# Latest papers in fluid mechanics

### Erratum: Drag force on spherical particles trapped at a liquid interface [Phys. Rev. Fluids <b>7</b>, 124001 (2022)]

Author(s): Zhi Zhou, Petia M. Vlahovska, and Michael J. Miksis

[Phys. Rev. Fluids 8, 089901] Published Mon Aug 28, 2023

### Spatial distribution of coherent structures in a self-similar axisymmetric turbulent wake

Author(s): Weijun Yin, Shancong Tao, Koji Nagata, Yasumasa Ito, Yasuhiko Sakai, and Yi Zhou

The coherent structures in a self-similar axisymmetric turbulent wake are orderly distributed and also regularly organized. There is an intrinsic connection between the self-similar distributions of the one-point statistics and the spatial distribution of the coherent structures. We confirm the well-known assertion that coherent structures are “the sinews and muscles of fluid motions”.

[Phys. Rev. Fluids 8, 084603] Published Thu Aug 24, 2023

### Enhancing the shear-stress-transport turbulence model with symbolic regression: A generalizable and interpretable data-driven approach

Author(s): Chenyu Wu and Yufei Zhang

Data-driven Reynolds averaged Navier-Stokes (RANS) turbulence models for separated flows based on black-box machine learning models have been widely researched in recent years. However, they often lack generalizability and interpretability. In this work, field inversion and symbolic regression (FISR) are used to develop an interpretable and generalizable data-driven RANS turbulence model. The proposed turbulence model shows good accuracy in various test cases that are completely distinct from its training set.

[Phys. Rev. Fluids 8, 084604] Published Thu Aug 24, 2023

### Underlying physics of mixing efficiency for shear-forced, stratified turbulence

Author(s): Young R. Yi and Jeffrey R. Koseff

Large scale ocean simulations employ eddy viscosities and diffusivities to represent the irreversible mixing of momentum and scalars due to unresolved scales of motion. These closures often require a prescription of the ratio of the vertical buoyancy flux to the rate of turbulence production (i.e., mixing efficiency). In this paper, we demonstrate that this ratio is strongly sensitive to the turbulence generation mechanism by considering the Reynolds stress and buoyancy flux budgets as a function of the turbulent Froude number.

[Phys. Rev. Fluids 8, 084803] Published Wed Aug 23, 2023

### Role of interfacial rheology on fingering instabilities in lifting Hele-Shaw flows

Author(s): Írio M. Coutinho and José A. Miranda

The lifting Hele-Shaw cell setup is a popular modification of the classic, fixed-gap, radial viscous fingering problem. In the lifting cell configuration, the upper cell plate is lifted such that a more viscous inner fluid is invaded by an inward-moving outer fluid. As the fluid-fluid interface cont…

[Phys. Rev. E 108, 025104] Published Tue Aug 22, 2023

### Measurements of an axisymmetric hypersonic shear-layer instability on a cone-cylinder-flare in quiet flow

Author(s): Elizabeth K. Benitez, Matthew P. Borg, Pedro Paredes, Steven P. Schneider, and Joseph S. Jewell

Compression corners are commonly present along aircraft, such as at control surfaces, and can cause shock/boundary-layer interactions when the vehicle moves at supersonic or hypersonic speeds. By generating a separation bubble at the corner, a shear layer is present in the flow, which can potentially amplify instabilities, leading to a boundary-layer transition downstream of reattachment. We show, through experiments conducted under low-disturbance Mach-6 flow, the amplification of such a shear-layer instability by comparing measurements from two cone-cylinder-flare geometries with computations of the same.

[Phys. Rev. Fluids 8, 083903] Published Tue Aug 22, 2023

### Reinforcement-learning-based parameter optimization of a splitter plate downstream in cylinder wake with stability analyses

Author(s): Chengyun Wang, Peng Yu, and Haibo Huang

We employ a single-step Deep Reinforcement Learning (DRL) algorithm to optimize the spatial location and length of a downstream splitter plate, aimed at suppressing vortex shedding behind a cylinder. Contrary to common sense, positioning the splitter plate laterally rather than directly in the posterior of the cylinder can make a greater influence on the stability of the entire flow field, even with a shorter length. By integrating DRL techniques, stability analysis via dynamic mode decomposition (DMD), and comparisons with Bayesian optimization (BO), the study highlights the potential of this DRL algorithm for advanced optimization in fluid dynamics.

