Latest papers in fluid mechanics

Physics of Fluids, Volume 33, Issue 5, May 2021.
The combined flow physics of several canonical flow configurations is experimentally studied. Here, we analyze an array of jets issuing into a crossflow, then immediately navigating past an array of cylinders. This is achieved with a 2 × 3 triangular pattern of jets and symmetric cylinders at three jets to crossflow velocity ratios, enabling near-complete optical access of each jet, with velocities measured by time-resolved particle image velocimetry. Jet trajectories reveal that each configuration adheres to a power-law trend and that greater penetration is achieved by the downstream and confined jets compared to the more conventional upstream one. Recirculation regions of the upstream and downstream jets are nearly independent, with the confined jet encompassing regions of overlap with both. Turbulent statistics reveal the influence of geometric placement and velocity ratio on the time-averaged velocity, anisotropy, and Reynolds stresses incurred by each jet. Galilean decomposition utilizes a supplemental crossflow-only velocity field to delineate the influence of each jet's low- and high-pressure regions on the otherwise uniform stream. Proper orthogonal decomposition suggests that increased jet penetration decreases the number of modes required for truncation in the investigated spanwise plane. Vortex identification algorithms are applied to the reconstructed flow fields, reaffirming that with increasing velocity ratio, the jets generate vortices of their own in similar statistical formations as the cylinders. This investigation provides a foundation to aid future modeling efforts toward characterizing flow physics of importance in designing and passively controlling transverse jets with varying blockage proximities in a crossflow.

Physics of Fluids, Volume 33, Issue 5, May 2021.
In the present study, simulations are directed to capture the dynamics of evacuating inner gas of a bubble bursting at the free surface, using Eulerian based volume of fluid (VOF) method. The rate by which surrounding air rushing inside the bubble cavity through the inner gas evacuation is estimated and compared by the collapsing bubble cavity during the sequential stages of the bubble bursting at the free surface. Further, the reachability of inner gas at different horizontal planes over the unperturbed free surface is estimated. The evacuating inner gas accompanies vortex rings, which entrains the surrounding gas-phase. During the successive stages of air entrainment, spatiotemporal characteristics of the vortex ring are obtained. At low Bond numbers (Bo < 1), the axial growth pattern of gas jet and the radial expansion of jet tip are studied with the phase contour of evacuating inner gas. Furthermore, the axial growth of rising inner gas over the free surface and the radial expansion of vortex rings of a bubble bursting at the free surface is compared with the quiescent surrounding air under the respiration process. At last, the effects of various possible asymmetric perforation of the bubble cap keeping the same Bo are studied. The cause of the bent gas jet, as a consequence of the perforation of the bubble cap, asymmetrically, is explained by plotting the velocity vectors. The effect of miscibility on the spreading dynamics of inner gas has been found to be minimal at the early stage of the bursting process.

Physics of Fluids, Volume 33, Issue 5, May 2021.
The concept of added (virtual) mass is applied to a vast array of unsteady fluid-flow problems; however, its origins in potential-flow theory may limit its usefulness in separated flows. A robust framework for modeling instantaneous fluid forces is proposed, named Energized Mass. The energized-mass approach is tested experimentally by acquiring the fluid kinetic-energy history around an accelerating sphere at both subcritical and supercritical terminal velocities. By tracking the energized-mass volume, the force response is shown to be related to changes in shear-layer growth as a function of acceleration moduli and Reynolds number. The energized-mass framework is then used to develop a low-order force model, requiring only body geometry and kinematics as input. An analytical expression for the instantaneous force on a sphere due to energized-mass growth is derived based on shear-layer mass flux arguments. Instantaneous forces determined experimentally, and modeled using the energized-mass approach, show strong agreement with direct force measurements. The results of this investigation thus demonstrate that the energized-mass framework provides a viable low-order modeling approach, and in tandem, can provide new insights into the origin of forces on accelerating bodies.

