Physical Review Fluids

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Spectra of supersaturation and liquid water content in cloud turbulence

Tue, 11/30/2021 - 10:00

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

Active nematic flows confined in a two-dimensional channel with hybrid alignment at the walls: A unified picture

Tue, 11/30/2021 - 10:00

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

Instability and self-propulsion of active droplets along a wall

Tue, 11/30/2021 - 10:00

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

Turbulence structure and scales in canopy-wake reattachment

Tue, 11/30/2021 - 10:00

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

Slug bubble growth and dissolution by solute exchange

Mon, 11/29/2021 - 10:00

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

Uniform blowing and suction applied to nonuniform adverse-pressure-gradient wing boundary layers

Mon, 11/29/2021 - 10:00

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

Error estimation of a homogenized streamwise periodic boundary layer

Mon, 11/29/2021 - 10:00

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

Wall modeling for large-eddy simulation on non-body-conforming Cartesian grids

Wed, 11/24/2021 - 10:00

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

Penetration and secondary atomization of droplets impacted on wet facemasks

Tue, 11/23/2021 - 10:00

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

Quantifying the effect of a mask on expiratory flows

Tue, 11/23/2021 - 10:00

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

Forced imbibition in stratified porous media: Fluid dynamics and breakthrough saturation

Tue, 11/23/2021 - 10:00

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

OnsagerNet: Learning stable and interpretable dynamics using a generalized Onsager principle

Tue, 11/23/2021 - 10:00

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

Quantitative uncertainty metric to assess continuum breakdown for nonequilibrium hydrodynamics

Tue, 11/23/2021 - 10:00

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

Nonlocal effects in the shear banding of a thixotropic yield stress fluid

Mon, 11/22/2021 - 10:00

Author(s): M. Raquel Serial, Daniel Bonn, Thom Huppertz, Joshua A. Dijksman, Jasper van der Gucht, John P. M. van Duynhoven, and Camilla Terenzi

A simple analytical model is introduced for describing the combined effect of thixotropic and nonlocal flow in a homogeneous stress field. The model adequately describes experimental rheo-MRI velocimetry profiles, measured for a milk microgel suspension in a cone-and-plate geometry.


[Phys. Rev. Fluids 6, 113301] Published Mon Nov 22, 2021

Bifurcation aspect of polygonal coherence over transitional Reynolds numbers in wide-gap spherical Couette flow

Mon, 11/22/2021 - 10:00

Author(s): Fumitoshi Goto, Tomoaki Itano, Masako Sugihara-Seki, and Takahiro Adachi

In a wide gap spherical Couette flow with η=1/2, the nonlinear branches of the 4,3,2-fold spiral states and the critical Reynolds numbers at which they sequentially bifurcate from the axisymmetric state are solved. The evidence that these states are established as stable states even over the successive critical Reynolds numbers is discussed based on the formation of their unstable manifolds and basins of attraction in the state space. This result substantiates that both the 4-fold and 3-fold spiral state may coexist stably in experiment.


[Phys. Rev. Fluids 6, 113903] Published Mon Nov 22, 2021

Nanobubble-induced flow of immersed glassy polymer films

Mon, 11/22/2021 - 10:00

Author(s): Christian Pedersen, Shuai Ren, Yuliang Wang, Andreas Carlson, and Thomas Salez

Glassy thin films have been abundantly studied in the past decades and shown to exhibit a liquid-like surface mobile layer. When immersed in water, nanobubbles spontaneously form at their free surface. Here, from a combination of glassy lubrication theory and finite-element numerical integration, we demonstrate that, and study how, the surface is dynamically deformed through a surface flow driven by the inner pressure of the bubble. As a consequence, the film eventually undergoes a dewetting event, for which we predict a typical time scale from the model.


[Phys. Rev. Fluids 6, 114006] Published Mon Nov 22, 2021

Relaxation dynamics of a flexible rod in a fluid

Mon, 11/22/2021 - 10:00

Author(s): Ayrton Draux, Hoa-Ai Béatrice Hua, Pascal Damman, and Fabian Brau

The relaxation dynamics of a bent rod immersed in a fluid is strongly affected by its bending stiffness and the fluid viscosity. For large bending stiffness or low viscosity, the dynamics is underdamped and the rod oscillates around its equilibrium position with a well-defined frequency and a damped amplitude. For low bending stiffness or large viscosity, the dynamics is overdamped and the rod relaxes to its equilibrium position without oscillating. We show the existence of two overdamped regimes where the relaxation dynamics is characterized by two different polynomial decays showing that the viscous force is not proportional to the rod velocity in our experiments.


[Phys. Rev. Fluids 6, 114102] Published Mon Nov 22, 2021

Nonsymmetric ionic transport in a nonbinary electrolyte at high voltage

Fri, 11/19/2021 - 10:00

Author(s): Arnon Ratzabi, Tal Eluk, Avi Levy, and Yuri Feldman

A phenomenological study focusing on investigation of ionic transport within one- and two-dimensional electrolytic cells operating at high voltage and confined by two open electrodes is presented. The cell is filled with a nonsymmetric electrolyte consisting of one positively charged and two negatively charged species. The study offers a physical interpretation and detailed discussion of the origin and development of electroconvective instability typical of configurations relevant to digital printing technologies.


[Phys. Rev. Fluids 6, 113701] Published Fri Nov 19, 2021

Effects of parietal suction and injection on the stability of the Blasius boundary-layer flow over a permeable, heated plate

Fri, 11/19/2021 - 10:00

Author(s): Mushrifah Al-Malki, Zahir Hussain, Stephen Garrett, and Sophie Calabretto

We investigate the effects of wall suction and injection on Chemical Vapour Deposition (CVD), a microfabrication process in which a gas mixture is pumped into a reactor and flows over a heated reactant porous surface to chemically deposit a thin film. Regular and cohesive film growth requires that laminar flow be maintained. The energy balance with wall suction for temperature dependence parameter ϵ<0 leads to increasing total mechanical energy due to reducing energy production and dissipation with increasing flow suction; for ϵ>0 there is a stabilization effect on the Tollmien-Schlichting waves mainly due to a large reduction in the energy contribution term.


[Phys. Rev. Fluids 6, 113902] Published Fri Nov 19, 2021

Thermocapillary instability of an ionic liquid-water mixture in a temperature gradient

Thu, 11/18/2021 - 10:00

Author(s): Marc Pascual, Axelle Amon, and Marie-Caroline Jullien

This paper reports the separation dynamics of a binary solution placed in a temperature gradient. We show the existence of three separation regimes, depending on the volume fractions of the components. One of the regimes is associated with a capillary instability of which a typical pattern is shown in this picture. All the observed regimes are rationalized.


[Phys. Rev. Fluids 6, 114203] Published Thu Nov 18, 2021

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