Physical Review Fluids

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Nature of branching in electrohydrodynamic instability

Mon, 05/10/2021 - 11:00

Author(s): B. Dinesh and R. Narayanan

Using weak nonlinear analysis, it is shown that an electric field imposed on a bilayer of fluids in the presence of gravity can only lead to subcritical instability of the interface. We see that no matter the horizontal dimension of the fluid layers or the Bond number, subcritical branching necessarily results. A simple expression for the case of deep layers reveals that the subcritical nature of the instability is contributed dominantly by electrostatic forcing, which is of O(k), and diminished curvature which is of O(k3), both being enough to offset the supercritical nature offered by gravity, which is of O(k2).

[Phys. Rev. Fluids 6, 054001] Published Mon May 10, 2021

Finite-rate chemistry effects in turbulent hypersonic boundary layers: A direct numerical simulation study

Mon, 05/10/2021 - 11:00

Author(s): D. Passiatore, L. Sciacovelli, P. Cinnella, and G. Pascazio

The accurate prediction of hypersonic flows is a key challenge for the design of reentry bodies or hypersonic aircraft. Knowledge concerning the wall-bounded turbulence is especially needed. The present work is a fundamental study of a turbulent spatially developing flat-plate boundary layer subjected to high-enthalpy effects. The influence of air chemical reactions, typical of high-speed regimes, is investigated by means of direct numerical simulations. First- and second-order statistics are accurately examined, as well as spectral content and classical correlations.

[Phys. Rev. Fluids 6, 054604] Published Mon May 10, 2021

Theoretical model for the separated flow around an accelerating flat plate using time-dependent self-similarity

Mon, 05/10/2021 - 11:00

Author(s): A. C. DeVoria and K. Mohseni

The prediction of unsteady forces on an accelerating plate is improved by including the sweeping effect of the free-stream flow in the equations that approximate the inviscid evolution of the separated flow structures. This effect is represented by a similarity variable that combines the temporal variation of the length scale and angle of attack, and which introduces an implicit time scale into the vortex dynamics and force histories.

[Phys. Rev. Fluids 6, 054701] Published Mon May 10, 2021

Grid resolution requirement for resolving rare and high intensity wall-shear stress events in direct numerical simulations

Fri, 05/07/2021 - 11:00

Author(s): Xiang I. A. Yang, Jiarong Hong, Myoungkyu Lee, and Xinyi L. D. Huang

Wall-shear stress becomes more intermittent as the Reynolds number (Re) of a flow increases. To properly resolve wall shear stress events in direct numerical simulations thus requires finer grids at higher Re. In this work we examine the grid resolution required to resolve a given percentage of wall shear stress events as a function of Re. We find that the standard grid resolution does not capture a fraction of high intensity events which increases with Re and quantify the grid resolution needed to do so.

[Phys. Rev. Fluids 6, 054603] Published Fri May 07, 2021

Relation between the spectral properties of wall turbulence and the scaling of the Darcy-Weisbach friction factor

Thu, 05/06/2021 - 11:00

Author(s): Francesco Coscarella, Roberto Gaudio, Gabriel G. Katul, and Costantino Manes

Empirical formulae describing the Darcy-Weisbach friction factor remain indispensable for applications in sciences and engineering dealing with turbulent flows. Despite their practical significance, these formulae have remained without theoretical interpretation for many decades. To close this knowledge gap we provide, using a co-spectral budget model, a clarification of the link between spectral properties of velocity fluctuations and the scaling of friction factors in turbulent pipe flows in the hydraulically smooth and fully rough regimes.

[Phys. Rev. Fluids 6, 054601] Published Thu May 06, 2021

Stochastic dynamical model for space-time energy spectra in turbulent shear flows

Thu, 05/06/2021 - 11:00

Author(s): Ting Wu and Guowei He

A dynamic autoregressive (DAR) random forcing model is proposed for space-time energy spectra in turbulent shear flows. This model starts with Taylor’s convection model and introduces the DAR random forcing to represent the random sweeping effect. The DAR model is further combined with linear stochastic estimation (LSE) to reconstruct the near-wall velocity fluctuations.

