# Latest papers in fluid mechanics

### Evaporation of active drops: Puncturing drops and particle deposits of ring galaxy patterns

Author(s): Ghansham Rajendrasingh Chandel, Vishal Sankar Sivasankar, and Siddhartha Das

Active drops through active stresses alter drop evaporation dynamics, puncture evaporating drops, and induce an inside-out evaporation. For an active drop with vortex defect, activity can extend contractile drop evaporation lifetimes by 50% and can accelerate extensile drop evaporation by 33%. Non-intuitive deposition patterns emerge due to evaporation induced fluid flows where the fluid is dragged towards both outer and newly formed inner contact lines. Potential applications of such active drop evaporation include enhanced drop longevity in biological contexts and customizable thin film deposits.

[Phys. Rev. Fluids 9, 033603] Published Wed Mar 27, 2024

### Effective water/water contact angle at the base of an impinging jet

Author(s): Théophile Gaichies, Anniina Salonen, Arnaud Antkowiak, and Emmanuelle Rio

The base of a jet impinging on an ultrapure water bath is studied experimentally. At the impact point, a train of capillary waves develops along the jet. A striking observation is the existence of an effective nonzero water/water contact angle between the jet and the meniscus. The rationalization of this finite contact angle requires a full description of the shape of the interface. By doing an analytical matching between the meniscus and the jet, we show that the capillary waves can be considered as reflected waves present to ensure pressure continuity. It is finally shown that the value of the apparent contact angle is fixed by energy minimization.

[Phys. Rev. Fluids 9, 034003] Published Wed Mar 27, 2024

### Finite volume fraction effect on self-induced velocity in two-way coupled Euler-Lagrange simulations

Author(s): Jungyun Kim and S. Balachandar

The present work addresses the effect of non-zero volume fraction in predicting the self-induced velocity of particles in an Euler-Lagrange (EL) simulation. Hundreds of EL simulations of flow over a random distribution of stationary particles, covering a range of Reynolds number and volume fraction, are performed to calculate the undisturbed flow and the self-induced velocities of particles in the presence of neighbors. The most significant finding is that the self-induced correction procedure of an isolated particle can be applied even at finite volume fraction, with a simple volume fraction dependent modification. However, the perturbation induced by neighbors often has a much larger effect.

[Phys. Rev. Fluids 9, 034306] Published Wed Mar 27, 2024

### Model for the cyclonic bias of convective vortices in a rotating system

Author(s): Jenny Dingwall and John R. Taylor

We address the long-standing mystery surrounding the rotational bias of convective vortices in the atmosphere (dust devils) and the ocean. We investigate the bias using large-eddy simulations of free convection configured for the ocean, but the idealization of our simulations makes the results more broadly relevant to a wide range of flows. We propose a theory that the addition of many small convective vortices, each of which exhibit a small bias, leads to a much more significant bias for large convective vortices. We apply this new theory to typical convective conditions in the ocean and the terrestrial and Martian atmospheres.

[Phys. Rev. Fluids 9, 033503] Published Tue Mar 26, 2024

### Film drop production over a wide range of liquid conditions

Author(s): Daniel B. Shaw and Luc Deike

A bursting bubble’s production of film drops - liquid from the bubble’s cap - is experimentally measured as a function of salinity, temperature, surfactant concentration, and viscosity. Existing theories for the film drainage rate and number of film drops are shown to be robustly consistent with the experimental results, but the lifetime of a bubble across various conditions remains poorly described by existing scalings. The reported relationships describing the importance of various physico-chemical variables on ocean spray emissions are of particular significance to the atmospheric and ocean science community as sea spray aerosols impact radiative transfer and cloud seeding.

[Phys. Rev. Fluids 9, 033602] Published Tue Mar 26, 2024

### Suppression of viscous fingering during perfect dielectric fluid displacement using transverse electric fields

Author(s): Jiachen Zhao, Zhongzheng Wang, and Emilie Sauret

This study numerically and theoretically investigates the control of viscous fingering using a transverse electric field. Viscous fingering can be delayed in the presence of an electric field due to the electrical force distribution along the fluid interface and completely suppressed when the electric field strength exceeds a certain value. A non-dimensional parameter ϕ is defined based on the force balance, which shows good capability for predicting the transition from an unstable viscous fingering to stable displacement under various fluid properties and flow conditions.

[Phys. Rev. Fluids 9, 033701] Published Tue Mar 26, 2024

### Flow kinematics model for universal Strouhal number in the separated flow past a bluff body

Author(s): A. Barrero-Gil and A. Velazquez

This letter revisits the problem of the existence of a universal Strouhal number in the separated flow past a stationary bluff body. Theoretical considerations grounded in flow kinetics yield a concise mathematical expression. The resultant model anticipates a universal Strouhal number equal to 1/2π≈0.159, aligning closely with experimental finding. Furthermore, the model has been expanded to forecast the Strouhal – Reynolds relationship in the supercritical regime, demonstrating a universal nature as it relies solely on critical Strouhal and Reynolds numbers. The circular, square, and triangular cross-section shapes have been used for validation purposes.

