# New Papers in Fluid Mechanics

### Low-frequency unsteadiness in hypersonic swept shock wave-boundary layer interactions

Author(s): Alessandro Ceci, Andrea Palumbo, Johan Larsson, and Sergio Pirozzoli

We carry out a numerical study of swept shock wave/turbulent boundary layer interaction (SBLI) in the hypersonic regime, where a crossflow velocity component is added to the incoming flow to mimic three-dimensional interactions with cylindrical symmetry. The spatiotemporal dynamics of wall pressure well conform with the previously introduced formula for swept supersonic interactions, extending the validity of the model to SBLIs in the hypersonic regime.

[Phys. Rev. Fluids 9, 054603] Published Tue May 07, 2024

### Expediting viscous spreading with liquid-infused solids

Author(s): Saurabh Nath and David Quéré

A viscous drop spreads slowly on a solid at a velocity selected by its viscosity. We show here that liquid-infused solids – a class of materials with properties in between a solid and a liquid – can expedite the spreading dynamics due to interfacial slip, which we investigate at short time.

[Phys. Rev. Fluids 9, 054001] Published Mon May 06, 2024

### Boundary-layer flows over deforming surfaces

Author(s): N. Hanevy, J. Ferguson, P. M. J. Trevelyan, and P. T. Griffiths

In this paper a formulation of the incompressible Navier-Stokes equations is introduced which allows one to model boundary-layer flows induced by the motion of a deforming surface. Such a formulation may be used to model flows relevant in a wide variety of industries from polymer processing to glass manufacturing. We show that for particular sheet geometries and velocities, similarity solutions may be obtained that account for sheet thinning (or thickening) and roughness patterns observed in extrusion-type processes.

[Phys. Rev. Fluids 9, 054101] Published Mon May 06, 2024

### Effect of aspect ratio on the unlimited flow-induced vibration of an elliptical cylinder-plate assembly

Author(s): Ying Wu, Fue-Sang Lien, Eugene Yee, and Guang Chen

We report numerical simulations for the transverse flow-induced vibration (FIV) of an elastically supported elliptical cylinder-plate assembly in the regime of low Reynolds number. We investigate the combined effect of the aspect ratio of the elliptical cylinder (namely, AR = 0.5-2), the reduced velocity (namely, Ur = 2-30), and the splitter-plate length (namely, LSP/D = 0.75 and 2.5). Among numerous factors that exert impacts on the assembly’s FIV over an unlimited range of reduced velocity, the aspect ratio determines the nature and width of the synchronization branch in the amplitude response.

[Phys. Rev. Fluids 9, 054102] Published Mon May 06, 2024

### Identifying the body force from partial observations of a two-dimensional incompressible velocity field

Author(s): Aseel Farhat, Adam Larios, Vincent R. Martinez, and Jared P. Whitehead

An algorithm is developed, rigorously justified, and numerically implemented that is capable of determining the full body force used to generate chaotic, turbulent dynamics in two-dimensional Navier-Stokes fluid dynamics. The primary contribution of this result is that the accurate reconstruction of the force requires only partial observation of the state, i.e. sparse observations of the state are sufficient to recover not only the state itself but the unknown forcing function as well even in the fully developed turbulent setting.

[Phys. Rev. Fluids 9, 054602] Published Mon May 06, 2024

### Generation of interfacial waves by rotating magnetic fields

Author(s): Gerrit Maik Horstmann, Yakov Nezihovski, Thomas Gundrum, and Alexander Gelfgat

We present an experimental and theoretical study of interfacial waves generated in a two-phase swirling flow by a low-frequency (1 - 10 Hz) rotating magnetic field (RMF) oriented parallel to the interface. In contrast to surface waves excited by axial magnetic fields, we find that the first and dominant wave mode resembles a hyperbolic paraboloid. A good agreement of experiment with a linearized model was obtained. These results can have important implications for metallurgical processes and point the way to further research in the dynamics of the swirl flow, particularly by extending into the nonlinear regime.

[Phys. Rev. Fluids 9, 054801] Published Mon May 06, 2024

### Two-layer baroclinic turbulence with arbitrary layer depths

Author(s): Gabriel Hadjerci and Basile Gallet

While heat transport by baroclinic turbulence in oceans and planetary atmospheres is well described by a two-layer model, the relative depth of the two layers varies greatly depending on the situation of interest, making it an important parameter governing the transport properties of the system. Focusing on the low-drag turbulent regime, we extend the vortex-gas scaling theory to address the case of arbitrary layer depths.

