Is the full odd viscous (anomalous) stress tensor responsible for the normal pressure experienced by a cylinder rotating in a viscous liquid? The answer is negative, in general. When liquid velocities (and not stresses) are prescribed on the boundaries, it is the null divergence part of the odd viscous stress that generates this pressure.

[Phys. Rev. Fluids 8, 014104] Published Thu Jan 26, 2023

]]>[Phys. Rev. Fluids 8, 010001] Published Tue Jan 24, 2023

]]>Droplet deformation at finite capillary numbers is accurately studied in shear flows via Immersed Boundary - Lattice Boltzmann (IB-LB) simulations. A reduced model is introduced to capture nonlinear effects in droplet deformation and orientation.

[Phys. Rev. Fluids 8, 013603] Published Tue Jan 24, 2023

]]>A Hankel transform based model is developed to study water wave interaction with an array of thin submerged horizontal perforated and/or elastic circular plates. The integral equations are formulated in terms of unknown functions related to the jump in velocity potential across each plate. A Galerkin method is adopted to the solution of these integral equations and the velocity potential jump across the plate is expressed in terms of Fourier-Gegenbauer series. The present model is found to be valid for multiple plates distributed arbitrarily, including the staggered arrangement, for which the traditional eigenfunction matching method would not work.

[Phys. Rev. Fluids 8, 014803] Published Mon Jan 23, 2023

]]>We investigate the self-propelling motion of a bent rod. Our analysis reveals that the planar motion of the rod is always circular, and the radius of the circle and the speed of the particle can be tuned by changing the relative length of the two arms and the angle between them.

[Phys. Rev. Fluids 8, 014103] Published Fri Jan 20, 2023

]]>This study investigates the flow physics in some of the most inhomogeneous and complex regions of a separated flow around a wing tip and the near wake. The region is highly convoluted, strongly three-dimensional, multiscale, and far from being self-similar. We elucidate the early formation mechanisms of vortices close to the leading edge and investigate the intensification/suppression mechanisms of the different types of vortical structures as they evolve in space. The production of turbulent kinetic energy and Reynolds stresses is also investigated and discussed in conjunction with the identified vortex patterns.

[Phys. Rev. Fluids 8, 014704] Published Fri Jan 20, 2023

]]>Various continuum models have been developed to describe granular flows, but most of these were developed based on discrete simulations of isochoric flows, such as shear cells, where flow is parallel and there is no variation in velocity or packing fraction along streamlines. Many real granular flows are nonisochoric and may exhibit significant packing variations along streamlines. In our work we use discrete element modeling to simulate granular flows in nonisochoric flow geometries. Using this discrete data, we compare the performance of some existing compressible continuum models.

[Phys. Rev. Fluids 8, 014304] Published Thu Jan 19, 2023

]]>Numerical simulations of superfluid helium turbulent flows show the emergence of hydrodynamic cooperation between quantum vortices leading to dynamics almost without dissipation. This energy saving mechanism accounts for the enhanced vortex lifetime observed in experiments and is similar to the collective dynamics observed in active fluids, such as bacteria in aqueous suspensions, fungal spores in the atmosphere, and cyclists in pelotons. This active fluid characteristic of superfluid helium potentially determines properties of turbulence in both the inviscid and the viscous fluid components.

[Phys. Rev. Fluids 8, 014702] Published Thu Jan 19, 2023

]]>We conduct experiments on the structure of the 3D flow fields for unswept cantilevered wings at high angle of attack. Stall cell counter-rotating vortices on the suction surface form for a range of aspect ratios (AR), though AR changes the angle of attack at which the flow separates. Analysis of mean flow volume over the suction surface and near wake shows that the arch vortex forms in the wake and connects to the surface at the stall cell foci. Reynolds stress peaks indicate a strong reliance on the arch vortex location in the mean flow. Spectral analysis of the velocity field at select spanwise planes shows that shedding is coherent but intermittent, and varies strongly along the wingspan.

[Phys. Rev. Fluids 8, 014703] Published Thu Jan 19, 2023

]]>We identified a new dynamic stall mechanism in surging flows on thick airfoils at low angles-of-attack. Counterintuitively, the laminar separation bubble bursts during early imposition of the favorable temporal pressure gradient, which rapidly drives the bubble aft, rendering it unable to reattach. We also introduced a new generalized pressure coefficient, without which the experimental data are impossible to interpret.

[Phys. Rev. Fluids 8, L012102] Published Thu Jan 19, 2023

]]>We present a general mapping approach between the microscopic molecular dynamics model and a mesoscopic model based on the thin-film equation for the description of the dynamics of droplets on partially-wettable, switchable substrates. We apply the mapping approach to the adaption of a droplet to a new wettability upon switching and the coalescence of two droplets making the first step toward quantitative comparisons of dynamics between models acting on mesoscopic and microscopic scales.

[Phys. Rev. Fluids 8, 013902] Published Wed Jan 18, 2023

]]>Linear and nonlinear analyses on miscible viscous fingering (VF) in a cylindrical packed column is conducted. The effects of boundary conditions on the onset conditions of instability are suggested as a function of log-viscosity ratio and Péclet number. Through two- and three-dimensional (2D and 3D) simulations based on the finite element method, we clearly visualize the effect of the lateral boundary on the onset and the growth of VF. Our 3D simulations explain the experimental results available in the literature by suitably choosing the parameters.

[Phys. Rev. Fluids 8, 013901] Published Tue Jan 17, 2023

]]>The gravitational collision efficiency of a pair of cloud droplets settling in quiescent air is computed using various models for the aerodynamic interaction forces. The employed models consider (i) the effect of spherical fluid drops with mobile interfaces, and (ii) the noncontinuum molecular effect which changes short-range lubrication forces when the gap size between a pair is comparable to the air mean free path. These cases are compared with the widely used case that considers the droplets to be spherical rigid particles. We find that assuming rigid particles is accurate for water droplets interacting in air, but noncontinuum lubrication has to be taken into account.

[Phys. Rev. Fluids 8, 014102] Published Tue Jan 17, 2023

]]>Suspended particles in the atmosphere, such as water droplets or pollen, flowing over obstacles such as an aircraft wing or a tree branch may not follow the same path as the air, due to the particle’s inertia. The particles may then collide with and deposit onto the obstacle. This is particularly significant in the case of water droplets that freeze to ice on an aircraft wing, as an ice deposit on the wing can cause a crash. In this paper we identify a scaling law for the amount of deposition, above a critical particle Stokes number, for particles in inviscid, irrotational flow past a cylinder.