Physics of Fluids, Volume 33, Issue 5, May 2021.
First, a new reconstruction strategy is proposed to improve the accuracy of the fifth-order weighted essentially non-oscillatory (WENO) scheme. It has been noted that conventional WENO schemes still suffer from excessive numerical dissipation near-critical regions. One of the reasons is that they tend to under-use all adjacent smooth substencils thus fail to realize optimal interpolation. Hence in this work, a modified WENO (MWENO) strategy is designed to restore the highest possible order interpolation when three target substencils or two target adjacent substencils are smooth. Since the new detector is formulated under the original smoothness indicators, no obvious complexity and cost are added to the simulation. This idea has been successfully implemented into two classical fifth-order WENO schemes, which improve the accuracy near the critical region but without destroying essentially non-oscillatory properties. Second, the tangent of hyperbola for interface capturing (THINC) scheme is introduced as another reconstruction candidate to better represent the discontinuity. Finally, the MWENO and THINC schemes are implemented with the boundary variation diminishing algorithm to further minimize the numerical dissipation across discontinuities. Numerical verifications show that the proposed scheme accurately captures both smooth and discontinuous flow structures simultaneously with high-resolution quality. Meanwhile, the presented scheme effectively reduces numerical dissipation error and suppresses spurious numerical oscillation in the presence of strong shock or discontinuity for compressible flows and compressible two-phase flows.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Self-similar solutions to converging (implosions) and diverging (explosions) shocks have been studied before, in planar, cylindrical, or spherical symmetry. Here, we offer a unified treatment of these apparently disconnected problems. We study the flow of an ideal gas with adiabatic index γ with initial density [math], containing a strong shock wave. We characterize the self-similar solutions in the entirety of the parameter space [math] and draw the connections between the different geometries. We find that only type II self-similar solutions are valid in converging shocks, and that in some cases, a converging shock might not create a reflected shock after its convergence. Finally, we derive analytical approximations for the similarity exponent in the entirety of parameter space.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Despite growing interest in the focusing and manipulation of particles in non-Newtonian fluids in confined flows, the combined effect of viscoelastic and shear-thinning effects on particle dynamics is not well understood. Herein, we report the dynamics of rigid microparticles in confined flows of strongly shear-thinning viscoelastic (STVE) fluids at very low Reynolds numbers. Our experiments with different STVE fluids reveal five different regimes: original streamline, bimodal, center migration, defocusing, and wall migration (WM), depending upon the fluid properties and flow rates. It is found that the occurrence of the different regimes depends on the STVE parameter [math] and average strain rate ([math]). We find that the dynamics of particles in the different regimes is underpinned by the synergy between viscoelastic lift force ([math]) and shear-thinning lift force ([math]). Numerical simulation results of strain rate and viscosity profiles at different [math] and [math] enable estimation of the forces and explaining the dynamics observed. We expect that our study will find relevance in applications involving positioning and manipulation of particles in confined flows of STVE fluids.

Physics of Fluids, Volume 33, Issue 5, May 2021.
This paper discusses the flow-induced vibration of a freely vibrating trapezoidal cylinder with a mass ratio of 10 at low Reynolds numbers (Re = 60–250). Over this range of the Reynolds number, we discuss the inflow and transverse amplitudes, frequency ratios, hydrodynamic forces, phase differences, and vortex modes. Comparing to square/circular cylinders with the same flow conditions, responses of the trapezoidal cylinder are much different. In both the vortex-induced vibration (VIV) and galloping regimes, double rise-up of the amplitudes and hydrodynamics forces is observed with respect to Re, as well as the two lock-ins for the frequency ratios. The phase differences and vortex modes in the wake are also found to be different from the square/circular cylinders. Thus, seven flow branches are identified, i.e., the initial branch, upper branch, lower branch, desynchronization region, initial galloping, upper galloping, and high galloping. Then, in order to interpret these branches, the evolutions of vortex formation and shedding in the wake are analyzed. It appears that the asymmetry of the trapezoidal cylinder to the inflow is the direct cause. In addition, a small degree of hysteresis is observed in the VIV regime and a larger degree is observed in the galloping regime.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Fluid–acoustic interactions are important in a variety of applications and typically result in adverse effects. We analyze the influence of Mach number on such interactions and their input–output characteristics by combining resolvent analysis with Doak's momentum potential theory. The specific problem selected is the flow over an open cavity of L∕D = 6 at Re = 10 000 and M∞ = 0.6 and 1.4, respectively. The resolvent forcing and response modes are decomposed into their hydrodynamic, acoustic, and thermal components. Although the results depend quantitatively on Mach number, some trends remain consistent. In particular, at lower frequencies, the acoustic component appears primarily at the trailing edge of the cavity. When the frequency is increased, the acoustic response moves toward the leading edge and overlaps with its hydrodynamic component. Inspired by actual cavity flow control, the forcing is then localized to two regions—the leading edge and front wall of the cavity—and filtered to consider notional actuators that can separately introduce each component of velocity, density, and temperature forcing, respectively. Among these different types of actuation perturbations, regardless of Mach number, streamwise velocity forcing achieves the largest energy amplification at the leading edge. For both flows, beyond a certain forcing frequency threshold value, the nature of the acoustic vs hydrodynamic response becomes independent of the forcing type; however, the amplification continues to be strongly impacted by the forcing frequency. The present work provides an alternative approach to examine input–output flow–acoustic characteristics and evaluate the relative effectiveness of different types and locations of actuation.