[Phys. Rev. Fluids 8, 083904] Published Tue Aug 22, 2023

### Drag on a partially immersed sphere at the capillary scale

Author(s): Robert Hunt, Ze Zhao, Eli Silver, Jinhui Yan, Yuri Bazilevs, and Daniel M. Harris

Drag on a sphere in a steady flow is an important and thoroughly studied problem in fluid dynamics, yet relatively little work includes the effects of a free surface. Through experiments and simulations, we characterize the drag on a sphere partially immersed at an air-water interface as a function of submergence depth. The presence of the free surface can induce drag forces several times greater than the equivalent fully submerged case, with pronounced hysteretic effects strongly influenced by the sphere’s wettability. We ascribe the drag increase to an asymmetric pressure loading on the sphere that persists for flows both above and below the minimum capillary-gravity wave speed.

[Phys. Rev. Fluids 8, 084003] Published Tue Aug 22, 2023

### Mechanisms of drag reduction by semidilute inertial particles in turbulent channel flow

Author(s): Himanshu Dave and M. Houssem Kasbaoui

We show that significant drag reduction can be achieved in a turbulent channel flow using appropriately scaled mono-disperse particles. We show that drag-reducing particles form very long ropes along the channel wall. These particle ropes interfere with the near-wall turbulence regeneration mechanisms, which causes partial re-laminarization of the flow and significant reduction of skin-friction drag.

[Phys. Rev. Fluids 8, 084305] Published Mon Aug 21, 2023

### Quantifying inner-outer interactions in noncanonical wall-bounded flows

Author(s): Mogeng Li, Woutijn J. Baars, Ivan Marusic, and Nicholas Hutchins

We investigate the underlying physics behind the change in amplitude modulation coefficient in noncanonical wall-bounded flows in the framework of the inner-outer interaction model (IOIM). An analytical relationship between the amplitude modulation coefficient and IOIM parameters is derived, which is shown to capture the increasing trend of the amplitude modulation coefficient with an increasing Reynolds number in a smooth-wall dataset. This relationship is then applied to classify and interpret the results of a turbulent boundary layer after a rough-to-smooth change in this work, as well as other noncanonical flow in the literature.

[Phys. Rev. Fluids 8, 084602] Published Mon Aug 21, 2023

### Swirling against the forcing: Evidence of stable counterdirected sloshing waves in orbital-shaken reservoirs

Author(s): Alice Marcotte, François Gallaire, and Alessandro Bongarzone

We report robust experimental evidence of the bistability of sloshing waves in circular cylindrical containers following planar and time-harmonic elliptic orbits. Specifically, we observed for the first time the counterintuitive existence of stable counter-swirling waves. Such novel experimental findings support previous theoretical studies and are here further rationalized by an asymptotic analysis, which shows that this archetypal resonant sloshing system can be well described by 4 degrees-of-freedom only.

[Phys. Rev. Fluids 8, 084802] Published Mon Aug 21, 2023

### Coalescence of bubbles in a viscoelastic liquid

Author(s): Alexandros T. Oratis, Vincent Bertin, and Jacco H. Snoeijer

The coalescence of bubbles dissolved in liquids occurs in systems like foams, magma, and various other bubbly flows. Here, we investigate the coalescence of two bubbles of identical size in semidilute aqueous polymer solutions. The rheological properties are very different from those of water; yet, the early-time dynamics of coalescence are not altered by the presence of polymers. Even though the polymers get highly stretched in the azimuthal direction, we find that the resulting hoop force is insufficient to modify the rate of coalescence or the shape of the growing neck.

[Phys. Rev. Fluids 8, 083603] Published Fri Aug 18, 2023

### Sharp depletion of radial distribution function of particles due to collision and coagulation inside turbulent flow: A systematic study

Author(s): Xiaohui Meng and Ewe-Wei Saw

In turbulence, particle collision-and-coagulation significantly reduce the number of particle pairs at separation distances closer than about 2.5 times particle diameter. This results in a noticeable decrease in the particle radial distribution function (RDF) compared to when collisions are absent. By examining the changes in the RDF and its relationship with particle and turbulence parameters, we can see how these physical factors influence the RDF.

