# Physical Review Fluids

### Lagrangian analysis for turbulent transport in variable-density turbulence

Author(s): G. S. Sidharth and J. R. Ristorcelli

It is shown that the basis functions of gradient transport theory are different when there is an additional materially conserved variable. The turbulent fluxes now depend on the mean density gradient indicating the possibility of counter gradient transport from first principles as seen in some laboratory experiments. It is shown that arguments by analogy from constant density transport for the Favre fluxes are not consistent with the Lagrangian results.

[Phys. Rev. Fluids 6, 023202] Published Thu Feb 25, 2021

### Modeling the nonlinear aeroacoustic response of a harmonically forced side branch aperture under turbulent grazing flow

Author(s): Tiemo Pedergnana, Claire Bourquard, Abel Faure-Beaulieu, and Nicolas Noiray

The response of a side branch aperture to harmonic forcing is a key element of the feedback loop describing flow-induced aeroacoustic instability in deep cavities. We derive and validate two physics-based models which, after calibration at a given condition, predict the influence of frequency, mean flow speed, and acoustic pressure amplitude on the response. Notably, these low-order models enable robust analytical amplitude predictions of self-sustained oscillations in deep cavities under turbulent grazing flow

[Phys. Rev. Fluids 6, 023903] Published Thu Feb 25, 2021

### Vorticity transfer in a leading-edge vortex due to controlled spanwise bending

Author(s): Kun Jia, Tyler Scofield, Mingjun Wei, and Samik Bhattacharya

The effect of dynamic spanwise bending on the vortex dynamics of an accelerating flat plate is studied with experiments and numerical simulations. A flat plate, held at an angle of attack of 30 degrees, is accelerated from rest to Reynolds number 2400. It was bent dynamically along the span in a controlled manner with a bending ratio of 0.65. We find that a dynamic spanwise bending induces a change in the effective shear layer velocity along the span’s bent part and creates spanwise vorticity convection. As a result, the growth of circulation in the leading-edge-vortex gets delayed along the bent part, and the final circulation is smaller than the no bending case.

[Phys. Rev. Fluids 6, 024703] Published Thu Feb 25, 2021

### Robust propagation of internal coastal Kelvin waves in complex domains

Author(s): Chenyang Ren, Xianping Fan, Yiling Xia, Tiancheng Chen, Liu Yang, Jin-Qiang Zhong, and H. P. Zhang

Internal coastal Kelvin waves were experimentally generated in a two-layer fluid system on a rotating table. Waves are exponentially localized near the tank boundary and propagate in the same direction as the table rotation along boundaries of complex geometries without being scattered. Our experiments suggest a connection between these unusual wave characteristics and topological properties of the underlying governing equations.

[Phys. Rev. Fluids 6, L022801] Published Thu Feb 25, 2021

### Numerical study of the influence of wall roughness on laminar boundary layer flashback

Author(s): Shuyu Ding, Kai Huang, Yifan Han, and Damir Valiev

Boundary layer flame flashback is a phenomenon that may constitute a key challenge for efficient combustion of novel fuels at gas turbine conditions. In the present work, the effect of wall roughness on the laminar boundary layer flashback is studied systematically using numerical simulation. The results indicate that the wall roughness can attenuate flashback speed due to enhanced heat loss in case of low thermal resistance of the wall. The critical velocity gradient of the oncoming flow is shown to decrease with wall roughness level and increase with gas thermal expansion ratio.

[Phys. Rev. Fluids 6, 023201] Published Wed Feb 24, 2021

### Flow in a weakly curved square duct: Assessment and extension of Dean's model

Author(s): Leonardo Rigo, Damien Biau, and Xavier Gloerfelt

The laminar flow in a weakly bent pipe exhibits very rich dynamical properties. The flow is stationary, periodic or chaotic depending on one control parameter. A very practical simplification inspired by Dean is capable of reproducing this behavior with remarkable accuracy.

[Phys. Rev. Fluids 6, 024101] Published Wed Feb 24, 2021

### Effect of stratification on the mixing and reaction yield in a T-shaped micro-mixer

Author(s): A. Mariotti, C. Galletti, R. Mauri, M. V. Salvetti, and E. Brunazzi

Experiments, i.e. micro-PIV and flow visualization, and direct numerical simulations, are used jointly to investigate how stratification affects mixing and chemical reaction in a T-shaped microreactor fed with two miscible liquids exhibiting a small density difference. The work analyzes the dependence of the degree of mixing on the Reynolds number and correlates the reaction yield with the Damköhler number to help to devise strategies for the practical operation of microreactors with fluids of practical interest.

