlovitz number (Ka) is less than 100. However, in many lean combustion applications, Ka is large (Ka > 100) due to the strong interactions of small-scale turbulence and flames. In the present work, a direct numerical simulation (DNS) database was used to evaluate the performance of algebraic FSD models in high Ka premixed flames in the context of large eddy simulations. Three DNS cases, i.e., case L, case M and case H, were performed, where case L is located in the TRZ regime with Ka < 100 and case M and case H are located in the broken reaction zones regime with Ka > 100. A convolutional neural network (CNN) model was also developed to predict the generalized FSD, which was trained with samples of case H and a small filter size, and was tested in various cases with different Ka and filter sizes. It was found that the fraction of resolved FSD increases with increasing filtered progress variable [math] and decreasing subgrid turbulent velocity fluctuation [math]. The performance of CNN and algebraic models was assessed using the DNS database. Overall, the results of algebraic models are promising in case L and case M for a small filter size; the CNN model performs generally better than the algebraic models in high Ka flames and the correlation coefficient between the modeled and actual generalized FSD is greater than 0.91 in all cases. The effects of [math] and [math] on the performance of different models for various cases were explored. The algebraic models perform well with large values of [math] and small values of [math] in high Ka cases, which indicates that they can be applied to high Ka flames in certain conditions. The performance of the CNN model is better than the algebraic models for a large filter size in high Ka cases.

### Static mesh adaptation for reliable large eddy simulation of turbulent reacting flows

The design challenge of reliable lean combustors needed to decrease pollutant emissions has clearly progressed with the common use of experiments as well as large eddy simulation (LES) because of its ability to predict the interactions between turbulent flows, sprays, acoustics, and flames. However, the accuracy of such numerical predictions depends very often on the user's experience to choose the most appropriate flow modeling and, more importantly, the proper spatial discretization for a given computational domain. The present work focuses on the last issue and proposes a static mesh refinement strategy based on flow physical quantities. To do so, a combination of sensors based on the dissipation and production of kinetic energy coupled to the flame-position probability is proposed to detect the regions of interest where flow physics happens and grid adaptation is recommended for good LES predictions. Thanks to such measures, a local mesh resolution can be achieved in these zones improving the LES overall accuracy while, eventually, coarsening everywhere else in the domain to reduce the computational cost. The proposed mesh refinement strategy is detailed and validated on two reacting-flow problems: a fully premixed bluff-body stabilized flame, i.e., the VOLVO test case, and a partially premixed swirled flame, i.e., the PRECCINSTA burner, which is closer to industrial configurations. For both cases, comparisons of the results with experimental data underline the fact that the predictions of the flame stabilization, and hence the computed velocity and temperature fields, are strongly influenced by the mesh quality and significant improvement can be obtained by applying the proposed strategy.

### Experimental investigation on the impingement of synthetic jet vortex rings onto a porous wall

This paper presents an experimental study on the effects of the Reynolds number (Resj = 300, 600, and 900) and porosity (ϕ = 20%–85%) on synthetic jet vortex rings impinging onto a porous wall. Laser-induced fluorescence and particle image velocimetry are used to acquire flow information qualitatively and quantitatively. When Resj is low (Resj = 300), ϕ plays a key role in determining the formation of transmitted vortex rings downstream. For the first time, a row of individual small-scale vortex rings that form at the lowest porosity (ϕ = 20%) have been observed in the synthetic jet/porous wall interaction. As Resj increases to 900, the triggered Kelvin–Helmholtz instability promotes the vorticity cancellation at a low porosity (ϕ = 30%), and thus contributes to the formation of a transmitted vortex ring. It is concluded that the vorticity cancellation is the dominant factor affecting the generation of a transmitted vortex ring. Time-averaged characteristics indicate that for a low Resj, the incoherence of the vortex ring is mainly due to the viscous effects. However, for a high Resj, it is the transition that leads to a significant enhancement in the turbulent kinetic energy. Measurements of flow macroscopic parameters show that the loss of the momentum flux exhibits a linear relationship with ϕ for all Resj, while the loss of the kinetic energy transport is nonlinearly dependent on ϕ. Incorporating ϕ, this study presents a more comprehensive similarity parameter, ϕln(Resj2[math]), to characterize the synthetic jet/porous wall interaction.