[Phys. Rev. Fluids 8, 014302] Published Tue Jan 17, 2023

]]>Point-particle closure models that are utilized in Euler-Lagrange simulations play an important role in replicating true dynamics of particle-laden flows. The accuracy of these point-particle models depends on how well they incorporate the local microstructural information of neighboring particles. The current work presents a physics-based hierarchical machine learning approach for developing robust N-body closures. The inclusion of ternary interactions, in addition to binary interactions, enabled by the hierarchical approach leads to improved predictions.

[Phys. Rev. Fluids 8, 014303] Published Tue Jan 17, 2023

]]>Flow around bluff bodies has been studied for more than 200 years, especially since the discovery of Kármán vortex streets, however less attention has been paid to polygonal cylinders. In this study we used Large Eddy Simulations (LES) to investigate flow behavior, and especially shear layer separation, of polygonal cylinders with side number of N=5-8 in various incidence angles. The flow separation mechanism is then explained with a focus on the separation pattern as well as the flapping motion of the separated shear layer and its signature in time mean fields.

[Phys. Rev. Fluids 8, 014701] Published Tue Jan 17, 2023

]]>Crossing sea states, with two or more wave trains propagating at an oblique angle to each other, constitute a plausible generation mechanism of rogue waves in the oceans. This underlying principle of enhanced modulation instability is extended to layered and stratified fluids. Long wave-short wave resonance in a two-layer flow and three-wave interaction in a fluid with constant buoyancy frequency are studied. Maxima in the growth rates imply preferred configurations of crossing sea states. A cascading mechanism is employed to demonstrate the emergence of breathers beyond linear modulation instability.

[Phys. Rev. Fluids 8, 014802] Published Tue Jan 17, 2023

]]>Intermittent swimming has been recognized as a strategy for fish to enhance their energetical efficiency. In this study, a hybrid computational fluid dynamic model is used to assess the swimming performance in intermittent swimming parametrically and quantitatively. The results show that the energetical performance of intermittent swimming can be better than that of continuous swimming, but also that an unoptimized intermittent gait may become very energetically expensive.

[Phys. Rev. Fluids 8, 013101] Published Fri Jan 13, 2023

]]>Statistical equations for inertial particle transport in turbulent boundary layers are usually closed using the quasi-Normal approximation (QNA), but this leads to large errors when the particle inertia is significant. Here we develop a new closure based on an asymptotic solution to the exact transport equations, referred to as the ACA. Results show that the ACA is in far better agreement with direct numerical simulation (DNS) data than the QNA.

[Phys. Rev. Fluids 8, 014301] Published Fri Jan 13, 2023

]]>More than two decades ago it was discovered that very viscous fluids can become turbulent even when the flow speed is vanishingly small. We perform challenging three-dimensional direct numerical simulation of this purely elastic turbulence in the prototypical Taylor-Couette flow with the aim to unravel the underlying mechanism. We demonstrate that in this unique inertialess turbulent state, large-scale solitary vortices and anisotropic elastic traveling waves as well as random velocity perturbations delicately sustain the turbulent dynamic cycle.

[Phys. Rev. Fluids 8, 014602] Published Fri Jan 13, 2023

]]>We investigate the effect of a vertically sheared current on wave statistics, including the probability of rogue waves, and apply it to a real-world case using measured spectral and shear current data from the mouth of the Columbia River. A theory for weakly nonlinear waves valid to second order in wave steepness is derived and used to analyze statistical properties of surface waves. With the wave spectrum and velocity profile measured in the Columbia River estuary, our theory predicts that the probability of rogue waves is significantly reduced and enhanced during ebb and flood, respectively, supporting the need for shear currents to be accounted for in wave modeling and prediction.

[Phys. Rev. Fluids 8, 014801] Published Fri Jan 13, 2023

]]>Wettability exerts fundamental control over multiphase flow in porous media, which has been extensively studied in uniform-wet porous media. In contrast, multiphase flow in mixed-wet porous media is less well-understood. We combine microfluidic experiments and pore-scale simulations to study the displacement of oil by water in a mostly oil-wet porous media patterned with discrete water-wet clusters. Our work demonstrates the complex nature of wettability control in mixed-wet porous media, and it presents experimental and numerical platforms upon which further insights can be drawn.

[Phys. Rev. Fluids 8, L012301] Published Wed Jan 11, 2023

]]>Theoretical calculations and experimental measurements reveal that, in any array of identical sessile droplets, the appropriately scaled drying dynamics of the 1st, 2nd, and n-th fastest evaporating droplets are (almost) universal across different arrangements.

[Phys. Rev. Fluids 8, 013602] Published Tue Jan 10, 2023

]]>Viscous fingering patterns that occur at the interface between two immiscible fluids of differing viscosities in a Hele-Shaw cell are normally studied mathematically via a nonlinear moving boundary problem with a single interface. Here we study a more realistic model which involves a doubly connected region of the more viscous fluid bounded by two interfaces. We simulate this model numerically using a level set method, supported by linear stability analysis and some experiments. Various results are presented for configurations in which the flow is driven by either a pressure difference or by rotating the entire Hele-Shaw cell.

[Phys. Rev. Fluids 8, 014001] Published Tue Jan 10, 2023

]]>Two-dimensional square-bars roughness significantly influences the drag coefficient and coherent structures of turbulent boundary layer flow. A pitch ratio of 8 can induce the maximum drag effect for the turbulent boundary layer compared to other pitch ratios at a moderate friction Reynolds number. Hot-wire experiment results show that a wide range of pitch ratios has similar energy modulation by transferring energy from the largest scale structures to the smaller ones.

[Phys. Rev. Fluids 8, 014601] Published Tue Jan 10, 2023

]]>We performed a new experiment of mechanical impact on a plate supporting a breath figure on its bottom surface. Although breath figures mature usually slowly, we show that the droplets pattern evolves here dramatically in a few milliseconds leading to a strong droplets number reduction above a threshold in acceleration. We interpret this by the droplets radii oscillations produced which then induce droplets contacting and coalescing with each other in a fascinating manner. Introducing an effective Bond number, coupling the droplets’ average initial radii and the acceleration amplitude, leads to a data collapse on a single master curve.

