# Physical Review Fluids

### Characteristics of shearing motions in incompressible isotropic turbulence

Author(s): T. Watanabe, K. Tanaka, and K. Nagata

Shearing motions in isotropic turbulence are studied with a triple decomposition of velocity gradient tensor. A mean flow around the shearing motions exhibits a thin shear-layer pattern sustained by a biaxial strain. The thickness of each shear layer is well predicted by Burgers’ vortex layer. Interplay between the shear and biaxial strain causes enstrophy production and strain self-amplification.

[Phys. Rev. Fluids 5, 072601(R)] Published Tue Jul 07, 2020

### Dynamic evolution of a transient supersonic trailing jet induced by a strong incident shock wave

Author(s): Mohammad Rezay Haghdoost, Daniel Edgington-Mitchell, Maikel Nadolski, Rupert Klein, and Kilian Oberleithner

The dynamic evolution of a highly underexpanded transient supersonic jet is investigated via high-resolution time-resolved schlieren and numerical simulations. Experimental evidence is provided for the presence of a second triple shock configuration along with a shocklet between the reflected shock and the slipstream. A model is developed and applied to the numerical simulations to reveal the mechanism leading to the formation of the second triple point.

[Phys. Rev. Fluids 5, 073401] Published Tue Jul 07, 2020

### Mean flow scaling in a spanwise rotating channel

Author(s): X. I. A. Yang, Z.-H. Xia, J. Lee, Y. Lv, and J. Yuan

While it is known that the mean flow in a spanwise rotating channel follows a linear law at the pressure side with an additive constant C, the exact dependence of this additive constant on the Reynolds number and the rotation speed was not entirely clear. It is shown that this additive constant C is a logarithmic function of a rotating induced length scale. After determining the mean-flow scaling, this knowledge is used for wall modeling and for relating the skin friction and the flow rate.

[Phys. Rev. Fluids 5, 074603] Published Tue Jul 07, 2020

### Doing more with less: The flagellar end piece enhances the propulsive effectiveness of human spermatozoa

Author(s): Cara V. Neal, Atticus L. Hall-McNair, Jackson Kirkman-Brown, David J. Smith, and Meurig T. Gallagher

Sperm have evolved to perform a difficult but crucial task, swimming thousands of times their body length to the egg through highly viscous fluids. This is achieved through their beating tail, a beautiful structure consisting of sliding filaments, powered by the action of motor proteins. Scientists have spent decades studying sperm propulsion but have tended to ignore the end piece of the tail, characterizing it as a “ragged end” with no motor activity. Mathematical modeling shows that the end piece helps the sperm to perform a faster and more efficient swimming stroke.

[Phys. Rev. Fluids 5, 073101] Published Mon Jul 06, 2020

### Physics and modeling of trailing-edge stall phenomena for wall-modeled large-eddy simulation

Author(s): Yoshiharu Tamaki, Yuma Fukushima, Yuichi Kuya, and Soshi Kawai

Predictability of trailing-edge stall phenomena using wall-modeled large-eddy simulations (LES) is investigated with a wall-resolved LES database. An analysis based on the momentum integral relation shows that the skin friction accumulation effect near the leading edge to the mid chord dominates boundary layer development, and thus, affects flow separation prediction near the trailing edge. The results indicate that accurate wall modeling near the leading edge to the mid chord is essential for predicting stall phenomena, but not necessarily required near and downstream of the separation.

[Phys. Rev. Fluids 5, 074602] Published Mon Jul 06, 2020

### Drop-in additives for suspension manipulation: Colloidal motion induced by sedimenting soluto-inertial beacons

Author(s): Anirudha Banerjee, Huanshu Tan, and Todd M. Squires

A 1-micron Brownian colloid on a random walk can take more than a month to traverse a 1 mm distance. We present a strategy for a drop-in additive to induce spontaneous migration of particles in a suspension at a rate orders of magnitude faster than simple diffusion. The additive, referred to as a solutoinertial beacon, releases a solute as it sediments within the suspension. This solute flux propels colloids to migrate via diffusiophoresis. Theoretical and scaling analyses capture the experimental observations well and reveal design parameters that govern the dynamics of particle motion.