[Phys. Rev. Fluids 6, 054602] Published Thu May 06, 2021

Input-output framework for actuated boundary layers

Wed, 05/05/2021 - 11:00

Author(s): I. Gluzman and D. F. Gayme

The input-output approach is expanded to investigate actuated wall-bounded shear flows whose geometries and input signals span a range of pulse-width modulated signals common in experimental flow control studies. The model is validated through comparisons to experiments and simulations of three different plasma actuator geometries. An important benefit of this analytical method is the low computational cost associated with its use, enabling efficient parametric studies.

[Phys. Rev. Fluids 6, 053901] Published Wed May 05, 2021

Vertical distribution and longitudinal dispersion of gyrotactic microorganisms in a horizontal plane Poiseuille flow

Wed, 05/05/2021 - 11:00

Author(s): Bohan Wang, Weiquan Jiang, Guoqian Chen, Luoyi Tao, and Zhi Li

A more concise and accurate generalized Taylor dispersion theory is applied to dispersion of active gyrotactic microorganisms in a plane Poiseuille flow. The joint effect of boundary conditions, cell shape anisotropy, swimming speed, and flow speed leads to the nonmonotonic variations of the phenomenological dispersion coefficients.

[Phys. Rev. Fluids 6, 054502] Published Wed May 05, 2021

Positively buoyant jets: Semiturbulent to fully turbulent regimes

Tue, 05/04/2021 - 11:00

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

Drag analysis with a self-propelled flexible swimmer

Mon, 05/03/2021 - 11:00

Author(s): David Gross, Yann Roux, Christophe Raufaste, and Argentina Médéric

Fish swim by undulating their body to ensure propulsion. In a steady state, thrust is balanced by the total drag force, for which the dominant terms depend on the Reynolds number and the flow regime. In this article we propose a set of simple scaling laws to determine the contribution of each mechanism to the drag exerted on the swimmer.

[Phys. Rev. Fluids 6, 053101] Published Mon May 03, 2021

Scaling laws for extremely strong thermals

Mon, 05/03/2021 - 11:00

Author(s): Alex Skvortsov, Timothy C. DuBois, Milan Jamriska, and Martin Kocan

Since the seminal results of Batchelor, Morton and Turner who laid the foundations of classical convective plume theory, it has been recognized that convective thermals exhibit remarkable scaling properties. In this paper, the authors identify dynamics of strong thermals which can be drastically different from weak Boussinesq-type thermals. In general, the evolution of thermals is affected by the interplay of two processes: entrainment flux caused by thermal expansion and solid-body acceleration of the thermal centroid. As a result, depending on the density contrast between the thermal and the ambient environment, scaling laws are modified with different power-law exponents.

[Phys. Rev. Fluids 6, 053501] Published Mon May 03, 2021

Erratum: Material transport in the left ventricle with aortic valve regurgitation [Phys. Rev. Fluids <b>3</b>, 113101 (2018)]

Mon, 05/03/2021 - 11:00

Author(s): Giuseppe Di Labbio, Jérôme Vétel, and Lyes Kadem

[Phys. Rev. Fluids 6, 059901] Published Mon May 03, 2021

Thixotropy in viscoplastic drop impact on thin films

Fri, 04/30/2021 - 11:00

Author(s): Samya Sen, Anthony G. Morales, and Randy H. Ewoldt

We report the first-ever experimental study of thixotropic aging in viscoplastic drop impact. A new dimensionless group is proposed and validated. The results will be useful in predicting splash behavior in a variety of applications from spray coating to fire suppression.

[Phys. Rev. Fluids 6, 043301] Published Fri Apr 30, 2021

Dynamic arrest during the spreading of a yield stress fluid drop

Fri, 04/30/2021 - 11:00

Author(s): Grégoire Martouzet, Loren Jørgensen, Yoann Pelet, Anne-Laure Biance, and Catherine Barentin

We study the spreading of drops made of yield-stress fluids. In contrast to what is observed in Newtonian fluids, the final contact angle reached by the drop depends on the drop size, on its yield stress, and on the liquid/solid hydrodynamic boundary condition. This highlights the crucial role of dynamic history. We then extend the classical Young’s law to the case of yield stress fluids. In particular, by considering that the final shape of the drop is set by a dynamical arrest, we predict the observed final contact angle.