[Phys. Rev. Fluids 8, 013601] Published Mon Jan 09, 2023

]]>This paper addresses shear flows over a superhydrophobic grating made up of a periodic array of grooves separated by solid slats, with air bubbles trapped in the grooves. At large pressures, the liquid partially invades the grooves, and the menisci are no longer pinned at the edges of the slats. Asymptotic methods and conformal maps are used to derive approximations for the slip length in the limit of small solid fractions.

[Phys. Rev. Fluids 8, L012101] Published Fri Jan 06, 2023

]]>Freshwater has been shown to have a maximum density at about four degrees Celsius, and this leads to a phenomenon known as cabbeling. Cabbeling occurs when masses of water on different sides of the temperature of maximum density mix and create a denser mass. What happens when intruding and ambient temperatures in a gravity current are on opposite sides of the temperature of maximum density? How does cabbeling affect the evolution characteristics of gravity currents, and what sort of long term behavior arises?

[Phys. Rev. Fluids 8, 014502] Published Thu Jan 05, 2023

]]>At steep ice faces submerged in the ocean, convection is driven by differences in salt concentration, yet ablation of the ice is controlled by a mixture of heat and salt fluxes. The differing molecular diffusivities of heat and salt play a key role in setting these fluxes due to thin, diffusive boundary layers at the ice-ocean interface. We analyze such boundary layers by simulating convection between two vertical plates of fixed temperature and salinity, and varying their relative diffusivities, finding that the ratio of fluxes transitions between scaling regimes at a critical Prandtl number. These results provide physical insight for future parameterization of steep ice-ocean interfaces.

[Phys. Rev. Fluids 8, 013501] Published Wed Jan 04, 2023

]]>Starting from a slender-body formulation, we numerically and asymptotically characterize the steady-state deformation of a tethered, elastic filament in viscous flows with uniform, shear, and parabolic profiles.

[Phys. Rev. Fluids 8, 014101] Published Tue Jan 03, 2023

]]>High-speed experiments studying the Blast Driven Instability are used to validate Reynolds-Averaged Navier-Stokes (RANS) and Large eddy simulation (LES) turbulent mixing models. The work helps to elucidate the three mixing regimes of the instability and shows that the LES can successfully capture, at lease to the first order, the complex physics therein. The work also highlights the limitations of RANS models for transitioning instabilities and how LES results remain highly sensitive to the characterization of initial conditions.

[Phys. Rev. Fluids 8, 014501] Published Tue Jan 03, 2023

]]>The addition of polymers is known to fundamentally affect capillary singularities such as drop pinch-off. In this study, we wonder how polymeric drops spread onto a solid. Although the dynamics of the liquid bridge connecting the drop to the solid is similar to that of pure water, we find that the interface gets significantly sharper, which is quantified by measuring the curvature of the bridge. These findings are reminiscent of what is found for drop coalescence, even though the spreading-coalescence analogy breaks down with viscoelastic liquids.

[Phys. Rev. Fluids 7, 123604] Published Fri Dec 30, 2022

]]>With experiments and theory the effect of acoustic parameters on the stability and break up of a planar liquid sheet under the influence of a standing acoustic wave are studied. Atomization characteristics of slit nozzles and the effects of acoustic pressure and injector position are analyzed. Above a critical acoustic pressure the standing wave acoustic field generates a Faraday wave on the liquid sheet surface. At a specific flow rate an empirical coefficient is found which enables linear instability theory to predict the wavelength of the Faraday waves and the breakup length of the liquid sheet.

[Phys. Rev. Fluids 7, 124004] Published Thu Dec 29, 2022

]]>We conduct an experimental study of axisymmetric gravity-capillary standing waves generated by a vertically vibrating ring on a fluid surface. Different regimes of standing waves are highlighted at the basin center: linear, nonlinear, and ejection regimes. For moderate forcing, the spatial profile of standing waves breaks the up-down symmetry as predicted by a third-order nonlinear theory, whereas for stronger forcing, the maximum height reached by the axisymmetric gravity-wave crest, at the basin center, is found to increase linearly with its wavelength, as a consequence of the saturation of its steepness.

[Phys. Rev. Fluids 7, 124801] Published Thu Dec 29, 2022

]]>Using the Reynolds scaling analysis, we simplify the Landau-de Gennes (LdG) equations for the flow of liquid crystals (LCs) in thin gaps. Then, both the simplified LdG equations and the full LdG equations are applied to a Couette geometry and a slider bearing geometry for the evaluation and verification of the simplified LdG equations by comparing results. The use of the simplified LdG equations results in a large decrease in wall time, which has the potential to significantly reduce the computational resources needed for simulating LC flows in complex geometries.

[Phys. Rev. Fluids 7, 123303] Published Wed Dec 28, 2022

]]>The presented work introduces a novel cancelation technique, based on the linear superposition of stationary crossflow instabilities (CFI) through the application of a streamwise series of optimally positioned discrete roughness elements (DRE) arrays on a swept wing surface. The DRE arrays are designed and arranged with suitable amplitude and phase shift to induce velocity disturbance systems that destructively interact, ultimately damping the developing CFI. This results in the suppression of monochromatic CFI, reducing their amplitude and growth and delaying the boundary layer transition to turbulence.

[Phys. Rev. Fluids 7, 123902] Published Wed Dec 28, 2022

]]>According to the definition by $O\phantom{\rule{0}{0ex}}x\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}d$ $L\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$, style is $a$ $p\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r$ $p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e$ $b\phantom{\rule{0}{0ex}}y$ $w\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}h$ $s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ $i\phantom{\rule{0}{0ex}}s$ $d\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e$ and thus perfectly fits with what we express in a technical paper; but it also designates $a$ $d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ $a\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}e,$ $t\phantom{\rule{0}{0ex}}y\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y$ $d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d$ $b\phantom{\rule{0}{0ex}}y$ $t\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}e$ $p\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$ $a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ $t\phantom{\rule{0}{0ex}}o$ $w\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}h$ $s\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ $i\phantom{\rule{0}{0ex}}s$ $d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d.$ This, that might be less clear in a scientific context, is precisely what we discuss in our paper devoted to the question of style in science. The figure shows a tear by van der Weyden, circa 1435.