Physics of Fluids, Volume 33, Issue 5, May 2021.
The classical Plateau problem of finding minimal surfaces supported by two noncircular coaxial rings is studied theoretically and experimentally. Using a fluid dynamics analogy, we generalize the classical catenoid solution for a film on circular rings to the general cases of noncircular rings. Some examples of analytical solutions for elliptical, polygonal, and ovoidal rings are presented. The shapes of a tubular film and a film separated by a lamella at the wrist are obtained in an analytical form. The stability of these films is analyzed and compared with the classical catenoid. The data on critical parameters of all minimal surfaces are collected in the tables that can be used in practical applications. The theory is experimentally validated using soap films on elliptical identical frames. Moreover, the shapes of soap films on two different elliptical frames demonstrate a new feature: a flat separating lamella lying parallel to the rings was never observed in experiments. All lamellae appeared deformed suggesting the existence of a new family of minimal surfaces which does not exist in the case of frames of the same sizes.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Weakly nonlinear propagation of pressure waves in initially quiescent compressible liquids uniformly containing many spherical microbubbles is theoretically studied based on the derivation of the Korteweg–de Vries–Burgers (KdVB) equation. In particular, the energy equation at the bubble–liquid interface [Prosperetti, J. Fluid Mech. 222, 587 (1991)] and the effective polytropic exponent are introduced into our model [Kanagawa et al., J. Fluid Sci. Technol. 6, 838 (2011)] to clarify the influence of thermal effect inside the bubbles on wave dissipation. Thermal conduction is investigated in detail using some temperature-gradient models. The main results are summarized as follows: (i) Two types of dissipation terms appeared; one was a well-known second-order derivative comprising the effect of viscosity and liquid compressibility (acoustic radiation) and the other was a newly discovered term without differentiation comprising the effect of thermal conduction. (ii) The coefficients of the KdVB equation depended more on the initial bubble radius rather than on the initial void fraction. (iii) The thermal effect contributed to not only the dissipation effect but also to the nonlinear effect, and nonlinearity increased compared with that observed by Kanagawa et al. (2011). (iv) There were no significant differences among the four temperature-gradient models for milliscale bubbles. However, thermal dissipation increased in the four models for microscale bubbles. (v) The thermal dissipation effect observed in this study was comparable with that in a KdVB equation derived by Prosperetti (1991), although the forms of dissipation terms describing the effect of thermal conduction differed. (vi) The thermal dissipation effect was significantly larger than the dissipation effect due to viscosity and compressibility.

Physics of Fluids, Volume 33, Issue 5, May 2021.
The gravitational debris flow, such as the agent forming alluvial cones in the mouths of mountain canyons, could bring about devastating disaster to downstream structures in mountainous areas. In the present study, a series of model tests were conducted on the sand and the ceramsite to systematically explore the interaction between debris flow and baffles. During the runout process, the impact force exerted by debris flow was measured by dynamometers. The runout distance, velocity of the flow, and flow depth were monitored by a video camera and a high-speed camera in a real time. The dynamic interaction under different particle sizes of dry granular materials, slop angles, and baffle configurations was simulated. Experimental results show that the smaller size material is favorable for the frictional energy dissipation during the sliding process, giving rise to the smaller runout distance. The present findings provide important references for the debris flow control engineering.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Micro/nanofluidic devices integrated with ion concentration polarization (ICP) phenomenon have been used to preconcentrate low-abundant molecules for separation and detection purposes. This work reviews ICP-based devices focused on electrokinetic fundamentals of ICP in microfluidics and related design factors. We discuss various designs of ICP devices and then provide insight on the role of design factors in ICP function. In addition, fabrication methods and relevant materials for making ICP devices and nanojunctions are explained. This work provides the most up-to-date applications of ICP with emphasis on active and passive methods in controlling and stabilizing streams of preconcentrated molecules to enhance the separation and detection efficiency in diagnostics, desalination, and electrodialysis.