[Phys. Rev. Fluids 9, L032701] Published Tue Mar 26, 2024

### Correlations in a weakly interacting two-dimensional random flow

Author(s): I. V. Kolokolov, V. V. Lebedev, and V. M. Parfenyev

We analytically examine fluctuations of vorticity excited by an external random force in two-dimensional fluid. We develop the perturbation theory enabling one to calculate nonlinear corrections to correlation functions of the flow fluctuations found in the linear approximation. We calculate the cor…

[Phys. Rev. E 109, 035103] Published Mon Mar 25, 2024

### Equilibrium bridge solution from a sessile drop partially covered by another fluid

Author(s): P. D. Ravazzoli, A. G. González, and J. A. Diez

Liquid bridges are present in different four-phase systems, like the one studied here, which is formed by a liquid connecting a horizontal solid substrate with a gas phase while surrounded by another immiscible liquid. We obtain the equilibrium solutions by treating each interface as a simple curve with axial symmetry and constant curvature, and satisfying the boundary conditions given by Neumann’s and Young’s laws. Therefore, the final equilibrium solution is formed by a union of a spherical cap, a catenoid, and portions of onduloids or nodoids. We find that the γ-angle is the control parameter of the problem, as it defines the existence and shape of the final equilibrium solution.

[Phys. Rev. Fluids 9, 033601] Published Mon Mar 25, 2024

### Nonaxisymmetric modes of magnetorotational and possible hydrodynamical instabilities in the upcoming DRESDYN-MRI experiments: Linear and nonlinear dynamics

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

Magnetorotational instability (MRI) is responsible for angular momentum transport in astrophysical disks. However, its definitive experimental confirmation is still missing. The upcoming DRESDYN-MRI experiments using a liquid sodium Taylor-Couette flow are a new effort to detect MRI. In preparation for this, here we study the dynamics of nonaxisymmetric MRI via simulations for the DRESDYN-MRI device. It is shown that nonaxisymmetric modes are important for large Re≳4×104 and relevant in these experiments with Re≳106. These modes are, however, of nonmagnetic nature, which grow and form a turbulent boundary layer near the cylinders, while axisymmetric MRI dominates in the bulk flow.

[Phys. Rev. Fluids 9, 033904] Published Mon Mar 25, 2024

### Model for the dynamics of the large-scale circulations in two-layer turbulent convection

Author(s): Yu Sun, Yi-Chao Xie, Jin-Xiao Xie, Jin-Qiang Zhong, Jianwei Zhang, and Ke-Qing Xia

A physically motivated low-dimensional model describes properly the interaction of two vertically-aligned large-scale circulations (LSC) in two-layer turbulent convection, and predicts their preferred flow states of thermal and viscous coupling. The model reveals that flow reversals can be achieved when turbulent fluctuations drive the LSC azimuthal diffusion into a flow state such that the two LSC planes are orthogonal to each other, the strength of the LSC in the high Rayleigh number fluid layer then reduces to zero deterministically. The model provides satisfactory interpretation for the high occurrence frequency of flow reversals observed in two-layer turbulent convection.

[Phys. Rev. Fluids 9, 033501] Published Fri Mar 22, 2024

### Turbulent thermal convection across a stable liquid-liquid interface

Author(s): Hailong Huang, Yin Wang, Wei Xu, Xiaozhou He, and Penger Tong

In this work, we conducted a systematic experimental study of turbulent two-layer convection in an upright cylinder, as illustrated. From the measured mean temperature and temperature variance profiles, we find a unique twin-boundary-layer structure across the liquid interface with one of the twin boundary layers (BLs) on each side of the interface. The functional form of the measured mean temperature and temperature variance profiles is well described by the equations for a BL attaching to a solid conducting plate, so long as a thermal slip length is introduced to account for the convective heat flux passing through the liquid interface.

[Phys. Rev. Fluids 9, 033502] Published Fri Mar 22, 2024

### Wake characteristics behind a tidal turbine with surface waves in turbulent flow analyzed with large-eddy simulation

Author(s): Pablo Ouro, Hannah Mullings, Aristos Christou, Samuel Draycott, and Tim Stallard

Understanding of the influence of waves on a tidal turbine wake in a turbulent channel flow is developed using large eddy simulations with modeling of the air-water interphase deformation. For wavelengths between two- and sixteen-times channel depth, the rate of wake recovery is faster than in the absence of waves with only small differences in turbine mean loading. The smallest wavelengths cause fastest wake recovery, and hence shortest wake length. This is due to enhanced vertical and transverse components of convection and turbulent transport of mean kinetic energy. Advection of helical tip vortices shed by the turbine blades is impacted by vertical and streamwise wave kinematics.