[Phys. Rev. Fluids 7, 120501] Published Mon Dec 26, 2022

]]>Filaments or elongated drops are common in daily life, nature, and technological applications. They contract due to surface tension. It is known from experiments that the velocity at which their tips retract can be increased in viscoelastic filaments that contain polymer additives. Here, it is shown from simulations that the velocity with which the tips of prestressed filaments retract is greatly increased compared to filaments in which the polymer molecules are relaxed. The enhancement can be understood in terms of a quantity that is positive when the flow does work on the polymer molecules but negative when the molecules do work on the flow, i.e. when elastic recoiling or unloading takes place.

[Phys. Rev. Fluids 7, L121601] Published Mon Dec 26, 2022

]]>We develop a rigorous and robust framework which allows for control of interfacial flows. In particular, we use a Model Predictive Control methodology that computes Optimal Controls based on a high fidelity Reduced Dimensional Model initialized with observations of an experimental proxy. The results are excellent and, while the actuation mechanism used here is an electric field, the versatility of the framework affords easy extension to other flows, control mechanisms, and geometries.

[Phys. Rev. Fluids 7, L122001] Published Mon Dec 26, 2022

]]>The collision behavior of droplet pairs subjected to confined shear flows at high density and viscosity ratios remains unexplored and unquantified, and therefore, the critical density and viscosity ratios for the collision mode of droplets under geometric parameters were unknown. Simulations have been performed using a free-energy-based lattice Boltzmann method, with the aim of determining new regimes compared to the previous study. The results demonstrate that the interaction between viscous and inertia forces plays a vital role in the coalescence nature of droplets, their deformation, course of trajectory, as well as their expedition toward the steady state.

[Phys. Rev. Fluids 7, 123603] Published Thu Dec 22, 2022

]]>A local rise of the surface temperature of a liquid layer of only a few tenths of degrees results in a thermocapillary flow, which can decrease the thickness of a supported liquid film down to tens of nanometers over millimetric distances. We show that accurate measurements of the thinning dynamics of this ultra-thin film can be used to finely probe the molecular interactions at stake.

[Phys. Rev. Fluids 7, 124003] Published Thu Dec 22, 2022

]]>In the study of sprays, a theoretical gap exists between droplet modeling and reality, especially in high pressure applications like jet engine combustion. In this paper, we use interface-capturing direct numerical simulations of freely deforming droplets to demonstrate the impact of droplet deformation and liquid internal circulation on drag. An increase of over 200% in drag can be seen, depending on the conditions studied. We further showed that these phenomena can be quantified by the pressure and the Weber number, and a correlation for droplet drag was obtained based on these factors.

[Phys. Rev. Fluids 7, 123602] Published Wed Dec 21, 2022

]]>This article explores the fluid dynamics underlying voice production. The key parameters explored in this paper are frequency (from adult male to children and adult females) and the degree of closure of the vocal folds during phonation. The latter can be a pathological condition, but is common in children and females without being problematic. The findings of this study indicate that changes in jet dynamics across the human frequency range may explain the fundamental differences between male and female voices.

[Phys. Rev. Fluids 7, 123102] Published Tue Dec 20, 2022

]]>We develop a parallel-plate hydrodynamic model to study the influence of thermal Marangoni flow on dynamic wetting failure. We show that flow toward the dynamic contact line delays wetting failure to a higher critical capillary number by changing stress gradients in the vicinity of the contact line. Through this work, we provide a potential framework for developing faster coating processes using thermal Marangoni flow.

[Phys. Rev. Fluids 7, 124002] Published Mon Dec 19, 2022

]]>Leveraging tools from graph signal processing and work on Lagrangian coherent structures, we introduce a graph-based scale transform that is fully Lagrangian and based on the transport properties of the flow. Applying this tool to several different flows, we are able to assess the dynamic consequences of kinematic coherence in a variety of contexts. This approach allows us to extend traditional Eulerian techniques (filtering, compression) to the analysis of Lagrangian quantities.

[Phys. Rev. Fluids 7, 124401] Published Mon Dec 19, 2022

]]>The purpose of this work is to investigate whether a cascading process can be associated with the rotational motions of compressible three-dimensional turbulence. This question is examined through the lens of circulicity, a concept related to the angular momentum carried by large turbulent scales. Point-splitting and coarse-graining analyses are performed in order to uncover some properties of the circulicity nonlinear transfer flux. These properties are established independently from the expression of the equation of state.

[Phys. Rev. Fluids 7, 124604] Published Mon Dec 19, 2022

]]>A Lagrangian stochastic model for the orientation evolution of inertialess, rod-shaped particles is proposed and studied in a two-dimensional homogeneous shear turbulent flow. The model consists of superposing a short-correlated random component to the steady large-scale mean shear and also accounts for the anisotropies of velocity gradient fluctuations. Analytical results on a few key orientation statistics are derived, highlighting the model’s ability to reproduce the long-term effects and elucidate its limits at intermediate times to catch violent fluctuations when compared to direct numerical simulations.

[Phys. Rev. Fluids 7, 124605] Published Mon Dec 19, 2022

]]>Turbulent diffusion and mixing of heat, introduced passively by a thin ribbon in grid-generated turbulence, was enhanced drastically by a small array of thin cylinders, positioned closely downstream of the ribbon. A multi-structure flow region was formed behind the array and relaxed towards grid turbulence, albeit maintaining a turbulence intensity and an integral length scale that were twice as large as those in the absence of the array. This configuration has potential for use in applications that would benefit from intense turbulent mixing.

[Phys. Rev. Fluids 7, 124502] Published Fri Dec 16, 2022

]]>In this work we present an inexpensive model that unifies bubble collapse properties in arbitrary geometries. The model produces a single value that quantifies the asymmetry known as the ‘anisotropy parameter’. We show that bubble collapse properties, such as bubble displacement, depend primarily on this parameter.

[Phys. Rev. Fluids 7, 123601] Published Wed Dec 14, 2022

]]>Modeling molecular mixing remains a key issue for the filtered density function (FDF) method. We propose a new closure for the mixing timescale of each individual chemical species. A posteriori tests demonstrate the advantages of the proposed model, highlighting its potential to be employed in FDF simulations of turbulent reacting flows.

[Phys. Rev. Fluids 7, 124603] Published Wed Dec 14, 2022

]]>Surface waves have been observed to be brought to rest in finite time when a layer at the surface is impregnated with floating particles. A theoretical model is presented to explore whether a similar nonlinear damping process operates if the surface is covered by a thin plate of yield-stress fluid. The model demonstrates that the yield stress again arrests motion in finite time, although the final decay of the wave amplitude is faster than with floating particles.

