Latest papers in fluid mechanics

Author(s): Toshiyuki Gotoh, Izumi Saito, and Takeshi Watanabe

A set of equations for supersaturation (SS) and liquid water content (LWC) fluctuations in cloud turbulence are derived and their spectra are analyzed by using the Lagrangian renormalized approximation. The SS spectrum has three power law ranges, while the LWC spectrum has two power law ranges before the exponential roll off as the wavenumber increases.

[Phys. Rev. Fluids 6, 110512] Published Tue Nov 30, 2021

Author(s): C. Rorai, F. Toschi, and I. Pagonabarraga

Active nematic fluids confined in narrow channels are known to generate spontaneous flows when the activity is sufficiently intense. Recently, it was demonstrated [R. Green, J. Toner, and V. Vitelli, Phys. Rev. Fluids 2, 104201 (2017)] that if the molecular anchoring at the channel walls is conflict...

[Phys. Rev. Fluids 6, 113302] Published Tue Nov 30, 2021

Author(s): Nikhil Desai and Sébastien Michelin

In experiments, chemically active drops most often swim along a rigid wall, the impact of which on self-propulsion is in general completely overlooked in models. Using linear stability analysis, we demonstrate here that the proximity to a rigid surface promotes droplet propulsion as a result of the localization of the strongest interfacial flows within the thin lubrication gap separating the droplet from the wall.

[Phys. Rev. Fluids 6, 114103] Published Tue Nov 30, 2021

Author(s): Hayoon Chung and Jeffrey Koseff

To better understand turbulent reattachment downstream of canopy edges we conducted analysis of velocity and visual data from flume experiments. We find that the mean flow and turbulence statistics suggests the presence of both canopy shear and backward facing step (BFS) dynamics. Depending on the canopy characteristics, the dominance of either dynamics varies spatially within the wake. In regions dominated by canopy-shear turbulence, the separation induced by the canopy edge (BFS dynamics) modifies the canopy signal by introducing both larger and smaller scales to the flow. We also find that turbulence development over the upstream canopy influences the reattachment length in the wake.

[Phys. Rev. Fluids 6, 114605] Published Tue Nov 30, 2021

Author(s): Yashwant Rawat, Sachit Kalia, and Pranab Kumar Mondal

We develop a mathematical model to quantitatively describe the imbibition dynamics of an elastic non-Newtonian fluid in a conical (nonuniform cross section) microfluidic assay. We consider the simplified Phan-Thien-Tanner viscoelastic model to represent the rheology of the elastic non-Newtonian flui...

[Phys. Rev. E 104, 055106] Published Mon Nov 29, 2021

Author(s): Konrad Gizynski, Karol Makuch, Jan Paczesny, Yirui Zhang, Anna Maciołek, and Robert Holyst

We analyze a compressible Poiseuille flow of ideal gas in a plane channel. We provide the form of internal energy U for a nonequilibrium stationary state that includes viscous dissipation and pressure work. We demonstrate that U depends strongly on the ratio Δp/p0, where Δp is the pressure differenc...

[Phys. Rev. E 104, 055107] Published Mon Nov 29, 2021

Author(s): Marco Vona and Eric Lauga

The four-roll mill, wherein four identical cylinders undergo rotation of identical magnitude but alternate signs, was originally proposed by G. I. Taylor to create local extensional flows and study their ability to deform small liquid drops. Since an extensional flow has an unstable eigendirection, ...

[Phys. Rev. E 104, 055108] Published Mon Nov 29, 2021

Author(s): Daniël P. Faasen, Devaraj van der Meer, Detlef Lohse, and Pablo Peñas

Mass transfer of gases in liquid solvents is a fundamental process during bubble generation for specific purposes or, vice versa, removal of entrapped bubbles. In our work, we address the growth dynamics of a trapped slug bubble in a vertical glass cylinder under a water barrier after replacing the ambient air atmosphere by a CO2 atmosphere at the same or higher pressure. The asymmetric exchange of the gaseous solutes between the CO2-rich water barrier and the air-rich bubble always results in net bubble growth, which we call solute exchange. We compare and explain the experimental results with a simple numerical model, with which the underlying mass transport processes are quantified.

[Phys. Rev. Fluids 6, 113501] Published Mon Nov 29, 2021

Author(s): Marco Atzori, Ricardo Vinuesa, Alexander Stroh, Davide Gatti, Bettina Frohnapfel, and Philipp Schlatter

We study different flow control methods on turbulent flow around a NACA4412 airfoil, using resolved large-eddy simulation (LES). We find that changes in total skin friction due to blowing and suction are not very sensitive to different pressure-gradient conditions nor the Reynolds number. However, the boundary-layer thickness, the intensity of the wall-normal convection, and turbulent fluctuations are much more affected, mostly due to the adverse-pressure-gradient conditions, as skin-friction decompositions show. Overall, we conclude that it is not possible to simply separate pressure-gradient and control effects, which is important for control design in practical applications.

