# Physical Review Fluids

### Linear global stability of a downward flow of liquid metal in a vertical duct under strong wall heating and transverse magnetic field

Author(s): Jun Hu

A three-dimensional unstable oscillatory instability is found for a downward flow of liquid metal in a vertical duct under strong wall heating and a transverse magnetic field. The unstable oscillatory mode first occurs at the specific flow structure which has an upward reverse flow near the heating wall and a downward flow near the opposite wall. The existence of an inflection point is the key instability mechanism of the three-dimensional oscillatory mode which may be regarded as an alternative physical explanation of the high-amplitude, low-frequency pulsations of temperature in experiments and related numerical simulations.

[Phys. Rev. Fluids 6, 073502] Published Wed Jul 21, 2021

### Lagrangian and Eulerian accelerations in turbulent stratified shear flows

Author(s): Frank G. Jacobitz and Kai Schneider

The Lagrangian and Eulerian acceleration (LA and EA) properties of fluid particles in homogeneous turbulence with uniform shear and uniform stable stratification are studied with direct numerical simulations. A wavelet-based scale-dependent decomposition of LA and EA is performed. Joint probability density functions of LA and EA show a trend of stronger correlation with increasing stratification strength and at larger turbulent scales. From the Navier–Stokes equation, LA is dominated by the pressure-gradient term, and EA by the nonlinear convection term. From geometrical statistics, the magnitude of EA is larger than LA due to mutual cancellation of the Eulerian and convective acceleration.

[Phys. Rev. Fluids 6, 074609] Published Wed Jul 21, 2021

### Simultaneous velocity and density measurements of fully developed Rayleigh-Taylor mixing

Author(s): Mark Mikhaeil, Prasoon Suchandra, Devesh Ranjan, and Gokul Pathikonda

The dynamics of molecular mixing and the energy transfer process in the Rayleigh-Taylor instability (RTI) are studied through the collection of simultaneous velocity-density measurements using particle image velocimetry (PIV) and laser induced fluorescence (LIF). Statistically stationary experiments are performed in a convective-type gas tunnel facility which allows long experimental times and enables collection of statistically important turbulence data. The data and analyses presented in this paper are expected to help validate variable-density turbulence models and further our understanding of instability-driven flows.

[Phys. Rev. Fluids 6, 073902] Published Mon Jul 19, 2021

### Asymptotic theory of hydrodynamic interactions between slender filaments

Author(s): Maria Tătulea-Codrean and Eric Lauga

Hydrodynamic interactions are important in biophysics because they influence the collective behaviour of microorganisms and active particles, and also play a key role in the emergence of swimming gaits. We determine the hydrodynamic interactions between slender filaments by means of asymptotic calculations and numerical simulations, for the case when two filaments are separated by a distance greater than their contour length (d > L). We then show how our theory explains the collective dynamics of two rigid helices rotating side-by-side.

[Phys. Rev. Fluids 6, 074103] Published Mon Jul 19, 2021

### Large eddy simulation of transitional channel flow using a machine learning classifier to distinguish laminar and turbulent regions

Author(s): Ghanesh Narasimhan, Charles Meneveau, and Tamer A. Zaki

Breakdown to turbulence in wall-bounded flows takes place through sporadic bursts of turbulent spots. Wall-modelled large-eddy simulations (LES) of transition to turbulence must dynamically identify the nascent turbulent regions, track their evolution, and apply the appropriate wall stress within and outside the turbulent/non-turbulent (T-NT) interface. Self-organized maps (SOM), a machine learning classifier, objectively and efficiently captures the T-NT interface. Wall-modeled LES with SOM interface identification predicts both orderly and bypass transition.

[Phys. Rev. Fluids 6, 074608] Published Mon Jul 19, 2021

### Editorial: Machine Learning and <i>Physical Review Fluids</i>: An Editorial Perspective

Author(s): Michael P. Brenner and Petros Koumoutsakos

[Phys. Rev. Fluids 6, 070001] Published Fri Jul 16, 2021

### Scaling of hypersonic shock/turbulent boundary layer interactions

Author(s): Clara M. Helm and M. P. Martín

A large database of shock/turbulent boundary layer interactions is compiled to study the separation length scaling over the range of flow conditions including hypersonic interactions. Experimental and computational data of two-dimensional and axisymmetric geometries are included with Mach number from 2 to 10 and ratio of wall to adiabatic recovery temperature from 0.3 to 1.9. A new scaling shows weak interactions collapse by the upstream boundary layer properties, strong interactions do not, and strong separation cases depend on the structure of the downstream flow.