[Phys. Rev. Fluids 7, 123302] Published Tue Dec 13, 2022

]]>Systematic experiments of particle clouds settling in quiescent water are performed in the absence and in the presence of background rotation, varying the size of particles to assess the influence of particle inertia on the clouds’ dynamics. In the absence of rotation, the particle inertia enhances the growth rate of turbulent clouds until particles decouple from eddies. In the presence of background rotation, the latter interrupts the clouds’ growth as it constrains clouds to become vortical columns.

[Phys. Rev. Fluids 7, 124302] Published Tue Dec 13, 2022

]]>The formation of large drops via collisions of droplets in warm clouds constitutes a major unsolved problem in rain prediction studies. Often typical turbulent accelerations of parcels of air that generate collisions are much smaller than gravitational acceleration. We manage to provide a complete theory in the limit of small accelerations and confirm it by direct numerical simulations of the Navier-Stokes turbulence. However, the theory is limited to Reynolds numbers much smaller than those in clouds, where, as we show, huge Reynolds number can overturn many existing paradigms.

[Phys. Rev. Fluids 7, 124303] Published Tue Dec 13, 2022

]]>The flow field around an axisymmetric slender body at an angle of attack is complex and typically contains extensive regions of three-dimensional crossflow separation; although the body is axisymmetric the side force induced by the flow on the body is significant and may change greatly along the body. This study provides a new perspective on the development of flow unsteadiness and the side-force distribution along the body for a wide range of angles of attack.

[Phys. Rev. Fluids 7, 124101] Published Mon Dec 12, 2022

]]>Composition gradients in the interior of Jupiter can affect and even suppress convective motions. In some situations, a composition gradient can trigger the formation of multiple convective layers separated by sharp diffusive interfaces, preventing further mixing. This fluid state, called layered convection, has been proposed to occur in giant planets. However, it is not guaranteed that secondary convective layers can form and survive underneath a turbulent convection zone. Our simulations find that below an evolving convection zone, layer formation is difficult and the fluid always fully mixes. This may have bearing on the survival of composition gradients in Jupiter’s interior.

[Phys. Rev. Fluids 7, 124501] Published Mon Dec 12, 2022

]]>Turbulence models for scalar and momentum transport are often analogous to each other, but roughness breaks this similarity. The turbulent Prandtl number, the momentum and scalar time-scales, and the dissipation budget are among the central quantities in the modeling of scalar transport. This study examines how roughness affects them by analyzing the results of direct numerical simulations of channel flow with resolved roughness. The flow field below the crest of the elements, which is very difficult to access experimentally, provides answers to long-standing questions.

[Phys. Rev. Fluids 7, 124601] Published Mon Dec 12, 2022

]]>Localized wall injection exists both in naturally occurring flows and in industrial applications, such as seepage of water from permeable soil layers in a river and film cooling of turbine blades. In these cases, the incoming flow is practically always subjected to disturbances, which in turn increases the turbulence level. In this experimental study, the combined effects of localized wall injection and background turbulence on the structure of turbulent channel flow are investigated for the first time.

[Phys. Rev. Fluids 7, 124602] Published Mon Dec 12, 2022

]]>The geometrical properties of the sets of points in the Poincaré maps associated to the Lagrangian orbits are quantified using topological data analysis. The number of segments in the sets of points and the occurrence of holes were determined with the 0- and 1- persistent homologies, respectively. Left: Poincaré map for Lagrangian orbits obtained with Ra = ${10}^{4}$, Right: Corresponding Vietoris-Rips radii of 1-homologies for points with several average distances to the center of the point distribution.

[Phys. Rev. Fluids 7, 123501] Published Fri Dec 09, 2022

]]>Transitional flow within an arteriovenous fistula (AVF) leads to vascular disease, which can be treated by implanting a stent. Large Eddy Simulations of blood flow within a patient-specific AVF revealed the significantly higher turbulent behavior in the Stent-absent AVF when compared to the Stented AVF. This finding provides key understanding of the reasons behind the success of this treatment strategy from a fluid dynamics perspective.

[Phys. Rev. Fluids 7, 123101] Published Thu Dec 08, 2022

]]>In this article we provide a comparative study of rate laws for internal energy excitation and dissociation of nitrogen. These thermochemical properties are obtained by studying molecular interactions on two independently developed $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ potential energy surfaces developed at the University of Minnesota and NASA Ames Research Center. Furthermore, a comparison of a canonical hypersonic flow field is provided, where the only modeling input to the flow simulation are the $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ potential energy surfaces.

[Phys. Rev. Fluids 7, 123401] Published Thu Dec 08, 2022

]]>The drag past spherical particles attached to a deformable fluid-fluid interface in the limit of small Capillary number and small deviation of contact angle from 90 degrees is determined. Analytical results are given for a single particle in flow while an analytical investigation supplemented with a straightforward numerical investigation is done for pairs of particles on an interface. Asymptotic results are given for large Bond number.

[Phys. Rev. Fluids 7, 124001] Published Thu Dec 08, 2022

]]>Focusing on the evolution of wake structure in a flow around an impulsively stopped sphere in an incompressible viscous fluid with a streamwise magnetic field, a complicated vortex structure system containing a primary vortex ring, a fragmented secondary vortex, and an accompanying vortex is discussed, summarized in the {N, Re} phase diagram. The scaling laws of drag coefficient and peak azimuthal vorticity are given and their behaviors are investigated under the influence of a magnetic field.

[Phys. Rev. Fluids 7, 123701] Published Wed Dec 07, 2022

]]>[Phys. Rev. Fluids 7, 129901] Published Wed Dec 07, 2022

]]>Experiments are performed on gas entrainment and leakage mechanisms of various ventilated cavity flow patterns around a conical axisymmetric body. As the ventilation rate increases, the size of shedding foam grows while the shedding frequency reduces, leading to balanced gas injection and leakage. The mechanism controlling cavity hysteresis is quantitatively explained using a validated gas leakage model in which the foam is periodically shed as a vortex ring. The closure discrepancy between fully developed cavities (recirculating vortex) and free-standing supercavities (continuous vortex tube) is explained by the adverse pressure gradient difference in the cavity streamwise direction.