Physics of Fluids, Volume 33, Issue 5, May 2021.
We investigate pressure driven pipe flow of Laponite suspension, as a model thixotropic fluid. The tendency of the suspension to age is controlled by addition of sodium chloride salt to vary the ionic strength. We use a syringe pump to prescribe the flow and observe that a steady state flow is obtained. Unusually, the steady state pressure drop required to maintain a constant flow rate decreases with an increase in the flow rate, in qualitative contrast to the expectation for Poiseuille flow. We demonstrate that experimental results obtained by varying the flow rate, salt concentration, and flow geometry (pipe diameter and length) can be collapsed onto a single universal curve that can be rationalized by invoking slip of the suspension at the tube walls. The Laponite suspension exhibits plug-like flow, yielding at the tube walls. Our results suggest that the slip length varies linearly with the flow rate and inversely with the tube diameter.

Physics of Fluids, Volume 33, Issue 5, May 2021.
We demonstrate a computational study used to evaluate drop-on-demand printability of liquid metals via a contactless magnetohydrodynamic (MHD) pumping method. We show that the ejection regimes of pure liquid metal droplets can be categorized using two dimensionless quantities: We and a new dimensionless quantity [math]. By plotting We vs S, a linear relationship emerges which relates the velocity through the ejection orifice to the applied magnetic flux density. Additionally, satellite-free droplet generation is shown to be bounded by the ranges [math] and [math]. These ranges, coupled with the linear We vs S relationship, allow one to predict the critical magnetic flux necessary to eject a satellite-free liquid metal droplet for any liquid metal with a very low viscosity to surface tension ratio ([math]). We discuss the physics underlying the MHD ejection process and relate the pump action to the dimensionless quantities. We use an MHD finite element model to parametrically sweep through applied magnetic fields and explore two-phase ejection of Al, Cu, Fe, Li, Sn, Ti, Zn, and Zr droplets from a 200 μm orifice. The model is validated using experimental high speed video ejection of Zn and Al, and the reported relationship between We and S can be used to connect the input flux density to the resulting ejection regime.

Physics of Fluids, Volume 33, Issue 5, May 2021.
This paper presents a novel method to accelerate flame propagation and transition to detonation in a coiled channel. The objective is to bring to light the basic understanding of the phenomenon and to show its potential in the fields of highly efficient combustion or propulsion. It was found that the flame evolution in the coiled channel is significantly different from that in a straight channel. In the flame acceleration stage, the flame propagation velocity increases exponentially in the coiled channel while it increases linearly in the straight channel, primarily due to the existence of a strong velocity gradient in the transverse direction in the coiled channel. Deflagration to detonation transition (DDT) was only observed in the coiled channel under current settings, being triggered by a series of local explosions at the boundary layer. In general, the coiled channel can greatly accelerate the flame and shorten the distance of the DDT compared with the straight channel.

Author(s): H. Hassanzadeh, A. Eslami, and S. M. Taghavi

Using a high-speed camera, laser imaging and ultrasound velocimetry, we study positively buoyant miscible jets. Based on the appearance of the laminar length, we classify the flow into fully turbulent and semi-turbulent regimes. We quantify the regime transition boundaries and propose empirical correlations to predict the laminar length. To have a global view of the flow, we also analyze the quasi-steady jet characteristics (jet radius, spread angle, virtual origin, velocity profiles and energy dissipation) and starting jet characteristics (penetration length and tip velocity).

[Phys. Rev. Fluids 6, 054501] Published Tue May 04, 2021

Physics of Fluids, Volume 33, Issue 5, May 2021.
Surface engineering is an emerging technology to design antiviral surfaces, especially in the wake of COVID-19 pandemic. However, there is yet no general understanding of the rules and optimized conditions governing the virucidal properties of engineered surfaces. The understanding is crucial for designing antiviral surfaces. Previous studies reported that the drying time of a residual thin-film after the evaporation of a bulk respiratory droplet on a smooth surface correlates with the coronavirus survival time. Recently, we [Chatterjee et al., Phys. Fluids. 33, 021701 (2021)] showed that the evaporation is much faster on porous than impermeable surfaces, making the porous surfaces lesser susceptible to virus survival. The faster evaporation on porous surfaces was attributed to an enhanced disjoining pressure within the thin-film due the presence of horizontally oriented fibers and void spaces. Motivated by this, we explore herein the disjoining pressure-driven thin-film evaporation mechanism and thereby the virucidal properties of engineered surfaces with varied wettability and texture. A generic model is developed which agrees qualitatively well with the previous virus titer measurements on nanostructured surfaces. Thereafter, we design model surfaces and report the optimized conditions for roughness and wettability to achieve the most prominent virucidal effect. We have deciphered that the optimized thin-film lifetime can be gained by tailoring wettability and roughness, irrespective of the nature of texture geometry. The present study expands the applicability of the process and demonstrates ways to design antiviral surfaces, thereby aiding to mitigate the spread of COVID-19.