[Phys. Rev. Fluids 6, 074607] Published Fri Jul 16, 2021

### Heat transfer mechanism driven by acoustic body force under acoustic fields

Author(s): Varun Kumar, Mohammed Azharudeen, Charish Pothuri, and Karthick Subramani

We demonstrate a heat transfer (HT) mechanism based on the relocation of an inhomogeneous fluid under acoustic fields. The proposed HT mechanism is studied under gravity and microgravity conditions. When differentially heated fluid is subjected to ultrasonic waves perpendicular to the HT direction, HT is found to be enhanced up to one order. Depending on acoustic properties, different flow patterns are observed for different fluids. A modified Rayleigh number is proposed for HT characterization that accounts for both gravity and acoustic effects. Furthermore, suppression of natural convection HT is observed when acoustic waves are applied parallel to the HT direction.

[Phys. Rev. Fluids 6, 073501] Published Thu Jul 15, 2021

### Experimental investigation of flow around a ${45}^{∘}$ oriented cube for Reynolds numbers between 500 and 50 000

Author(s): Majid Hassan Khan, P. Sooraj, Atul Sharma, and Amit Agrawal

Particle imaging velocimetry (PIV) measurements were performed for flow around an oriented cube, to study the wake at various Reynolds numbers. The wake shows numerous small eddies and the velocity profiles have multiple peaks. At higher Reynolds number the streamwise to transverse root mean square velocity ratio Urms/Vrms ~ 1 indicates homogenizing and better mixing ability of an oriented cube as compared to a normal cube. Proper orthogonal decomposition (POD) has been used to examine the energy content of the flow and the evolution of coherent structure.

[Phys. Rev. Fluids 6, 074606] Published Thu Jul 15, 2021

### Path instability of a no-slip spheroidal bubble in isotropic turbulence

Author(s): Gihun Shim, Jongsu Kim, and Changhoon Lee

Path instability of a millimetric spheroidal bubble in isotropic turbulence is investigated by direct numerical simulation combined with an immersed boundary method. The zigzag frequency and the degree of obliquity of the bubble are enhanced with the strength of the background turbulence.

[Phys. Rev. Fluids 6, 073603] Published Tue Jul 13, 2021

### Lord Kelvin's isotropic helicoid

Author(s): Darci Collins, Rami J. Hamati, Fabien Candelier, Kristian Gustavsson, Bernhard Mehlig, and Greg A. Voth

Can a propeller be isotropic? Nearly 150 years ago, Lord Kelvin proposed the isotropic helicoid, but there are no published measurements on his particle. We 3D-printed his particle and unexpectedly found no measurable translation-rotation coupling. We explain these results by demonstrating theoretically and computationally that Kelvin’s proposed coupling exists, but it is small since it is only due to a weak breaking of a symmetry of non-interacting vanes in Stokes flow.

[Phys. Rev. Fluids 6, 074302] Published Tue Jul 13, 2021

### Pinch-off dynamics to elucidate animal lapping

Author(s): Sunghwan Jung

Most carnivorous mammals (e.g., cats and dogs) lap water with their tongues to drink water at high frequencies by creating a liquid column out of a bath. Presumably, the animals bite just before the pinch-off time of the water column to maximize the water intake. Such a pinch-off phenomenon in the liquid column can be described as the acceleration-induced (i.e., unsteady) inertia balances with the capillary force.

[Phys. Rev. Fluids 6, 073102] Published Mon Jul 12, 2021

### Small-scale flow topologies, pseudo-turbulence, and impact on filtered drag models in turbulent fluidization

Author(s): F. Dabbagh and S. Schneiderbauer

The small-scale flow topologies in a moderately dense (gas-particle) turbulent fluidization have been investigated using highly-resolved Eulerian two-fluid model simulations. Enhanced contraction of focal (enstrophy) and nodal (strain) gas phase structures are found as a result of the solid particles presence. The gas structures, thereby, revealed a tendency towards boundary-layer-like turbulence. In the solid phase, the focal topologies are arranged in elongated vortical tubes corresponding to dense clusters, which in turn induce a pseudo-turbulence on the gas phase.