[Phys. Rev. Fluids 7, 123901] Published Mon Dec 05, 2022

]]>Surface acoustic waves (SAWs) are used in droplet microfluidics for operations on sessile droplets, such as internal mixing, directional motion, or nebulization. An unsolved problem has been to understand how high frequency (MegaHz) SAWs can counterintuitively cause low frequency (10-500 Hz) oscillations on the drop free surface, which is known to enhance drop mobility. Using simultaneous measurements of the free-surface dynamics and internal acoustic pressure, we find that the drop oscillation instability results from coupling between an intracavity acoustic mode excited by the SAW, and an inertia-capillary surface eigenmode, through amplitude modulation and delayed radiation pressure feedback.

[Phys. Rev. Fluids 7, 124201] Published Mon Dec 05, 2022

]]>We present a simple extension of the suspension balance model (SBM) for viscous Stokes flow that allows for modeling of suspensions of particles of bidisperse size. In this work, we show a range of simulations to justify the assumptions made in the polydisperse suspension balance model. The aim is to study the effects of moderate size variation on the rheology of the suspension.

[Phys. Rev. Fluids 7, 124301] Published Fri Dec 02, 2022

]]>We present an analysis of pulsatile shear-thinning flows in two-dimensional channels. Characteristic viscosity is determined based on steady-state analysis of non-Newtonian flows and is used to non-dimensionalize the flow system by introducing the non-Newtonian Womersley number. Numerical analyses on various Carreau fluids reveal the existence of master curves related to the amplitude and phase lag of the flows, where the shape of the master curve is determined by the degree of shear-thinning. Furthermore, the pulsatile flow dynamics can be predicted accurately using pre-computed master curves, thus predicting shear-thinning pulsatile flow dynamics without explicit transient computations.

[Phys. Rev. Fluids 7, 123301] Published Thu Dec 01, 2022

]]>We conduct an experimental investigation of droplet sliding under the influence of a laminar or turbulent airflow for water and glycerin droplets. The onset of sliding is described with a critical Weber number that depends on the Reynolds number through the drag coefficient. During sliding, various shapes (oval, corner, and rivulet) are observed, and transitions are predicted using a capillary number.

[Phys. Rev. Fluids 7, 113605] Published Wed Nov 30, 2022

]]>Complex electrohydrodynamic behavior occurs during drop impact onto a solid surface under an external electric field. The process can be divided into three stages: (1) deformation of the drop in the electric field prior to contact; (2) initial contact of the drop with the substrate; and (3) the rich postcontact phenomena. Herein we provide a complete physical picture of the entire process of a drop impacting onto the solid surface under an external electric field. The various drop deposition modes are summarized in a phase diagram which sheds light on identifying appropriate electric fields for high-quality drop depositions without air bubble entrapments or jettings.

[Phys. Rev. Fluids 7, 113604] Published Tue Nov 29, 2022

]]>We study the dynamics of an oblate shape micro-particle exposed to standing bulk acoustic waves in a microchannel. A parametric study examines the effects of initial orientation, aspect ratio, size, and initial location of the particle on the translational and rotational motion. We find that the particle undergoes rotation to minimize the acoustic radiation torque potential. The direction of rotation of the particle was found to change from anticlockwise to clockwise beyond a critical aspect ratio and larger particles are found to rotate faster and closer to the nodal plane. These results can be relevant to the dynamics of an elongated microorganism or biological cells in an acoustic field.

[Phys. Rev. Fluids 7, 114204] Published Tue Nov 29, 2022

]]>Buoyant plumes are known to oscillate at a characteristic frequency, but little is known about how this frequency changes when two plumes are in close proximity. In this work, we conduct a series of high-fidelity simulations varying the spacing and width between two-dimensional helium plumes. We find that the global trends are similar to that of interacting reacting plumes, and four distinct regimes of interaction can be identified, most notably a scaling regime when the plumes are at moderate distances apart.

[Phys. Rev. Fluids 7, L111501] Published Tue Nov 29, 2022

]]>The dynamics of a gas bubble suspended in an axisymmetric straining flow is examined with modern tools of global stability analysis. The complete bifurcation diagram of a bubble constrained to remain trapped at the stagnation point is determined. It is made of a stable branch, along which the bubble oscillates, and an unstable branch along which it develops non-oscillating waisted shapes prefiguring breakup. Two unstable non-oscillating modes also allow a free bubble to drift away from the stagnation point, not only along the elongation axis but also within the compressional plane thanks to a specific self-propulsion mechanism.

[Phys. Rev. Fluids 7, 113603] Published Mon Nov 28, 2022

]]>We study numerically the effect of a soluble surfactant on the stability of two-phase flow in a finite-length microchannel. The two streams drag the surfactant molecules toward the downstream end of the interface against the action of the Marangoni stress. The sharp reduction of the interfacial tension at that end enhances the interface deformation and considerably destabilizes the system, translating into a sharp reduction of the critical capillary number even for very small surfactant volume concentrations.

[Phys. Rev. Fluids 7, 114003] Published Mon Nov 28, 2022

]]>A terahertz electric field (TEF) is employed to stimulate an active pump for water transportation by a bias applied in a nanochannel under no external pressure gradient. The excellent pumping ability is attributed to the resonance coupling between the TEF and water molecules. This proposed TEF-driven pump design will offer a guide in polar molecule transport through artificial or biological nanochannels, particularly in a controllable, noncontact, and large-scale process.

[Phys. Rev. Fluids 7, 114202] Published Mon Nov 28, 2022

]]>The hydrodynamic wall location at which the slip boundary condition is applied lies inside the fluid. Its distance from the fluid-solid interface is a strong function of the fluid state and system properties and cannot be neglected. Accounting for this distance holds the key to correct calculations of slip as, for example, when reconciling equilibrium and nonequilibrium measurements.

[Phys. Rev. Fluids 7, 114203] Published Mon Nov 28, 2022

]]>Typical isolated energy-conserving systems, including three-dimensional Euler turbulence, approach thermodynamic equilibrium, which is the most disordered state of the system. However, as shown in this paper, two-dimensional (2D) Euler turbulence remains out of equilibrium, with the asymptotic state more ordered than the intermediate state. We quantify the order of Euler turbulence using a unique measure called hydrodynamic entropy. The final state of 2D Euler turbulence exhibits a complex exchange of energy.