[Phys. Rev. Fluids 6, 024202] Published Wed Feb 24, 2021

### General method for determining the boundary layer thickness in nonequilibrium flows

Author(s): Kevin Patrick Griffin, Lin Fu, and Parviz Moin

In this work, a new method for computing the boundary layer thickness is proposed by reconstructing an approximate inviscid solution based on the Bernoulli equation. The viscous streamwise velocity profile U[y] agrees with this inviscid reconstruction UI[y] outside the boundary layer, and the solutions diverge from each other at the boundary layer edge. The boundary layer thickness is readily determined by examining the discrepancy between these profiles. Extensive validation suggests that the present method is more robust and more widely applicable than existing methods.

[Phys. Rev. Fluids 6, 024608] Published Wed Feb 24, 2021

### Weakly nonlinear Holmboe waves

Author(s): Joshua Cudby and Adrien Lefauve

Holmboe waves are long-lived traveling waves commonly found in environmental stratified shear flows. Here we study their finite-amplitude properties in the nonlinear but nonturbulent regime, with a weakly nonlinear temporal stability analysis. Using a versatile amplitude expansion method, we analyze supercritical bifurcation diagrams both in Reynolds number and Richardson number, transient phase portraits, and the vertical structures of all components modes up to third order. We believe these results provide a basis for a future fully nonlinear analysis of the Holmboe dynamical system.

[Phys. Rev. Fluids 6, 024803] Published Tue Feb 23, 2021

### Wet to dry self-transitions in dense emulsions: From order to disorder and back

Author(s): Andrea Montessori, Adriano Tiribocchi, Marco Lauricella, Fabio Bonaccorso, and Sauro Succi

Computer simulations show the self-transition between ordered and disordered emulsions in divergent microfluidic channels. The transition is driven by the nonlinear competition between viscous dissipation and surface tension forces as controlled by the device geometry. An unexpected route back to order is observed in the regime of large opening angles where a trend towards increasing disorder would be intuitively expected.

[Phys. Rev. Fluids 6, 023606] Published Mon Feb 22, 2021

### Physical invariance in neural networks for subgrid-scale scalar flux modeling

Author(s): Hugo Frezat, Guillaume Balarac, Julien Le Sommer, Ronan Fablet, and Redouane Lguensat

A physics informed approach is applied to neural networks for subgrid-scale scalar flux modeling. We show that several invariances of the scalar transport equation are not enforced by existing parametric models, which reduce their interpretability and question their application. A new architecture embedding these invariances as hard and soft constraints is proposed. Through different flow configurations, we show that the proposed constraints increase both the performances and the generalization capabilities of the model.

[Phys. Rev. Fluids 6, 024607] Published Mon Feb 22, 2021

### Coupled triads in the dynamics of internal waves: Case study using a linearly stratified fluid

Author(s): Q. Pan, N. N. Peng, H. N. Chan, and K. W. Chow

Coupled triads (two sets of resonant triads with one member in common) can arise in linearly stratified fluids. Such coupling may induce modulation instabilities which are otherwise absent for component triads in isolation themselves. Long wavelength instabilities will imply the occurrence of internal rogue waves which may attain amplitudes much larger than their surface wave counterparts.

[Phys. Rev. Fluids 6, 024802] Published Mon Feb 22, 2021

### Marginal regeneration in a horizontal film: Instability growth law in the nonlinear regime

Author(s): Alice Gros, Adrien Bussonnière, Sanjiban Nath, and Isabelle Cantat

The marginal regeneration process responsible for foam film drainage is revisited. It is shown that a horizontal, micron thick, foam film in contact with a meniscus destabilizes and that patches of thinner film grow along the meniscus, forming a very regular pattern.

[Phys. Rev. Fluids 6, 024004] Published Fri Feb 19, 2021

### Subgrid-scale characterization and asymptotic behavior of multidimensional upwind schemes for the vorticity transport equations

Author(s): Daniel Foti and Karthik Duraisamy

We establish subgrid-scale (SGS) characteristics of a finite volume vorticity-transport-based approach for large-eddy simulations. Modified equation analysis indicates that dissipation can be controlled locally via nonlinear limiting of the gradient employed for the vorticity reconstruction. The enstrophy budget highlights the remarkable ability of the truncation terms to mimic the true SGS dissipation and diffusion. Numerical dissipation in under-resolved simulations can be characterized by diffusion terms discovered in the modified equation analysis.