[Phys. Rev. Fluids 6, 044006] Published Fri Apr 30, 2021

Diffraction of weakly unstable detonation through an obstacle with different sizes and shapes

Thu, 04/29/2021 - 11:00

Author(s): Yuan Wang, Zheng Chen, and Haitao Chen

Two-dimensional simulations considering detailed chemistry are conducted to investigate the weakly unstable detonation diffracting through an obstacle. Subcritical, critical, and supercritical regimes are identified and their distributions are significantly affected by the obstacle size and shape. In contrast, the mixtures with different nitrogen dilution have little influence on the regime distributions.

[Phys. Rev. Fluids 6, 043201] Published Thu Apr 29, 2021

Large-eddy simulation of bubble plume in stratified crossflow

Thu, 04/29/2021 - 11:00

Author(s): Shuolin Xiao, Chen Peng, and Di Yang

Bubble-driven plume in stratified crossflow is modeled using Eulerian-Eulerian large-eddy simulation. Various bubble sizes and crossflow velocities are considered, and noticeable differences in the plume characteristics and material transport are observed. Statistical analysis of the simulation results provides insights on how the crossflow affects the exchanges of momentum and mass between the bubble-driven plume and the surrounding water.

[Phys. Rev. Fluids 6, 044613] Published Thu Apr 29, 2021

Air-cushioning effect and Kelvin-Helmholtz instability before the slamming of a disk on water

Thu, 04/29/2021 - 11:00

Author(s): Utkarsh Jain, Anaïs Gauthier, Detlef Lohse, and Devaraj van der Meer

A solid plate about to slam onto a water surface makes it presence felt before the actual contact by squeezing out a mediating air cushioning layer. This air cushioning layer has regions of low and high pressures. At the point of high (stagnation) air pressure, the water surface is deflected away from the impactor. While in the low pressure region, a Kelvin-Helmholtz instability initiates the suction of the water surface towards the impactor. Using a new measuring technique we measure such deflections, of the order of 10-300 microns, and explain the mechanisms driving them.

[Phys. Rev. Fluids 6, L042001] Published Thu Apr 29, 2021

Droplet splashing on rough surfaces

Wed, 04/28/2021 - 11:00

Author(s): Thijs de Goede, Karla de Bruin, Noushine Shahidzadeh, and Daniel Bonn

Drop splashing on surfaces is important for a wide variety of processes ranging from inkjet printing to pollination by rain and forensic blood pattern analysis. The critical impact velocity beyond which the drop disintegrates is well understood for smooth surfaces, but remained a puzzle for rougher ones that are often encountered in practice. We find that the splashing threshold on rough surfaces is lower, which can be understood as an interplay between the surface roughness and the viscous, inertial, and capillary forces on the drop.

[Phys. Rev. Fluids 6, 043604] Published Wed Apr 28, 2021

Electrical voltage by electron spin-vorticity coupling in laminar ducts

Wed, 04/28/2021 - 11:00

Author(s): Hamid Tabaei Kazerooni, Georgy Zinchenko, Jörg Schumacher, and Christian Cierpka

A tiny electrical voltage can be generated by the collective coupling of the electron spins to the flow vorticity in laminar and turbulent liquid metal flows. Here, we demonstrate a linear scaling law between electrical voltage and pressure drop for laminar flows through capillaries with different cross sections, both, analytically and experimentally.

[Phys. Rev. Fluids 6, 043703] Published Wed Apr 28, 2021

Deformation of soap bubbles in uniform electric fields

Tue, 04/27/2021 - 11:00

Author(s): S. Mawet, H. Caps, and S. Dorbolo

Soap bubbles are easy to deform: a child blowing, the wind, or an electric field imposed by a plane capacitor are some possible examples. When an electric field is applied, a bubble elongates along the direction of the field, deforming into a spheroid. For a sufficiently high electric field, bubbles eventually become conical, forming a so-called Taylor cone. In addition to the dependence on the electric field and the soapy solution used, the shape of a bubble is also related to the substrate on which it rests, namely a solid plate or a liquid bath.

[Phys. Rev. Fluids 6, 043603] Published Tue Apr 27, 2021