[Phys. Rev. Fluids 7, 114608] Published Mon Nov 28, 2022

]]>Large Eddy Simulation (LES) is used to study the wind veer effects on the wake of a yawed wind turbine. The veer deflects the wake in the spanwise direction, and this effect can be captured in the wake model by adding a veer correction term. The counter-rotating vortex pair structures previously observed behind yawed turbines can also be recovered by subtracting the background veer vorticity.

[Phys. Rev. Fluids 7, 114609] Published Mon Nov 28, 2022

]]>The motion of point vortices that preserves the initial geometry of the vortex arrangement is an important class of vortex motion associated with N-vortex systems, referred to as the self-similar motion. While the self-similar motion of three-point vortices is well understood, larger vortex systems still need to be explored. Here, we use simple concepts from linear algebra to numerically investigate the distribution of initial conditions that lead to the self-similar motion of point vortices.

[Phys. Rev. Fluids 7, 114701] Published Mon Nov 28, 2022

]]>We examine the breakup of low-viscosity ($\mu $) liquid filaments. When $\mu $ is small, the filament initially thins as if it were inviscid and its minimum radius ${h}_{m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n}$ obeys a universal scaling law. Here, we use simulations to show that for fluids of sufficiently small $\mu $, a coefficient value in the scaling law predicted from computations agrees with theory to three decimal places and inviscid power-law behavior can be observed over 2-3 decades in ${h}_{m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n}$ as ${t}_{b}-t\to 0$ where ${t}_{b}$ is the filament break up time. Transition from the inviscid regime to a viscous one is also demonstrated from simulations.

[Phys. Rev. Fluids 7, L112001] Published Wed Nov 23, 2022

]]>We investigated the four-wave resonant and quasi-resonant interactions in a special degenerated case, wherein bichromatic mother waves are generated to give birth to a daughter wave. Through theoretical and numerical analyses, we first discover the threshold value of water depth when the four-wave resonance diminishes. Our work can shed light on the limiting shallow water depth where the energy exchange due to four wave resonance creases.

[Phys. Rev. Fluids 7, 114803] Published Tue Nov 22, 2022

]]>How do microorganisms move in fluids that contain particles and/or polymers? Such fluids often display nonlinear rheological behavior such as shear-rate dependent viscosity and viscoelasticity. The effects of such rheological behavior on the swimming behavior of microorganism at low Reynolds number are only recently being elucidated. This article highlights a few of the key developments in the field of swimming in complex fluids.

[Phys. Rev. Fluids 7, 110515] Published Mon Nov 21, 2022

]]>Filaments rotating in fluids arise throughout nature and human engineering, from the thin helices that drive bacterial swimming to methods for laying deep sea cables. Previous work examined the instability that occurs when a filament of circular cross-section rotates in a viscous fluid. Here we extend that analysis to probe the dynamics beyond onset of the instability and how the behavior is affected by cross-sectional shape. Our analysis reveals a rich phase space and elucidates the past discrepancy between immersed boundary simulations and other analytic and computational approaches.

[Phys. Rev. Fluids 7, 113101] Published Mon Nov 21, 2022

]]>Corners formed between any two surfaces enhance the capillary flow along the corners resulting in the rise of thin liquid streams known as rivulets. In this work, we investigate the rivulet dynamics inside a square capillary under forced wetting. The continuously rising rivulets start decreasing in length linearly when the force is first applied and eventually reach a finite length above the central region of fluid in the capillary. We explain our observations using a simple model based on capillary rise.

[Phys. Rev. Fluids 7, 114002] Published Mon Nov 21, 2022

]]>A novel force-partitioning method is used to quantify the lift induced on finite-span wings at low Reynolds numbers by streamwise and spanwise oriented vorticity over the wing and in its wake. Although spanwise vortices, such as the leading-edge vortex, are thought to be the dominant lift producing mechanism, we show that streamwise vorticity induces a higher net lift on the wing and spanwise vorticity in the wake can produce negative lift. We quantify the lift induced by vortex cores and their associated strain-dominated flow, and show that they exhibit very different vortex tilting and stretching dynamics that can diminish the influence of spanwise and augment streamwise vorticity.

[Phys. Rev. Fluids 7, 114102] Published Mon Nov 21, 2022

]]>The present study investigates the effects of impurities on convective mixing processes in geological sequestration of CO${}_{2}$ at pore scale. We focus on the concentration expansion coefficient and diffusion rate of different impurities, the effects of which on the fingering phenomena, dissolution flux, as well as the onset time of convection ${t}_{o\phantom{\rule{0}{0ex}}n}$ are mainly analyzed. Based on the numerical results, the mathematical form of effective Rayleigh number Ra${}_{e}$ in a multicomponent system is fit, which could ensure that the scaling relations between ${t}_{o\phantom{\rule{0}{0ex}}n}$ and Ra${}_{e}$ are consistent with that in a pure system.

[Phys. Rev. Fluids 7, 114501] Published Mon Nov 21, 2022

]]>Turbulent flow over a wavy wall represents many physical processes in natural environmental flow, such as aeolian sand or wind waves. We focus on the vortex generation and interaction in the two-dimensional wavy wall turbulent mass transfer. The centrifugal instability interprets the formation of the streamwise vortices. The spanwise vortices are verified to be generated through a Kelvin–Helmholtz instability deviating from the crest. A correlation of vorticity production with the statistics of the turbulent momentum (mass) flux demonstrates the likely mechanism of the vortex interaction and its effects on these statistics.

[Phys. Rev. Fluids 7, 114607] Published Mon Nov 21, 2022

]]>The interaction between turbulent axisymmetric wakes plays an important role in many industrial applications, notably in the modeling of wind farms. This work proposes an experimental study on the interaction between two bluff bodies. We find that a simple mathematical expression for the wake interaction length based on nonequilibrium turbulence scalings can be used to collapse the streamwise developments of the second, third, and fourth moments of the streamwise fluctuating velocity.

[Phys. Rev. Fluids 7, 114606] Published Fri Nov 18, 2022

]]>Many living microorganisms experience an affinity to populate boundaries. The reasons for such affinity can be complex. Here we show that a simple synthetic microswimmer (Janus catalytic colloidal particles) tends to accumulate in the vicinity of liquid interfaces in sessile droplets. We show that the main mechanism is related to their active swimming motion, which is dominating even in the presence of evaporation-driven flows within the sessile droplet.