### Investigation of turbulent inflow specification in Euler–Lagrange simulations of mid-field spray

The process of atomization of a liquid jet by a parallel high-speed gas stream results in a spray, whose downstream development is of considerable interest to several applications. The round jet spray can be spatially divided into (i) a near-field (near-nozzle) region of liquid atomization and (ii) a downstream mid-field region of fully-dispersed droplets. In order to accurately model mid-field droplet dispersion, this work aims at developing a rigorous and robust injection model for Euler–Lagrange spray simulations. Results from experiments are used to obtain the relevant droplet number density, size distribution, and mean and standard deviation velocity distributions of the injection model, systematically in a step-by-step process. Two-phase large eddy simulations are performed by stochastically generating the Lagrangian droplets at the inlet of the mid-field region. Number flux, diameter distribution, mean velocity, and other time-averaged statistics at several downstream locations are shown to agree well with the corresponding experimental data.

### Universal perturbation growth of Richtmyer–Meshkov instability for minimum-surface featured interface induced by weak shock waves

Experimental and theoretical investigations are performed to explore the development of Richtmyer–Meshkov (RM) instability for a minimum-surface featured (3D-) interface. The exact mathematical expression of 3D-interface perturbation is obtained for the first time by the spectrum analysis, describing as a superposition of transverse two-dimensional (2D) single-mode with three-dimensional (3D) multi-mode. In particular, the normalized 3D-interface profile is found to be solely determined by one dimensionless parameter related to the 3D-interface initial spectrum. The shock tube experiments are performed by varying the interface height to change the mode-composition of 3D-interfaces under weak shock conditions. It is found that the 3D multi-mode component of a 3D-interface promotes/suppresses the RM instability at the transverse boundary/symmetry plane in comparison with the classical 2D single-mode case. At the linear regime, the 3D perturbation growth can be well predicted by combining the amplitude growth of a 2D single-mode and a 3D dual-mode. At the nonlinear regime, as the interface height reduces, the nonlinear effect on the RM instability at the boundary plane becomes stronger. A generalized nonlinear model is established to predict the interface amplitude by considering the interface spectrum and the mode-coupling of 3D modes. It is found that the mode-coupling has an evident influence on the bubble evolution, and the first-order 3D mode leads to different behaviors for the bubble and spike width growths. This work may provide great insight into the physical mechanism of the 3D RM instability existing in practical applications.

### Simulation of aerosol transmission on a Boeing 737 airplane with intervention measures for COVID-19 mitigation

Identifying economically viable intervention measures to reduce COVID-19 transmission on aircraft is of critical importance especially as new SARS-CoV2 variants emerge. Computational fluid-particle dynamic simulations are employed to investigate aerosol transmission and intervention measures on a Boeing 737 cabin zone. The present study compares aerosol transmission in three models: (a) a model at full passenger capacity (60 passengers), (b) a model at reduced capacity (40 passengers), and (c) a model at full capacity with sneeze guards/shields between passengers. Lagrangian simulations are used to model aerosol transport using particle sizes in the 1–50 μm range, which spans aerosols emitted during breathing, speech, and coughing. Sneeze shields placed between passengers redirect the local air flow and transfer part of the lateral momentum of the air to longitudinal momentum. This mechanism is exploited to direct more particles to the back of the seats in front of the index patient (aerosol source) and reduce lateral transfer of aerosol particles to other passengers. It is demonstrated that using sneeze shields on full capacity flights can reduce aerosol transmission to levels below that of reduced capacity flights without sneeze shields.

### Spontaneous imbibition in tight porous media with different wettability: Pore-scale simulation

Spontaneous imbibition is significantly influenced by rock wettability, and it has been extensively studied in core-based experiments and numerical simulations owing to its important role in the development of oil/gas reservoir. Due to the fine pore structure and complex wettability of tight sandstone, an in-depth exploration of the effects of wettability on the pore-scale flow physics during spontaneous imbibition is of great value to complement traditional experimental studies and enhance the understanding of microscopic flow mechanisms during the development of tight oil reservoirs. Based on a X-ray computed tomography scanning experiment and a lattice Boltzmann multiphase model, in this work, we systematically investigate the effects of different hydrophilic strengths on the evolution of the imbibition fronts within the micropores and the degree of nonwetting fluid recovery during spontaneous imbibition of tight sandstone. The results show that the wettability significantly affects the morphological characteristics of the imbibition fronts. Under strong hydrophilic conditions, the wetting fluid preferentially invades the pore corner in the form of angular flow. As the contact angle increases, the hysteresis effect at the main terminal interface decreases, and the two-phase interface becomes regular and compact. Wettability also significantly affects the imbibition rate and the nonwetting fluid recovery degree. The smaller the contact angle, the faster the imbibition rate and the higher the recovery degree of nonwetting fluids during the cocurrent spontaneous imbibition.