[Phys. Rev. Fluids 6, 113904] Published Mon Nov 29, 2021

Author(s): Joseph Ruan and Guillaume Blanquart

This work concerns the simulation of a homogenized streamwise periodic boundary layer which reduces the computational cost of direct numerical simulations of incompressible flat plate turbulent boundary layers. It expands upon the understanding of how the transpiration velocity changes dependent on whether a blending function is used to account for inner and outer self-similar scaling effects. With this rescaling correction, lower Reynolds number effects on the transpiration velocity are captured by the homogenized streamwise periodic boundary layer simulation.

[Phys. Rev. Fluids 6, 114604] Published Mon Nov 29, 2021

Physics of Fluids, Volume 33, Issue 11, November 2021.
Contrary to the popular inertial number-based rheology of dense granular flows, recent studies suggest a non-monotonic variation of the effective friction coefficient [math] with the inertial number I in plane shear flows. While the popular rheology assuming monotonic variation of [math] with I suggests existence of an upper limit of inclination angle for steady chute flows, the non-monotonic variation suggests the possibility of two different flow states for chute flows at a given inclination angle. In this work, we perform DEM simulations of chute flow of frictional inelastic disks and show that steady, fully developed flows are possible at inclinations much higher than those predicted from the monotonic [math] rheology. We observe steady flows up to inertial number [math] and find non-monotonic variation of the effective friction at high inertial numbers for chute flow of disks. The flows at high inertial numbers exhibit a constant density bulk region supported on top of a very dilute energetic basal layer of particles. We show that, in addition to a modified effective friction law that accounts for the non-monotonic variation of [math] and the dilatancy law relating the solids fraction [math] with I, the rheological description also needs to account for the stress anisotropy by means of a normal stress difference law. By accounting for the presence of the normal stress difference, we also establish that only a single flow state is possible at any given inclination angle despite the non-monotonic variation of the effective friction coefficient.

Physics of Fluids, Volume 33, Issue 11, November 2021.
The interaction of a National Advisory Committee for Aeronautics 0012 wing-tip vortex with a grid-generated turbulent flow was experimentally investigated. The experiments were conducted in the near- and mid-wake regions at three free-stream turbulence (FST) levels, viz., 0.5%, 3%, and 6%, at a Reynolds number, based on the wing chord length, cw, of [math]. Stereoscopic particle image velocimetry measurements were carried out at four downstream positions: [math] = 1.25, 3.25, 6.25, and 7.75. Streamwise vorticity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. Turbulent surroundings were found to affect the meandering amplitude of the vortex, which increased with the dispersed scatter pattern of the vortex center motion, resulting in a meandering-induced turbulence. Meandering-corrected turbulent kinetic energy revealed the existence of a laminar core at the center of the vortex surrounded by low turbulence levels outside the core. This was attributed to the stabilizing Coriolis effects of the strong rotational motion inside the vortex core, which tend to re-laminarize the turbulent fluid crossing the periphery of the vortex. Snapshot proper orthogonal decomposition analysis on the coherent component of the velocity field revealed two dominant modes forming a helical dipole, consistent with the helical displacement of a Kelvin wave with an azimuthal wavenumber [math]. An analysis of the terms balancing the rate of decay of the mean enstrophy revealed that increasing FST increases the stretching of the mean enstrophy within the vortex core while it reduces both its transport and convection terms. Nonetheless, the latter contributions were larger in all cases studied acting as the main mechanism for mean enstrophy decay.

Author(s): Yoshiharu Tamaki and Soshi Kawai

In this paper a novel methodology is proposed for wall-modeled large eddy simulation (WMLES) on non-body-conforming Cartesian grids. The proposed WMLES employs a partial-slip velocity boundary condition to reduce conservation errors at the wall. In addition, since the slip velocity reduces the shear stress in the near-wall region, a modeled turbulence shear stress is introduced to maintain the shear-stress balance in the near-wall region. The proposed WMLES robustly predicts turbulence statistics in turbulent boundary layers developed on an inclined flat plate without showing log-layer mismatch.

[Phys. Rev. Fluids 6, 114603] Published Wed Nov 24, 2021

Physics of Fluids, Volume 33, Issue 11, November 2021.
Nanoparticle surfactants, formed at liquid–liquid interfaces by the interactions between functional groups on nanoparticles and polymers having complementary end-functionality, have been recently proposed as an excellent interface stabilizer to cover liquid droplets for applications of substance encapsulation and delivery. However, the effects of nanoparticle surfactants on the production of liquid droplets in a microfluidic channel have not been comprehensively understood yet, which is a key prerequisite for achieving various functions in real applications. In this study, we have performed a systematic investigation on the effects of nanoparticle surfactants on droplet formation in a flow-focusing microchannel by using microfluidic experiments and theoretical analysis. We have found that simultaneously adding carboxylated nanoparticles into the dispersed phase and amino-terminated polymers into the continuous phases significantly decreases the droplet size but increases the production rate. More importantly, we have indicated that the combined effect of nanoparticles and polymers is much greater than the sum of their individual effects, which is mainly attributed to the significant reduction of the oil–water interfacial tension by the formation of nanoparticle surfactants. Besides, via analyzing the competition between hydrodynamic and interfacial forces acting on the droplet, we have established a theoretical criterion for the prediction of the droplet size with considering the effects of nanoparticle surfactants, which shows a good agreement with the experimental data.