[Phys. Rev. Fluids 9, 034608] Published Fri Mar 22, 2024

### Unbounded two-dimensional wall turbulence induced by inverse cascade

Author(s): Xi Chen, Peng-Yu Duan, and Jianchao He

What would be the ultimate statistical invariance for wall turbulence? In 3D flows, whether turbulent fluctuations are bounded or unbounded for a series of quantities when the Reynolds number (Re) tends to infinity, is debated. Here, we focus on 2D wall turbulence and find that fluctuations in wall units such as root mean square of wall pressure, wall shear stress intensity, streamwise velocity fluctuation, etc., all exhibit a distinct Re1/3 scaling law. This scaling indicates an unbounded behavior for 2D flows due to the inverse energy cascade, which sheds light on the asymptotical behavior of 3D flows that fluctuations might be bounded because of the forward energy cascade.

[Phys. Rev. Fluids 9, 034609] Published Fri Mar 22, 2024

### Initial evolution of three-dimensional turbulence en route to the Kolmogorov state: Emergence and transformations of coherent structures, self-similarity, and instabilities

Author(s): Giorgio Krstulovic and Sergey Nazarenko

The development of singular structures and the path to turbulence in fluids is a fundamental problem that has puzzled researchers for a long time. In this work, we study numerically the evolution of a large-scale initial condition under the hyperviscous incompressible Navier-Stoke equations. Our observations revealed the emergence of shrinking vortex pancakes, which lead to vortex ribs and, eventually, vortex ropes, as depicted in the figure. Eventually, a fully developed turbulent state was achieved prior to its viscous decay.

[Phys. Rev. Fluids 9, 034610] Published Fri Mar 22, 2024

### Vortex dynamics: A variational approach using the principle of least action

Author(s): Nabil M. Khalifa and Haithem E. Taha

Vortices are found everywhere around us at every scale. Current models describing vortices are mainly kinematics-based, in which the resulting dynamics are limiting in their nature. Relying on these models, one can predict vortices response for a prescribed initial condition but cannot study their response under generic conditions and external disturbances. That is because current models are a well devised formulation. However, we propose a model from first-variational principles relying on the Principle of Least Action, and not only was it able to predict the vortices response in a generic sense, but also it can be extended to include varying strength vortices or any dynamical constraints.

[Phys. Rev. Fluids 9, 034701] Published Fri Mar 22, 2024

### Critical slope singularities in rotating and stratified fluids

Author(s): Stéphane Le Dizès

Gravito-inertial waves propagate in fixed directions and exhibit a critical slope singularity whenever one of these directions is tangent to a surface boundary. The nature of this singularity is analyzed. It is shown how it governs the scaling and the structure of the intense viscous beam that it generates.

[Phys. Rev. Fluids 9, 034803] Published Fri Mar 22, 2024

### Wake transition of an unconstrained self-propelled flexible flapping plate

Author(s): Kui Liu and Haibo Huang

This study numerically investigates the wake transition of an unconstrained self-propelled flexible flapping plate, which can move freely both longitudinally and laterally. Three distinct wake patterns, including symmetric, deflected, and chaotic, are identified quantitatively. The symmetry breaking will be triggered when the cruising and flapping Reynolds number as well as translational kinetic energy reach critical values, which follow some simple scaling laws. it is also revealed that passive lateral oscillation and bending deformation of the plate are two key mechanisms affecting wake symmetry properties.

[Phys. Rev. Fluids 9, 033102] Published Thu Mar 21, 2024

### Description of laminar-turbulent transition of an airfoil boundary layer measured by differential image thermography using directed percolation theory

Author(s): Tom T. B. Wester, J. Peinke, and G. Gülker

The laminar-turbulent transition still poses a challenging problem to fluid dynamic research. This study shows how differential image thermography can be utilized to capture the spatiotemporal aspects of this phenomenon along the curved surface of an airfoil. Further, an incredible agreement between the transition characteristics and the (1+1)D directed percolation theory is observed for a broad range of experimental parameters, namely angle of attack and inflow velocity.

[Phys. Rev. Fluids 9, 033903] Published Thu Mar 21, 2024

### Turbophoresis and preferential accumulation of inertial particles in compressible turbulent channel flow: Effect of Mach number

Author(s): Ping Wang, Bowen Zhou, and Xiaojing Zheng

This paper investigates the effects of the Mach number on turbophoresis and the preferential accumulation of particles. A particle relaxation time weighting transformation is proposed which collapses the concentration profiles not only for particles with small inertia, but also for turbulence with various compressibility at the studied low Reynolds number and in the inner region. Inertial particles in compressible wall turbulence tend to distribute in high-fluid-density and negative-fluid-dilatation regions. This is explained by the changes in turbulent structures at varying Mach number, which in turn also account for the varying scales of particle streaks.

[Phys. Rev. Fluids 9, 034305] Published Thu Mar 21, 2024