Latest papers in fluid mechanics

Author(s): Michael S. Dodd and Lluís Jofre

The small-scale flow topologies in droplet-resolved direct numerical simulations of isotropic turbulence are classified using tensor invariants. The results show that when approaching the droplet surface, the flow topologies fundamentally change from those found in isotropic turbulence to boundary-layer-like structures.

[Phys. Rev. Fluids 4, 064303] Published Thu Jun 13, 2019

Author(s): Benoît-Joseph Gréa and Antoine Briard

The horizontal vibrations of an interface between miscible fluids trigger a parametric instability, leading to frozen wave patterns. Theory and numerical simulations reveal that the mixing layer size varies as the square of the forcing amplitude in the turbulent regime.

[Phys. Rev. Fluids 4, 064608] Published Thu Jun 13, 2019

Author(s): Gustavo Düring, Christophe Josserand, Giorgio Krstulovic, and Sergio Rica

In the low viscosity liquid and zero thickness vibrating elastic sheet, turbulent behavior is observed. We find the same Kolmogorov spectrum cascade for energy redistribution per scale Ek∼P2/3k−5/3 in both, apparently, completely disparate physical situations.

[Phys. Rev. Fluids 4, 064804] Published Thu Jun 13, 2019

Physics of Fluids, Volume 31, Issue 6, June 2019.
The phenomenon of a droplet impacting on an elastic solid surface exists in wide and versatile natural and industrial areas, which is involved with the interplay between elasticity and droplet dynamics. In the present work, we have made a comprehensive study on the process of a droplet impacting on a cantilever resulting in large deformation. The morphology of the droplet is observed, and the maximum deflection of the cantilever with respect to the initial velocity, apparent contact angle, and surface tension of the droplet is calculated by the developed theoretical model, which matches the experimental results very well. These findings may aid to engineer new energy harvesting devices and microsensors, and are also promising for many agricultural and industrial applications.

Author(s): Peipei Zhao, Lipo Wang, and Nilanjan Chakraborty

A turbulent premixed flame anchored by a solid wall can reach a statistically stationary state. Overall the configuration is counterflowlike. Flame-wall interactions under such conditions are key to understanding combustion in a confined space and in developing wall flame models. A numerical study is presented.

[Phys. Rev. Fluids 4, 063203] Published Wed Jun 12, 2019

Author(s): Peter D. Yeh, Ersan Demirer, and Alexander Alexeev

Complex actuation of a caudal fin can generate turning moments enabling underwater swimmer navigation. A numerical study explores the hydrodynamics resulting from two actuation strategies leading to pitching and yaw moments.

[Phys. Rev. Fluids 4, 064101] Published Wed Jun 12, 2019

Author(s): Sofia Kuperman, Lilach Sabban, and René van Hout

Holographic cinematography is used to measure rotational and translational dynamics of inertial fibers in isotropic turbulence. Results show that for the present parameter range, translational motion is similar to that for spheres. In contrast, tumbling rates peaked at an intermediate Stokes number.

[Phys. Rev. Fluids 4, 064301] Published Wed Jun 12, 2019

Author(s): William Chèvremont, Bruno Chareyre, and Hugues Bodiguel

A numerical study of sheared suspensions of frictional and frictionless rigid spheres finds that macroscopic friction in granular suspensions is nearly independent of interparticle contact friction, and besides contact lubrication, the viscous contribution of the pore fluid is negligibly small.

[Phys. Rev. Fluids 4, 064302] Published Wed Jun 12, 2019

Author(s): Surupa Shaw and John P. McHugh

A pair of distributed vortices in stratified flow disintegrate sooner than a traditional vortex pair and form multiple coherent vortex structures.

[Phys. Rev. Fluids 4, 064803] Published Wed Jun 12, 2019

Physics of Fluids, Volume 31, Issue 6, June 2019.
In this paper, the basic ideas underlying the Direct Simulation Monte Carlo (DSMC) method are examined and a novel nonhomogeneous N-particle kinetic equation describing the randomized mathematical model of DSMC is derived. It is shown that different collision-partner selection schemes, including No-Time-Counter (NTC) and Bernoulli-trials schemes, are approximations of the general transition operator of the randomized model. The popular collision-partner selection schemes, represented by the standard NTC and Bernoulli-trials approximations of the general transition operator, represented by Simplified Bernoulli-trials and Generalized Bernoulli-trials schemes, are tested on the one-dimensional rarefied gas heat transfer problem against conditions of two approximation limits: first, leading to the Boltzmann equation and, second, leading to the novel N-particle kinetic one.