[Phys. Rev. Fluids 7, 110514] Published Thu Nov 17, 2022

]]>A simple model of hydrogen evolution at a horizontal cathode at the bottom of an acidic aqueous solution is proposed. The model accounts for convection due to suction of liquid through the electrode, as well as diffusion and migration of the charged species. The rate of growth of hydrogen bubbles and their spacing on the electrode are computed as functions of the mean current density and the suction velocity.

[Phys. Rev. Fluids 7, 113602] Published Thu Nov 17, 2022

]]>We derive the governing equation of flow for a three-dimensional fluid with a parity-broken viscosity tensor when confined to a Hele-Shaw cell. When such a fluid is pushed through a channel, a transverse force is exerted on the walls, and when a bubble of air expands into a region of such fluid, a circulation develops in the far field. The Saffman-Taylor stability condition is also modified, with these terms tending to stabilize the two fluid interface. Such experiments can in principle facilitate the measurement of parity odd coefficients in both synthetic and natural active matter systems.

[Phys. Rev. Fluids 7, 114201] Published Wed Nov 16, 2022

]]>Compression only is a counterintuitive response pattern of lipid coated microbubbles that are strain softening by nature and normally exhibit a bias towards expansion during volume pulsation. It is a result of shell viscoelastic behavior and arises for shells with low bending resistance and shear viscosity. During sonication such microbubbles buckle when the shell is prestressed, leading to oscillations around deformed shapes of lower volume. The latter are energetically favored and arise due to parametric mode excitation. Lower values of shear shell viscosity enhance this pattern that can be used to improve protocols for the acoustic characterization of shell mechanical properties.

[Phys. Rev. Fluids 7, 113601] Published Tue Nov 15, 2022

]]>We examine interaction mechanisms between the drag-reducing streamwise-traveling waves of spanwise wall velocity applied at the wall of a turbulent channel flow, and near-wall turbulent coherent structures. In particular, we focus our analysis on the drag-reduction-induced modifications of the quasi-streamwise vortices that populate the near-wall region of turbulent flows through a conditional averaging procedure. We observe that, while the streamwise-traveling waves share most of the drag-reducing mechanisms with the spanwise wall oscillations, some important differences related to the finite phase speed of the wave and the different properties of the generalized Stokes layer exist.

[Phys. Rev. Fluids 7, 114602] Published Mon Nov 14, 2022

]]>In wall turbulence, meandering behaviors of large-scale structures observed in the logarithmic layer is a crucial spatial feature for understanding the spatial organization of these structures and improving the structure-based turbulence model. These structures extend from the near-wall region to the edge of boundary layers. Their meandering motions leave an imprint on the two-point turbulence statistics across the flow, especially in the logarithmic region. Here, we demonstrate the influence of the meandering motions of wall-attached structures on the two-point correlation and premultiplied two-dimensional spectra by analyzing direct numerical simulation data of the turbulent boundary layer.

[Phys. Rev. Fluids 7, 114603] Published Mon Nov 14, 2022

]]>The present study investigates the scale characteristics of the log- and outer-region motions and structures in subsonic and supersonic wall turbulence. The energy distribution among the multiscale structures in the outer region is found to be dominated by the semilocal friction-Reynolds-number effects rather than the Mach-number effects. The geometrical characteristics of the self-similar structures populating the logarithmic region are also revealed by adopting a linear coherence spectrum.

[Phys. Rev. Fluids 7, 114604] Published Mon Nov 14, 2022

]]>Simulated fully localized internal gravity wave packets with moderately large initial amplitude exhibit finite transmission across a reflection level, due to the combined effects of the wave-induced shear locally cancelling the retrograde background shear, modulational instability, and the nonlinear generation and evolution of secondary wave packets.

[Phys. Rev. Fluids 7, 114802] Published Mon Nov 14, 2022

]]>Diffusiophoretic motion of particles is influenced by the pH of surrounding liquid, due to the varying zeta potential of surfaces at different pH. By using the particles with an isoelectric point (pI), we study pH-dependent diffusiophoresis under a pH gradient. Various particle behaviors in the absence and presence of wall diffusioosmosis are demonstrated using a dead-end pore geometry.

[Phys. Rev. Fluids 7, 110513] Published Thu Nov 10, 2022

]]>When water meets sunflower oil in a horizontal capillary tube, a water droplet can spontaneously form. The position of the breakup event can be manipulated by simply adjusting the oil content that prefills the capillary. For short length scales, the system advances with a constant velocity making it a good predictive tool for engineering applications.

[Phys. Rev. Fluids 7, 114001] Published Thu Nov 10, 2022

]]>Nonlinear Rayleigh-B ́enard convection in an infinite Prandtl number fluid layer between poorly conducting boundaries is considered as a model for convection in the Earth’s upper mantle. It is shown that accounting for the generally neglected impact of viscous dissipation may lead to the development of large-scale spatiotemporal chaotic dynamics governed by the familiar Kuramoto-Sivashinsky (KS) equation, ${\mathrm{\Phi}}_{\tau}+{\nabla}^{4}\mathrm{\Phi}+2{\nabla}^{2}\mathrm{\Phi}-{\left(\nabla \mathrm{\Phi}\right)}^{2}+\alpha \phantom{\rule{0}{0ex}}\mathrm{\Phi}=0,$ known to occur in various physical systems. The figure shows an irregular pattern of chaotically recombining cells developing at $\alpha =0.1$.

[Phys. Rev. Fluids 7, 113501] Published Wed Nov 09, 2022

]]>Liquid ammonia has been under investigation as a possible substitute of the current carbon-based fuels. However, its thermodynamical properties are responsible for a strong cooling effect of the surrounding environment and peculiar dispersion patterns that can affect the efficiency of a combustor. Through this computational study, it is highlighted how the rapid environmental cooling and change of carrier mixture make ammonia droplets condense, despite the ambient temperature being considerably higher than the saturation one, affecting the jet topology as well.

[Phys. Rev. Fluids 7, 114301] Published Wed Nov 09, 2022

]]>The formation of raindrops in clouds requires the collision and coalescence of water droplets. The rapidity of this process has long been a puzzle in atmospheric fluid dynamics. We review one possible explanation - inertial particle caustics around strong vortices. We apply this simple physical picture to data from direct numerical simulations of highly turbulent flow, and show that caustics around vortices can, at high Reynolds number, provide the seed for the rapid formation of larger droplets, and thus rain, in ice-free clouds.

[Phys. Rev. Fluids 7, 110512] Published Tue Nov 08, 2022

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