[Phys. Rev. Fluids 5, 073701] Published Thu Jul 02, 2020

### Actuation response model from sparse data for wall turbulence drag reduction

Author(s): Daniel Fernex, Richard Semaan, Marian Albers, Pascal S. Meysonnat, Wolfgang Schröder, and Bernd R. Noack

Drag reduction of an actuated turbulent boundary layer at a momentum-thickness-based Reynolds number Reθ = 1000 is computed, modeled, and predicted. The drag reduction for the set of actuation parameters is modeled using 71 large-eddy simulations. This drag model allows extrapolation outside the actuation domain for larger wavelengths and amplitudes. The modeling novelty combines support vector regression for interpolation, a parametrized ridgeline leading out of the data domain, a scaling for the drag reduction, and a discovered self-similar structure of the actuation effect.

[Phys. Rev. Fluids 5, 073901] Published Thu Jul 02, 2020

### Decaying two-dimensional turbulence undergoes statistical heating

Author(s): J. G. Esler and R. K. Scott

Simulations of decaying two-dimensional turbulence show a persistent trend in the statistical temperature, from “colder” states in which dipoles are prevalent, to “hotter” states dominated by clusters of like-signed vortices. The spontaneous heating effect is shown to be consistent with a decay law for the vortex number density that is faster than the t−2/3 law deduced from similarity arguments.

[Phys. Rev. Fluids 5, 074601] Published Thu Jul 02, 2020

### Hydrodynamic forces on a clean spherical bubble translating in a wall-bounded linear shear flow

Author(s): Pengyu Shi, Roland Rzehak, Dirk Lucas, and Jacques Magnaudet

Fully resolved simulations are conducted to determine hydrodynamic forces on clean spherical bubbles translating near a flat rigid wall in a linear shear flow. Flows range from low-but-finite Re to nearly inviscid situations. Based on simulation results, semi-empirical expressions for drag and lift forces at arbitrary Re, relative shear rate, and separation distance are found. These improve over current ‘point-particle’ models which ignore wall effects, and may be used to predict realistic bubble trajectories and distributions in wall-bounded flows.

[Phys. Rev. Fluids 5, 073601] Published Wed Jul 01, 2020

### Transient growth analysis of oblique shock-wave/boundary-layer interactions at Mach 5.92

Author(s): Anubhav Dwivedi, Nathaniel Hildebrand, Joseph W. Nichols, Graham V. Candler, and Mihailo R. Jovanović

Experimental studies of a transitional shock-wave–boundary-layer interaction identify robust streaklike flow structures. In order to account for the emergence of these three-dimensional flow features, growth of small initial perturbations around the two-dimensional laminar base flow is examined. The most significant transient amplification originates from the upstream streamwise vortices, which are not related to modal instabilities. Unique to separated high-speed flows, the impinging shock compresses the boundary layer, thereby causing the growth of the streaks.

[Phys. Rev. Fluids 5, 063904] Published Tue Jun 30, 2020

### Experimental investigation of the solid-liquid separation in a stirred tank owing to viscoelasticity

Author(s): Weheliye Hashi Weheliye, Giovanni Meridiano, Luca Mazzei, and Panagiota Angeli

Experiments demonstrate that viscoelastic-induced particle migration concentrates solids at the core of vortices in particle suspensions. The findings can successfully be applied to the separation of solids suspended in a viscoelastic liquid with small density difference between the phases, high liquid viscosity, and small-sized particles. The proposed separation method is low cost and relevant to many chemical engineering processes.

[Phys. Rev. Fluids 5, 063302] Published Mon Jun 29, 2020

### Cooperation and competition of viscoelastic fluids and elastomeric microtubes subject to pulsatile forcing

Author(s): Aimee M. Torres Rojas and E. Corvera Poiré

The cooperation and competition of viscoelastic fluids, subject to pulsatile forcing, with the elastomeric microtubes that confine them is explored. Tuning of system parameters allows for the excitation of different modes, and resonances can be achieved by driving the fluid with the appropriate pulsatile pressure drop. The results are relevant at microscales and potentially useful for tailoring composite microfluidic devices, where one can induce an increase or decrease of the amplitude of the longitudinally averaged flow, relative to the one of tubes made of a single material.