### Opaque and translucent films from aqueous microfiber suspensions by evaporative self-assembly

Films were prepared by casting microfiber (MF) suspensions on hydrophobic and hydrophilic substrates at controlled conditions (23 °C and 50% relative humidity). It was found that opaque films are formed on the hydrophilic surface, while translucent films are formed on the hydrophobic one. The physical and mechanical properties of the MF films were found to be comparable to those of nano-fibrillated cellulose and microfibrillated cellulose films. The observations from the microfiber film formation on the two substrates of different wettability are discussed in the context of the evaporation of water from sessile droplets containing nanoparticles.

### Cavitation causes brain injury

In this study, an artificial transparent head surrogate with high-speed photography discovers the formation and collapse of cavitation bubbles near the contrecoup regions as the head is exposed to a sudden translational impact. The cavitation damages the brain surface and produces a shock wave through the brain matter. Based on a novel experimental design, this new finding uncovers the mystery of the motion and deformation of the soft brain matter, which is not visible otherwise. It suggests that current brain injury criteria may underestimate the risk of head collision.

### Transport of vascular endothelial growth factor dictates on-chip angiogenesis in tumor microenvironment

On-chip investigations on tumor angiogenesis, hallmarked by the growth of new blood vessels from preexisting ones, have attracted significant interest in recent times, due to their exclusive capabilities of probing the detailed mechanisms of chemokine transport and visualization of cell-cell interactions that are otherwise challenging to capture and resolve under in vivo conditions. Here, we present a simulation study mimicking tumor angiogenesis microenvironment on-chip, with a vision of establishing the favorable conditions for stable and uniform gradients of vascular endothelial growth factor that plays a pivotal role in tumor progression. The model platform addresses different responses of endothelial cells such as chemotaxis, haptotaxis, and mitosis, under combined convection-diffusion transport in a micro-confined fluidic environment constituting collagen-based extracellular matrix. The model predictions emerge to be consistent with reported in vitro angiogenesis experiments and hold potential significance for the design of organ-on-a-chip assays, disease modeling, and optimizing anti-angiogenic therapies.

### Oscillatory pressure-driven rarefied binary gas mixture flow between parallel plates

Author(s): Alexandros Tsimpoukis, Steryios Naris, and Dimitris Valougeorgis

The rarefied, oscillatory, pressure-driven binary gas mixture flow between parallel plates is computationally investigated in terms of the mixture molar fraction and molecular mass ratio of the species, in a wide range of gas rarefaction and oscillation frequency. Modeling is based on the McCormack ...

[Phys. Rev. E 103, 033103] Published Mon Mar 15, 2021

### Theory of acoustophoresis in counterpropagating surface acoustic wave fields for particle separation

Author(s): Zixing Liu, Guangyao Xu, Zhengyang Ni, Xizhou Chen, Xiasheng Guo, Juan Tu, and Dong Zhang

Acousotophoretic particle separations in counterpropagating surface acoustic wave (SAW) fields, e.g., standing SAWs (SSAWs), phase modulated SSAWs, tilted angle SSAWs, and partial standing SAWs, have proven successful. But there still lacks analytical tools for predicting the particle trajectory and...

[Phys. Rev. E 103, 033104] Published Mon Mar 15, 2021

### Shear localization in large amplitude oscillatory shear (LAOS) flows of particulate suspensions

Author(s): Marko Korhonen, Kristian Wallgren, Antti Puisto, Mikko Alava, and Ville Vuorinen

Significant shear localization, observed as substantial concentration gradients, is discovered in large amplitude oscillatory shear (LAOS) simulations of a complex fluid in a planar Couette setup. The localization is demonstrated to occur due to the inertial effects imposed by the oscillatory shear in LAOS, which serve as perturbations in the nonlinear equations describing the structural response of the fluid to shear. Additionally, a criterion for the onset of this shear localization is presented.

[Phys. Rev. Fluids 6, 033302] Published Mon Mar 15, 2021