Author(s): Abdulafeez Adebiyi, Olatunde Abidakun, Gbolahan Idowu, Damir Valiev, and V’yacheslav Akkerman

The impact of the Lewis number on ultrafast premixed flame acceleration in channels equipped with comb-shaped arrays of tightly spaced obstacles attached to the walls is studied by the computational simulations of the reacting flow equations, including fully compressible hydrodynamics.

[Phys. Rev. Fluids 4, 063201] Published Tue Jun 11, 2019

Author(s): Z. M. Nikolaou, Y. Minamoto, and L. Vervisch

The application of an iterative deconvolution modeling framework for the unresolved stresses in turbulent and reacting flows is demonstrated using high-fidelity direct numerical simulation data of premixed rod-stabilized V-flames.

[Phys. Rev. Fluids 4, 063202] Published Tue Jun 11, 2019

Author(s): Kyohei Yamamoto, Kazumichi Kobayashi, Masao Watanabe, Hiroyuki Fujii, Misaki Kon, and Hiroyuki Takahira

A numerical study of the collapse of a vapor bubble finds that a small amount of noncondensable molecules affects the temperature profile inside the collapsing vapor bubble, preventing shock wave generation inside the collapsing bubble.

[Phys. Rev. Fluids 4, 063603] Published Tue Jun 11, 2019

Author(s): J. W. Kurelek, S. Yarusevych, and M. Kotsonis

Experiments find that vortex merging occurs naturally in a laminar separation bubble. Acoustic forcing at the subharmonic and fundamental vortex shedding frequency promotes and inhibits merging, respectively. In all cases, merging occurs in the aft portion of the bubble in a spanwise nonuniform manner.

[Phys. Rev. Fluids 4, 063903] Published Tue Jun 11, 2019

Author(s): E. Boujo and M. Sellier

Imagine a thin liquid film that must cover the substrate it spreads over before solidifying: tilting the substrate is useful to get a little push from gravity, but obtaining a uniform film thickness is challenging. Here, an adjoint method finds time-dependent motions that improve uniformity.

[Phys. Rev. Fluids 4, 064802] Published Tue Jun 11, 2019

Physics of Fluids, Volume 31, Issue 6, June 2019.
We study incipient motion of single beads on regular substrates made of spherical particles of a different size in steady shear flow at small particle Reynolds numbers. We cover a large range of sizes: from small beads that are highly shielded from the shear flow by the substrate spheres, and hence, are susceptible to the flow through the interstices of the substrate, to beads fully exposed to the flow, where the substrate rather acts like roughness of an otherwise flat surface. Numerical and experimental studies agree within measurement uncertainty. To describe the findings, we extend a recently derived model for particles of equal size which was validated over a wide range of substrates [Agudo et al., “Shear-induced incipient motion of a single sphere on uniform substrates at low particle Reynolds numbers,” J. Fluid Mech. 825, 284–314 (2017)]. The extended model covers the entire spectrum of size ratios, where the critical Shields number varies from about zero to infinity. The model properly describes the numerical and experimental data. For well exposed beads, we find a scaling law between the critical Shields number and the size ratio between mobile bead and substrate spheres with an exponent of −1.

Physics of Fluids, Volume 31, Issue 6, June 2019.
Extensive combined experimental and theoretical investigations of the linear evolution of unsteady (in general) Cross-Flow (CF) and three-dimensional (3D) Tollmien-Schlichting (TS) instability modes of 3D boundary layers developing on a swept airfoil section have been carried out. CF-instability characteristics are investigated in detail at an angle of attack of −5° when this kind of instability dominates in the laminar-turbulent transition process, while the 3D TS-instability characteristics are studied at an angle of attack of +1.5° when this kind of instability is predominant in the transition process. All experimental results are deeply processed and compared with results of calculations based on several theoretical approaches. For the first time, very good quantitative agreement of all measured and calculated stability characteristics of swept-wing boundary layers is achieved both for unsteady CF- and 3D TS-instability modes for the case of a boundary layer developing on a real swept airfoil. The first part of the present study (this paper) is devoted to the description of the case of CF-dominated transition, while the TS-dominated case will be described in detail in a subsequent second part of this investigation.