[Phys. Rev. Fluids 6, 024606] Published Fri Feb 19, 2021

### Energy transfer mechanisms and resolvent analysis in the cylinder wake

Author(s): Bo Jin, Sean Symon, and Simon J. Illingworth

We investigate discrepancies between the two-dimensional cylinder flow and a quasilinear model (i.e.~resolvent analysis) from an energy transfer perspective at Re=100. The energy balances achieved by the true flow are characterized and compared to predictions from resolvent analysis. The impact of the neglected nonlinear energy transfer on the resolvent mode shapes is clarified by analyzing the spatial distribution of the energy transfer mechanisms. This provides insights into the extent to which resolvent analysis correctly models energy transfer mechanisms, which is essential for understanding the limitations of quasilinear approximations and improving the modeling of nonlinear flows.

[Phys. Rev. Fluids 6, 024702] Published Thu Feb 18, 2021

### Nature of trapping forces in optically induced electrothermal vortex based tweezers

Author(s): Avanish Mishra, Kshitiz Gupta, and Steven T. Wereley

Precise manipulation of micro and nanosized particles has enabled investigations into various applications ranging from mechanobiology of biomolecules and cells to self-assembly of two-dimensional colloids. This work is focussed on studying the nature of a noninvasive electrothermal vortex based micro-manipulation tool called rapid electrokinetic patterning (REP). Using the equipartition method, we show that a REP trap is Hookean in nature and has an ultralow trap stiffness on the order of femtonewtons/μm. The dynamic tunability of an optically induced REP trap makes it a versatile tool for various biophysical applications.

[Phys. Rev. Fluids 6, 023701] Published Wed Feb 17, 2021

### Capillary transport from barrel to clamshell droplets on conical fibers

Author(s): J. Van Hulle, F. Weyer, S. Dorbolo, and N. Vandewalle

Droplets spontaneously move when they are placed at the tip of a cone surface. Using three-dimensionally-printed structures, an experimental exploration of a large panel of configurations regarding the aperture angle of the cone finds evidence for a change of the droplet geometry while moving along the conical fiber—from barrel to clamshell shape. The position of this geometrical transition is estimated and two models are proposed to describe the motion of the barrel and the clamshell droplets.

[Phys. Rev. Fluids 6, 024501] Published Tue Feb 16, 2021

### Transition from steady to oscillating convection rolls in Rayleigh-Bénard convection under the influence of a horizontal magnetic field

Author(s): J. C. Yang, T. Vogt, and S. Eckert

The effect of a horizontal magnetic field on the oscillatory instability of convection rolls in a finite liquid-metal layer is investigated. The flow measurements reveal that the first developing oscillations are of a two-dimensional nature. In particular, a mutual increase and decrease in the size of adjacent convection rolls is observed where the periodicity of the “breathing” convection rolls can be related to standing inertial waves. With gradual reduction of the magnetic-field strength, the oscillating convection rolls are increasingly affected by three-dimensional disturbances.

[Phys. Rev. Fluids 6, 023502] Published Mon Feb 15, 2021

### Large impact velocities suppress the splashing of micron-sized droplets

Author(s): Masashi Usawa, Yuta Fujita, Yoshiyuki Tagawa, Guillaume Riboux, and José Manuel Gordillo

An investigation shows that, counterintuitively, the splash of drops impacting with velocities of a few tens of meters per second is suppressed because the aerodynamic lift force that would cause the liquid film to separate from the substrate and to break into much finer droplets is inhibited. This occurs as a consequence of the fact that the thickness of the lamella becomes similar to the mean-free path of gas molecules.

[Phys. Rev. Fluids 6, 023605] Published Mon Feb 15, 2021

### Symmetry breaking in a turbulent environment

Author(s): Alexandros Alexakis, François Pétrélis, Santiago J. Benavides, and Kannabiran Seshasayanan

Symmetry breaking in laminar flows is well known in the transition to turbulence scenario. Here we consider the breaking of a remaining symmetry in an already turbulent flow, as in the transition from a two-dimensional turbulent flow shown in the figure to a three-dimensional turbulent flow. We show that such cases have critical exponents that differ from the mean-field predictions and our results indicate the possible existence of a new class of out-of-equilibrium phase transition controlled by the multiplicative turbulent noise.

[Phys. Rev. Fluids 6, 024605] Published Mon Feb 15, 2021