Physics of Fluids, Volume 33, Issue 5, May 2021.
The influence of natural convection cells on heat transfer and the evolution of melt interface is studied for low Prandtl number fluid (Pr = 0.025) in phase-change Rayleigh–Benard convection using the lattice Boltzmann method. The thermal lattice Boltzmann model is used to evaluate the effect of Rayleigh number (Ra = 6708, 11 708, and 21 708) and cavity aspect ratio (γ = 0.062 5, 0.125, 0.25, 0.5, and 1) on the onset of convection, number of convection cells, and Nusselt number in the classical Rayleigh–Benard convection. The modified equilibrium distribution function-based thermal lattice Boltzmann model is applied to evaluate the effect of Stefan number (Ste = 0.025, 0.05, and 0.1) in the phase change Rayleigh–Benard convection. Distinct flow configurations depend on the Rayleigh number, aspect ratio, and Stefan number. The number of convection cells follows an inverse relation with the aspect ratio. Nusselt number increases with decreasing cavity aspect ratio and increasing Rayleigh number in the classical Rayleigh–Benard convection. With the variation in the aspect ratio based on the melt layer height during melting of phase change material, the number of convection cells changes resulting in the change in the evolution of the melt interface and convective heat transfer. Melting in a cavity of aspect ratio less than 0.5, the evolution of melt interface remains symmetrical. For an aspect ratio greater than 0.5, the interface evolution becomes unsymmetrical depending on the transition to single convection cell-dominated heat transfer.

Physics of Fluids, Volume 33, Issue 5, May 2021.
Understanding the mechanisms that control the dynamics of bubble clouds is essential to many industrial processes in the energy and chemical realms. Due to the complexity of the two-phase flow configurations, modeling the physics of the phenomena driving the mixing of bubbles within a liquid matrix is still a major challenge. One of the weaknesses of most existing two-phase flow models is due to the incomplete handling of bubble dispersion. This difficulty comes from the fact that dispersion can be driven by numerous complex phenomena, such as turbulence, local pressure distribution, bubble to bubble interaction, etc. In this study, we introduce the effect of added mass fluctuations on the dispersion of small bubbles. Existing models based on the Euler–Euler approach do not take into account local flow variations due to bubble distributions. Therefore, these models do not correctly describe fine dispersion features. Solving the potential flow around N bubbles allows to take into account the effect of the added mass on bubble cloud distributions. To this aim, a complete added mass model, which includes local bubble configurations via the void fraction gradient, is developed. The void fraction gradient allows us to account for the asymmetry of the bubble cloud around a single central bubble. Consequently, the proposed model can only represent regular and irregular bubble cloud distributions. This methodology results in a more consistent consideration of the added mass effects as well as Meshchersky's force, which should be included in hydrodynamic two-phase flow models. The proposed approach can be implemented in Euler–Euler models intended to consider the dispersion of bubbles caused by the effect of added mass.

Physics of Fluids, Volume 33, Issue 5, May 2021.
The Mpemba effect occurs when two samples at different initial temperatures evolve in such a way that the temperatures cross each other during the relaxation toward equilibrium. In this paper, we show the emergence of a Mpemba-like effect in a molecular binary mixture in contact with a thermal reservoir (bath). The interaction between the gaseous particles of the mixture and the thermal reservoir is modeled via a viscous drag force plus a stochastic Langevin-like term. The presence of the external bath couples the time evolution of the total and partial temperatures of each component allowing the appearance of the Mpemba phenomenon, even when the initial temperature differences are of the same order of the temperatures themselves. Analytical results are obtained by considering multitemperature Maxwellian approximations for the velocity distribution functions of each component. The theoretical analysis is carried out for initial states close to and far away (large Mpemba-like effect) from equilibrium. The former situation allows us to develop a simple theory where the time evolution equation for the temperature is linearized around its asymptotic equilibrium solution. This linear theory provides an expression for the crossover time. We also provide a qualitative description of the large Mpemba effect. Our theoretical results agree very well with computer simulations obtained by numerically solving the Enskog kinetic equation by means of the direct simulation Monte Carlo method and by performing molecular dynamics simulations. Finally, preliminary results for driven granular mixtures also show the occurrence of a Mpemba-like effect for inelastic collisions.