Physics of Fluids, Volume 31, Issue 6, June 2019.
The control effect of micro-vortex generators (VGs) on the instability of attached cavitation was investigated in a series of experiments. The micro-VGs, located at the leading edge of a NACA0015 hydrofoil, were used to alter the near-wall flow and control the attached cavitation dynamics. The effect of the nondimensional height of micro-VGs on the nondimensional cavity length was quantitatively evaluated by regression equations through response surface methodology. The micro-VGs increased the nondimensional cavity length. The counter-rotating streamwise vortices induced by micro-VGs had a rectifying effect on the near-wall flow and withstood the flow disturbance in the spanwise direction. Additionally, the micro-VGs partially suppressed Rayleigh–Taylor instability and Kelvin–Helmholtz instability arising from reverse flow underneath the cavity. Under a partial cavity oscillation (PCO) condition, the growth of sheet cavitation was highly two-dimensional in the spanwise direction, and the cloud cavity shedding had a strict periodicity with a smaller Strouhal number (St) than for the smooth hydrofoil. The shedding cloud cavity was captured in a single spanwise vortex core, which was advected toward the trailing edge of the hydrofoil. The transition from PCO to transitional cavity oscillation (TCO) occurred when the cavity length was larger than 0.8 of chord length. Under the TCO condition, the concave cavity closure line of sheet cavitation on the hydrofoil showed perfect symmetry and the St was nearly constant. As a result of our investigation, the micro-VGs have high potential to manipulate and control the attached cavitation dynamics.

Physics of Fluids, Volume 31, Issue 6, June 2019.
The use of magnetism for various microfluidic functions such as separation, mixing, and pumping has been attracting great interest from the research community as this concept is simple, effective, and of low cost. Magnetic control avoids common problems of active microfluidic manipulation such as heat, surface charge, and high ionic concentration. The majority of past works on micromagnetofluidic devices were experimental, and a comprehensive numerical model to simulate the fundamental transport phenomena in these devices is still lacking. The present study aims to develop a numerical model to simulate transport phenomena in microfluidic devices with ferrofluid and fluorescent dye induced by a nonuniform magnetic field. The numerical results were validated by experimental data from our previous work, indicating a significant increase in mass transfer. The model shows a reasonable agreement with experimental data for the concentration distribution of both magnetic and nonmagnetic species. Magnetoconvective secondary flow enhances the transport of nonmagnetic fluorescent dye. A subsequent parametric analysis investigated the effect of the magnetic field strength and nanoparticle size on the mass transfer process. Mass transport of the fluorescent dye is enhanced with increasing field strength and size of magnetic particles.

Physics of Fluids, Volume 31, Issue 6, June 2019.
Based on the two-dimensional theory of a Newtonian incompressible fluid, an improved model is proposed by combining Reynolds stresses of new disturbance factors and velocity polynomials. It is used to solve the Reynolds averaged Navier-Stokes equation for flow through a severely narrow pipe at the continuous change of the Reynolds number from laminar flow to turbulence. Both axial and radial velocity polynomials are considered in the momentum integral method. Under boundary and symmetry conditions, a first-order differential equation for a coefficient of the axial velocity with the disturbance factors is derived. Using a numerical shooting method to solve the equation, the axial distributions of pressure are obtained in the range of Reynolds numbers from 20 to 105 when the degree of stenosis equals 0.4 or 0.9. Also, under a lower Reynolds number, the velocity profiles in axial and radial directions, the streamlines at downstream and the wall shear stresses (WSS) in narrow regions are illustrated. The disturbance factors introduced can sensitively regulate the variation of inertia, pressure gradient, and viscosity term in the Reynolds averaged Navier-Stokes equation. With an increase in the Reynolds number and the parameters from 0.02 to 20 in the disturbance factors, the axial and radial velocities reverse at some narrow regions gradually, the WSS falls to below zero downstream, and the pressure drop increases in the narrow section of the pipe. It is implied that the pressure drop plays an important role in artery collapse when it is less than 40% stenosis. When the percentage of stenosis is increased to more than 40% and the Reynolds number is only 200, WSS gradually exceeds the tolerance of endothelial cells in blood vessels. The increase in pressure drop at downstream and WSS at upstream leads to the aggravation of vascular stenosis and exfoliation of the atherosclerotic plaque.