[Phys. Rev. Fluids 9, L051802] Published Mon May 06, 2024

### Motion and deformation of capsules flowing through a corner in the inertial and non-inertial regimes

Author(s): Damien P. Huet, Antoine Morente, Guodong Gai, and Anthony Wachs

We investigate the inertial and noninertial dynamics of three-dimensional elastic capsules flowing through a square channel presenting a sharp corner. The channel Reynolds number Re ranges from 0.01 to 50 and the Capillary number Ca, which measures the ratio of the viscous and elastic stresses, ranges from 0.075 to 0.35. We report trajectory, surface area, velocity and membrane stress in the case of a single capsule, two capsules and a train of ten capsules released upstream of the corner. This study contributes to the elaboration of practical guidelines for controlling capsule breakup and predicting throughput in both inertial and noninertial microfluidic experiments.

[Phys. Rev. Fluids 9, 053601] Published Fri May 03, 2024

### Fluidic control of a precessing axisymmetric body by near-wake coupling

Author(s): Thomas J. Lambert, Bojan Vukasinovic, and Ari Glezer

The reciprocal coupled interactions between a forebody gimbaled axisymmetric bluff body, free to precess in pitch, yaw, and roll, and the body’s near wake are exploited for prescribing its attitude using fluidic actuation. It was shown that closed-loop control of pitch and yaw can significantly attenuate the inherent model’s baseline oscillations by more than 80%, or the control could be tuned to achieve a rapid large-amplitude response to amplify the natural yaw or pitch oscillations by more than 225% of the baseline motion. Either suppression or amplification of the natural oscillations of the model are associated with induced changes in the symmetry of the primary dynamical wake modes.

[Phys. Rev. Fluids 9, 053904] Published Fri May 03, 2024

### Noise-induced transitions past the onset of a steady symmetry-breaking bifurcation: The case of the sudden expansion

Author(s): Yves-Marie Ducimetière, Edouard Boujo, and François Gallaire

We consider flows subject to a steady symmetry-breaking bifurcation and forced by a weak noise acting on a slow timescale. By employing a multiple-scale weakly nonlinear expansion technique, we derive a stochastically forced Stuart-Landau equation for the dominant symmetry-breaking mode. The probability density function of the solution, and of the escape time from one attractor to the other, are then determined by solving the associated Fokker-Planck equation, which is made possible by the extremely low dimensionality of the amplitude equation. The validity of this reduced order model is then tested on the flow past a planar sudden expansion.

[Phys. Rev. Fluids 9, 053905] Published Fri May 03, 2024

### Barriers and chutes for mixing of active particles in a vortex chain flow

Author(s): Nghia Le, Casey M. Miller, Julianna S. Detrick, Cameron R. Lodi, Kevin A. Mitchell, and Thomas H. Solomon

Self-propelled (active) particles in laminar fluid flows are blocked by one-way barriers called “swimming invariant manifolds” (SwIMs). The SwIM theory is a generalization of manifold approaches previously applied to the mixing of passive tracers. We verify the blocking behavior of SwIMs experimentally for algae swimming in a vortex chain flow. The SwIMs also form chutes that transport swimmers between vortices with a predicted inter-vortex flux that is consistent with the experiments, despite noise and nonuniformities in the swimming directions of the microbes.

[Phys. Rev. Fluids 9, 054501] Published Fri May 03, 2024

### Spectrum of passive scalar carried by particles in isotropic turbulence

Author(s): Izumi Saito, Takeshi Watanabe, and Toshiyuki Gotoh

The turbulent transport of small particles (dust particles, cloud droplets, etc.) can be viewed macroscopically as a problem of passive scalar turbulence with extremely high Schmidt numbers. To investigate the spectrum of a passive scalar carried by such particles, we conducted Lagrangian particle simulations in homogeneous isotropic turbulence. The scalar variance spectrum obeys the -5/3 and -1 power laws in the lower and higher wavenumber ranges, respectively, with a clear transition at normalized wavenumber of about 0.04. The dimensionless constants for each range are also consistent with the estimations obtained from previous laboratory and simulation studies.

[Phys. Rev. Fluids 9, 054601] Published Fri May 03, 2024

### Rising droplets in a centrifugal field: A way to avoid interfacial contamination in liquid-liquid flow

Author(s): Hassan El Itawi, Benjamin Lalanne, Subhadarshinee Sahoo, Emmanuel Cid, Gladys Massiera, Nathalie Le Sauze, and Olivier Masbernat

Substituting a centrifugal field to gravity makes possible the study of the dynamics of rising deformable drops in another immiscible liquid, keeping free from contamination of the interface. In this experiment, impurities do not have time to adsorb during the very short residence time due to the strong acceleration. Experiments are used to validate direct numerical simulation results in liquid-liquid systems, and the aspect ratio of rising ellipsoidal droplets is found to be a growing function of the sole Weber number, with a smaller rate compared to clean bubbles.