[Phys. Rev. Fluids 6, 074301] Published Mon Jul 12, 2021

### Unsteady dynamics in the streamwise-oscillating cylinder wake for forcing frequencies below lock-on

Author(s): Maysam Shamai, Scott T. M. Dawson, Igor Mezić, and Beverley J. McKeon

Forcing a cylinder in the streamwise direction with a frequency much lower than the stationary shedding frequency significantly alters wake dynamics. During certain portions of the forcing cycle the wake resembles that of a stationary cylinder, while in others it exhibits strong unsteadiness. Although a range of flow phenomena are observed, it is shown that a novel time scaling framework can be used to relate the forced and unforced systems.

[Phys. Rev. Fluids 6, 074702] Published Mon Jul 12, 2021

### Variation of focusing patterns of laterally migrating particles in a square-tube flow due to non-Newtonian elastic force

Author(s): Naoto Yokoyama, Hiroshi Yamashita, Kento Higashi, Yuta Miki, Tomoaki Itano, and Masako Sugihara-Seki

Pressure-driven flow experiments demonstrate that spherical particles in dilute polymer solutions are focused on the midline and/or the diagonal in the channel cross-section, with details set by the polymer concentration. Numerical simulations based on the FENE-P constitutive model are then used to understand the variation of the focusing position as a function of the Weissenberg number.

[Phys. Rev. Fluids 6, L072301] Published Mon Jul 12, 2021

### Larger wavelengths suit hydrodynamics of carangiform swimmers

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

We examine the connection between the physiology and wavy kinematics of carangiform swimmers, such as Jack, Tuna, and Sunfish. Using high-fidelity numerical simulations for flows over Jack Fish models obtained through reconstruction of high-speed images of real natural swimmers, it was revealed that undulation with larger wavelengths improves the hydrodynamic performance of the carangiform swimmer in terms of better thrust production by the caudal fin, lower drag production on the trunk, and reduced power consumption by the trunk.

[Phys. Rev. Fluids 6, 073101] Published Fri Jul 09, 2021

### Learning unknown physics of non-Newtonian fluids

Author(s): Brandon Reyes, Amanda A. Howard, Paris Perdikaris, and Alexandre M. Tartakovsky

Non-Newtonian fluids have a shear-rate dependent viscosity that is difficult to measure in experiments. We present a physics-informed neural networks (PINN) approach for learning the viscosity using indirect measurements (such as velocity and pressure) subject to the momentum conservation and continuity equations constraints. We use the PINN approach to estimate viscosity of polymer melts and suspensions of particles using velocity measurements from two-dimensional shear flow simulations. The PINN-inferred viscosity models agree with empirical models for shear rates with large absolute values but deviate for shear rates near zero where the empirical models have an unphysical singularity.

[Phys. Rev. Fluids 6, 073301] Published Fri Jul 09, 2021

### Effects of resolution inhomogeneity in large-eddy simulation

Author(s): Gopal R. Yalla, Todd A. Oliver, Sigfried W. Haering, Björn Engquist, and Robert D. Moser

Discretizations with inhomogeneous resolution affect large-eddy simulations (LES) through the commutator of the filtering and differentiation operators. We employ a multiscale asymptotic analysis to investigate the statistical characteristics of this commutator, which can serve as a target for commutation models. Further, we demonstrate how the neglect of this commutator manifests numerically, drawing a connection between the so-called commutation error and the dispersion relation of the underlying numerics. A modeling approach for the commutator is proposed that is based on the numerical properties of the LES evolution equation.

[Phys. Rev. Fluids 6, 074604] Published Thu Jul 08, 2021

### Representing rectangular jet dynamics through azimuthal Fourier modes

Author(s): Surya Chakrabarti, Datta Gaitonde, and S. Unnikrishnan

Rectangular propulsion nozzles have advantages over circular nozzles, including easier thrust-vectoring and air-frame-integration. Jet noise is easier to study with circular jets (CJ), however, due to azimuthal homogeneity, which, together with low-rank acoustic dynamics enables simpler acoustic models. Using Large Eddy Simulations of rectangular jets (RJ) of various aspect ratios we show that acoustic fluctuation components exhibit comparably rapid convergence in azimuthal Fourier space even for high aspect ratios. A reduced-order model for RJ that retains near-field acoustic asymmetry can be constructed using only three leading azimuthal modes, but with two additional terms relative to CJ.

[Phys. Rev. Fluids 6, 074605] Published Thu Jul 08, 2021

### Exact trajectory solutions of a spherical microswimmer under flow and external fields

Author(s): M. Guedda, J. Chaiboub, M. Benlahsen, and C. Misbah

Exact trajectoires of microswimmers under flow are determined analytically exhibiting intriguing patterns such as a spherical helix.

[Phys. Rev. Fluids 6, 074102] Published Wed Jul 07, 2021