[Phys. Rev. Fluids 5, 063303] Published Mon Jun 29, 2020

### Electrowetting diminishes contact line friction in molecular wetting

Author(s): Petter Johansson and Berk Hess

In electrowetting, an electrostatic potential is applied to a droplet to increase its wettability. As the droplet rapidly spreads to its new equilibrium state, contact line friction is greatly diminished. Molecular dynamics simulations show that this effect is present at molecular scales and is related to how liquid molecules advance the contact line.

[Phys. Rev. Fluids 5, 064203] Published Mon Jun 29, 2020

### Microroughness-induced disturbances in supersonic blunt body flow

Author(s): Thomas Schilden, Alexej Pogorelov, Sohel Herff, and Wolfgang Schröder

To identify the mechanism triggering boundary layer transition on a spherical forebody of an Apollo type re-entry capsule, direct numerical simulations of perturbed flow are analyzed. The perturbations are generated by deterministic distributed surface roughnesses that resemble model surface imperfections that are mounted on a capsule model in corresponding experiments. The receptivity of the capsule boundary layer to the roughness and the subsequent disturbance growth are analyzed.

[Phys. Rev. Fluids 5, 063903] Published Fri Jun 26, 2020

### Flows induced by Coriolis-influenced vertically propagating two-dimensional internal gravity wave packets

Author(s): Bruce R. Sutherland, Wyatt Reeves, and Ton S. van den Bremer

In the absence of background rotation, the Eulerian flow induced by two-dimensional internal wave packets is well known to have the structure of long trailing internal waves. We show that rotation makes these waves evanescent and the structure of the flow over the waves in the wave packet qualitatively changes, with implications for the modulational stability of the waves.

[Phys. Rev. Fluids 5, 064805] Published Thu Jun 25, 2020

### Aeroacoustic noise generation due to vortex reconnection

Author(s): Hamid Daryan, Fazle Hussain, and Jean-Pierre Hickey

Compressible direct numerical simulation of vortex reconnection uncovers the source and far-field pattern of aeroacoustic noise.

[Phys. Rev. Fluids 5, 062702(R)] Published Wed Jun 24, 2020

### Experimental realization of broadband control of water-wave-energy amplification in chirped arrays

Author(s): A. J. Archer, H. A. Wolgamot, J. Orszaghova, L. G. Bennetts, M. A. Peter, and R. V. Craster

An experimental demonstration is presented that a chirped array of cylinders can be designed to control the spatial distribution of water wave energy and substantially amplify target frequencies at specified locations, over a broad range of frequencies, consistent with linear band-gap theory.

[Phys. Rev. Fluids 5, 062801(R)] Published Wed Jun 24, 2020

### Axisymmetric squirmers in Stokes fluid with nonuniform viscosity

Author(s): Patrick S. Eastham and Kourosh Shoele

Numerical techniques are employed to study the locomotion of a spheroidal squirmer in a complex fluid with a nutrient-dependent viscosity. It is found that the nonuniform viscosity significantly affects the pressure field around the swimmer rather than the velocity field. Results obtained here will be helpful in interpreting experimental observation where a microswimmer significantly affects a fluid’s local rheology.

[Phys. Rev. Fluids 5, 063102] Published Wed Jun 24, 2020

### Kinematics of a simple reciprocal model swimmer at intermediate Reynolds numbers

Author(s): Thomas Dombrowski and Daphne Klotsa

A study of the kinematics, power and recovery strokes, fluid flows, and efficiency of a reciprocal dumbbell swimmer with finite inertia finds that the swimmer’s average flow field is dominated by the flow during its power stroke, and it switches from pullerlike to pusherlike depending on the (finite) Reynolds number.

[Phys. Rev. Fluids 5, 063103] Published Wed Jun 24, 2020

### Flow transitions and mapping for undulating swimmers

Author(s): Muhammad Saif Ullah Khalid, Junshi Wang, Haibo Dong, and Moubin Liu

Unfolding the connection between kinematics and physiology of natural swimming species is of much value for bioinspired designs of autonomous underwater vehicles. Extensive numerical investigations find which wavelength of the undulating motion of carangiform and anguilliform swimmers is suitable to maximize their swimming performance in terms of thrust production and efficiency under different flow conditions. It is also explained how jet switching in the wake of a swimmer deteriorates the hydrodynamic efficiency of anguilliform and carangiform swimmers.

[Phys. Rev. Fluids 5, 063104] Published Wed Jun 24, 2020