[Phys. Rev. Fluids 9, L051601] Published Fri May 03, 2024

### Gyre turbulence: Anomalous dissipation in a two-dimensional ocean model

Author(s): Lennard Miller, Bruno Deremble, and Antoine Venaille

We unveil a gyre turbulence regime within a two-dimensional wind-driven ocean model, where energy dissipation becomes independent of fluid viscosity. This anomalous dissipation is driven by a vigorous two-dimensional vortex gas overlaying a low energy western-intensified gyre, shedding light on the effect of boundary instabilities in disrupting the inverse energy cascade.

[Phys. Rev. Fluids 9, L051801] Published Fri May 03, 2024

### Direct numerical simulation of backward-facing step turbulent flow controlled by wave-machine-like traveling wave

Author(s): Junichi Morita, Ryuichi Kimura, Hiroya Mamori, and Takeshi Miyazaki

A direct numerical simulation of turbulent flow over a backward-facing step is performed. To control flow separation, a wall-normal body force in the form of a wave-machine-like traveling wave is applied to the top surface of the step. In this control, the recirculation bubble is periodically released in time and in the streamwise direction. In addition, a pair of longitudinal vortices is generated above the step and in the recirculation bubble and the released separation bubble, which causes three effects that reduce the reattachment length: a decrease in the secondary bubble streamwise length, an enhancement of the negative wall-normal velocity, and an increase in Reynolds shear stress.

[Phys. Rev. Fluids 9, 053903] Published Thu May 02, 2024

### Amplification of the anomalous scaling in the Kazantsev-Kraichnan model with finite-time correlations and spatial parity violation

Author(s): E. Jurčišinová, M. Jurčišin, and R. Remecký

By using the field theoretic renormalization group technique together with the operator product expansion, simultaneous influence of the spatial parity violation and finite-time correlations of an electrically conductive turbulent environment on the inertial-range scaling behavior of correlation fun…

[Phys. Rev. E 109, 055101] Published Wed May 01, 2024

### Random walk model for dual cascades in wave turbulence

Author(s): Oliver Bühler

Dual cascades in turbulent systems with two conserved quadratic quantities famously arise in both two-dimensional hydrodynamic turbulence and also in wave turbulence based on four-wave interactions. Examples for the latter include surface waves and nonlinear Schrödinger equations with cubic nonlinea…

[Phys. Rev. E 109, 055102] Published Wed May 01, 2024

### Impact of harmonic inflow variations on the size and dynamics of the separated flow over a bump

Author(s): Himpu Marbona, Daniel Rodríguez, Alejandro Martínez-Cava, and Eusebio Valero

Direct numerical simulations examine the separated flow over a wall-mounted bump subjected to harmonic inflow oscillations, resembling the passage of wakes from the preceding stage along the suction side of low-pressure turbine blade conditions. Three scenarios unfold: (i) analog to the steady inflow where the separated-flow laminar-to-turbulent transition is initiated by self-sustained Kelvin-Helmholtz (KH) instability; (ii) intermittent large vortex cluster formation replacing the KH during a part of the inflow period; and (iii) a continuous vortex cluster formation and release that shorten the separated flow length compared to steady inflow, the desirable one for practical applications.

[Phys. Rev. Fluids 9, 053901] Published Wed May 01, 2024

### Measurement of pressure gradients near the interface in the viscous fingering instability

Author(s): Savannah D. Gowen, Thomas E. Videbæk, and Sidney R. Nagel

The iconic branching patterns of the viscous fingering instability, where one fluid invades another in a thin gap, are unconventionally visualized in an experiment that allows us to measure the velocity field throughout the fluid as the patterns form and grow. We find that local pressure gradients decay, both in front of and behind the interface, with a characteristic decay length. This elegant visualization allows us to see how the structure seeded at the interface continues to influence flow long after the instability onset. With this technique we capture features of the late-time flow field that have traditionally been hard to access by simulation or experiment.

[Phys. Rev. Fluids 9, 053902] Published Wed May 01, 2024

### Iterative methods for Navier-Stokes inverse problems

Author(s): Liam O'Connor, Daniel Lecoanet, Evan H. Anders, Kyle C. Augustson, Keaton J. Burns, Geoffrey M. Vasil, Jeffrey S. Oishi, and Benjamin P. Brown

Even when the partial differential equation underlying a physical process can be evolved forward in time, the retrospective (backward in time) inverse problem often has its own challenges and applications. Direct adjoint looping (DAL) is the defacto approach for solving retrospective inverse problem…

[Phys. Rev. E 109, 045108] Published Tue Apr 30, 2024