[Phys. Rev. Fluids 8, 084304] Published Fri Aug 18, 2023

### Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: Scaling properties and relation to upcoming DRESDYN-MRI experiment

Author(s): Ashish Mishra, George Mamatsashvili, and Frank Stefani

Magnetorotational instability (MRI) is the most likely mechanism driving angular momentum transport in astrophysical disks. However, there is no conclusive experimental evidence for MRI, despite many attempts to find it. The planned DRESDYN-MRI experiments are a new effort which use a magnetized Taylor-Couette flow of liquid sodium that mimics an accretion disk. We numerically study the nonlinear evolution and saturation of MRI and analyze its scaling behavior for the DRESDYN-MRI device. The obtained scaling laws with Reynolds number, yielding the magnitudes of velocity and magnetic field perturbations expected in these experiments, will be crucial for identifying MRI in the laboratory.

[Phys. Rev. Fluids 8, 083902] Published Thu Aug 17, 2023

### Curvature scaling of the transient convective boundary layer flow along a vertical cylinder: An improved explicit form

Author(s): Yang Liu, Yifeng Zhu, and Changhui Liu

The scaling method is used to study the transient natural convection boundary layer flow, and the curvature effect is investigated. Important scaling laws, such as boundary layer thickness δt and characteristic velocity umz of the transient and steady states and cut-off time of the initial growth are determined in the explicit form, where the curvature effect is accurately described by the proposed dimensionless coefficient Ψ′(A). The present explicit laws provide clearer physical interpretation, make the scaling laws much easier to utilize, and facilitate a better understanding of the underlying physical phenomena governing the curved boundary layer flow.

[Phys. Rev. Fluids 8, 084101] Published Thu Aug 17, 2023

### Impact of surface rheology on droplet coalescence in uniaxial compressional flow

Author(s): Natasha Singh and Vivek Narsimhan

When two droplets come close together to coalesce, a thin liquid film forms between the droplets. The drainage of this film alters the critical conditions under which coalescence occurs. In this study, we explore how the interfacial surface shear and dilational viscosity affect the thin film formation and drainage time in a head-on collision between two droplets. We model the surface rheology of the droplet using the Boussinesq–Scriven constitutive relationship for a Newtonian interface.

[Phys. Rev. Fluids 8, 083602] Published Wed Aug 16, 2023

### Transport of inertial ellipsoidal particles in turbulent flow over rough walls

Author(s): D. Saccone, M. De Marchis, B. Milici, and C. Marchioli

This study examines combined effects of particle inertia, particle aspect ratio, and wall roughness on dynamics of prolate ellipsoidal particles dispersed in turbulent channel flow bounded by rough walls. We use direct numerical simulation of the turbulence and Lagrangian tracking of the particles. The particles spatial distribution within the flow domain and their translational statistics is shown to depend almost exclusively on the combined effect of inertia and wall roughness, with weak aspect ratio effects. These effects are observed in the particle orientation statistics: Longer particles exhibit a stronger tendency to align with the mean flow direction, especially at low inertia.

[Phys. Rev. Fluids 8, 084303] Published Wed Aug 16, 2023

### Instabilities of rotating inclined buoyancy layers

Author(s): Y. Xiao, J. J. Tao, and F. H. Busse

The boundary layer near a cooled inclined plate, which is immersed in a stably stratified fluid rotating about an axis parallel to the direction of gravity, is a model for katabatic flows at high latitudes. In this paper the base flow of such an inclined buoyancy layer is solved analytically for arb…

[Phys. Rev. E 108, 025102] Published Tue Aug 15, 2023

### Pattern formation of spherical particles in an oscillating flow

Author(s): T. J. J. M. van Overveld, H. J. H. Clercx, and M. Duran-Matute

We study the self-organization of spherical particles in an oscillating flow through experiments inside an oscillating box. The interactions between the particles and the time-averaged (steady streaming) flow lead to the formation of either one-particle-thick chains or multiple-particle-wide bands, …

[Phys. Rev. E 108, 025103] Published Tue Aug 15, 2023

### Unified scaling law for wall friction in laterally confined flows of shape anisotropic particles

Author(s): Antonio Pol, Riccardo Artoni, and Patrick Richard

Using particle-based simulations, we study the wall friction weakening in three-dimensional dense, confined granular flows made of shape anisotropic particles. We show how the effective wall friction is affected by both particle shape and flow pattern. Then, we propose a new scaling law for the friction mobilization at the walls which is based on a balance between sliding and rolling motion of the particles. This study highlights the importance of angular motion of the particles for the understanding of granular flows at flat boundaries and may reignite the debate about the relevant variables in theories aiming to capture and predict the behavior of dense and confined granular flows.

[Phys. Rev. Fluids 8, 084302] Published Mon Aug 14, 2023