Physics of Fluids, Volume 33, Issue 11, November 2021.
Electrohydrodynamic atomization (EHDA) is carried out in the Taylor cone mode for generating unimodal particle distribution, which can be achieved by either constant voltage actuation (CVA) or alternating voltage actuation (AVA). The present study reports an experimental investigation of the flow field both inside and outside the Taylor cone using light sheet fluorescence imaging and time-resolved particle image velocimetry measurements. Liquid ethanol is used as the working fluid and the amplitude of both constant and alternating electric potential difference is set at the same value, i.e., [math] kV with an actuation frequency of 200 Hz in the case of alternating EHDA. The hydrodynamic behavior both inside and outside the Taylor cone is presented for the first time. The flow field measurements demonstrate meridional circulation from the nozzle exit toward the apex of the Taylor cone along the generatrix followed by flow from the apex of the Taylor cone along the central axis. A symmetric toroidal vortex is observed inside the Taylor cone in the case of CVA and an asymmetric toroidal vortex is observed for AVA. The flow field shows streamline-like flow in the ambient medium from the nozzle toward the ground electrode along the interface of the Taylor cone jet for CVA. In contrast, two vortical structures are observed around the apex of the Taylor cone for AVA. The velocity profile near the liquid–air interface of the Taylor cone indicates no direct correlation between the flow field inside the Taylor cone with the flow of the ambient medium. This difference may be attributed to the corona wind generated due to asymmetric electrode configuration. The unsteady flow field generated by alternating EHDA has great potential for enhanced heat transfer using spray cooling.

Author(s): Sombuddha Bagchi, Saptarshi Basu, Swetaprovo Chaudhuri, and Abhishek Saha

We present a study of wetted facemasks to evaluate their capability in blocking respiratory droplets. We show that the increase in wetness progressively weakens the penetration capability of the impacted droplets. Such behavior is observed for hydrophobic and hydrophilic masks, although the underlying mechanism is different.

[Phys. Rev. Fluids 6, 110510] Published Tue Nov 23, 2021

Author(s): Philippe Bourrianne, Nan Xue, Janine Nunes, Manouk Abkarian, and Howard A. Stone

In addition to their ability to filter pathogenic droplets, masks also represent a porous barrier to exhaled and inhaled air flow. In this study, we characterize the aerodynamic effect of a mask by tracking the air exhaled by a person through a mask. We show how a mask confines the exhaled flows within tens of centimeters in front of a person breathing or speaking.

[Phys. Rev. Fluids 6, 110511] Published Tue Nov 23, 2021

Author(s): Nancy B. Lu, Daniel B. Amchin, and Sujit S. Datta

Imbibition, the displacement of a nonwetting fluid by a wetting fluid, plays a central role in diverse energy, environmental, and industrial processes. While this process is typically studied in homogeneous porous media with uniform permeabilities, in many cases, the media have multiple parallel strata of different permeabilities. Here, we use numerical simulations to examine the fluid dynamics of imbibition in stratified media. Our results highlight how stratification can fundamentally change the dynamics of imbibition, and provide quantitative guidelines for predicting and controlling this process.

[Phys. Rev. Fluids 6, 114007] Published Tue Nov 23, 2021

Author(s): Haijun Yu, Xinyuan Tian, Weinan E, and Qianxiao Li

Machine learning is becoming an increasingly popular method for building mathematical models from observations of natural processes. Here, the central challenge is to impart structure into the model parameterization to enforce physical relevance, yet retain a degree of generality so that a large variety of dynamics can be learned. We introduce a novel methodology, based on a data-driven extension of the classical Onsager principle, that strikes a balance between these competing aspects. We demonstrate its efficacy by learning quantitatively accurate and qualitatively faithful reduced order models of the Rayleigh-Bénard convection equations.

[Phys. Rev. Fluids 6, 114402] Published Tue Nov 23, 2021

Author(s): Narendra Singh and Michael Kroells

Multiscale problems such as hypersonic flows with strong nonequilibrium due to strong shocks and expansions result in flow physics which is no longer accurately described by the Navier-Stokes equations (NSE). Similarly, the NSE break down in rarefied (low density) gas flows. Therefore, hybrid methods, which can combine the continuum description using NSE and the kinetic description (KD), are necessary for efficient high-fidelity numerical simulations. A key input to hybrid methods is a metric to identify regions in the flow-field where the NSE breaks down and the KD should be used. In this Letter, starting from kinetic theory, we develop a rigorous metric to assess where the NSE breaks down.

[Phys. Rev. Fluids 6, L111401] Published Tue Nov 23, 2021