Latest papers in fluid mechanics

Author(s): Nikolai Kubochkin, Tatiana Gambaryan-Roisman, and Vladimir S. Ajaev

Evaporation on rough or structured surfaces is of interest from both fundamental and practical points of view. In this work, we consider volatile droplets and rivulets with contact lines pinned to wedge-shaped elements of the substrate topography. We develop a local analytical model of the evaporation-induced flow in the vicinity of the contact line in both liquid and gas phases, analyze the influence of gas viscosity as well as of an opening angle of the wedge on the flow patterns, and obtain critical contact angles, above which the eigenmode solution dominates the evaporation-induced solution.

[Phys. Rev. Fluids 9, 094007] Published Mon Sep 23, 2024

Author(s): Lei Yan (严雷), Xingming Zhang (张星明), Jie Song (宋杰), and Gang Hu (胡钢)

This study proves the effectiveness of deep reinforcement learning (DRL) as a valuable tool for addressing complicated active flow control challenges, especially when employing flow fields characterized by strong nonlinearity and various wind attack angles. It demonstrates that employing multiple jets and surface pressure probes can achieve an ideal control performance, effectively diminishing aerodynamic forces and optimizing flow stability around the square cylinder under different wind attack angles. These findings enhance the potential for the practical application of DRL-based flow control strategies in engineering, and further progress toward real-world applications.

[Phys. Rev. Fluids 9, 094607] Published Mon Sep 23, 2024

Author(s): Revaz D. Chachanidze, Othmane Aouane, Jens Harting, Christian Wagner, and Marc Leonetti

Margination, the migration of leukocytes to the vessel wall, is a well-known physiological process. However, the role of cell stiffness in this phenomenon remains unclear. Our study investigates the segregation of stiffened red blood cells (RBCs) within healthy cell suspensions. Unexpectedly, we found central and corner peaks in the stiffened RBC distribution and a nonmonotonic dependency on the flow rate. Length scales for margination are found to be much longer than typical distances between bifurcations in-vivo, questioning the importance of margination in physiological situations.

[Phys. Rev. Fluids 9, L091101] Published Mon Sep 23, 2024

Author(s): Omair A. A. Mohamed and Luca Biancofiore

We investigate the role that vapor diffusion plays in the evolution of an evaporating liquid film using a coupled liquid-vapor system in which the evaporation rate is dictated by both the film’s thickness and its curvature. Under this kinetic-diffusion model, the thermocapillary Marangoni effect is split into two distinct components: the first results from surface tension gradients driven by uneven heating while the second arises from surface tension gradients caused by imbalances in vapor diffusion. Notably, these two components interact with evaporative mass loss and vapor recoil in a rich and complex manner, which we analyze within the temporal and spatiotemporal frameworks.

[Phys. Rev. Fluids 9, 094006] Published Fri Sep 20, 2024

Author(s): Wenqiao Jiao, David Scheidweiler, Nolwenn Delouche, Pietro de Anna, and Alberto Guadagnini

The traditional Kozeny-Carman formulation does not predict the permeability of complex porous structures. We develop an original model for the characterization of the intrinsic permeability of porous media with spatially heterogeneous pore size distributions. By conceptualizing the medium as a collection of smaller-scale porous units in series, our model captures spatial variability and aligns with microfluidics experiments on designed complex structures. Our model offers a fresh perspective beyond the traditional Kozeny-Carman formulation, enhancing our understanding of how pore size variability influences the overall medium permeability.

[Phys. Rev. Fluids 9, 094102] Published Fri Sep 20, 2024

Author(s): Jessie Liu, Florian Schäfer, Spencer H. Bryngelson, Tamer A. Zaki, and Ali Mani

Reynolds-averaged Navier—Stokes (RANS) closure operators are generally nonlocal and anisotropic, for example in wall-bounded turbulence. We introduce a computationally efficient approach to obtain these operators, using an adjoint formulation. We then quantify the streamwise and wall-normal nonlocal eddy viscosity in turbulent channel flow, which can be used to guide closure modeling.

[Phys. Rev. Fluids 9, 094606] Published Fri Sep 20, 2024

Author(s): Yifeng Yang and Luofeng Huang

Floating solar panels installed on water reservoirs are gradually becoming an increasingly popular renewable energy scenario. When the reservoir gate is opened to release water, complex interactions between an incoming current and the floating panels will occur. In this paper, by modeling the entire floating panel structure as a thin elastic plate, a mathematical model based on the linearized potential flow theory is established to investigate such interactions. Extensive analyses are conducted on the wave profile and plate deflection, revealing significant fluid resonance phenomena at certain current speeds.

[Phys. Rev. Fluids 9, 094804] Published Fri Sep 20, 2024

Author(s): Yuxue Zhong, Jingzhu Wang, Jianlin Huang, and Yiwei Wang

A new and interesting phenomenon is found during the interaction between cavitation bubbles and elastic membranes: The collapse of the spark-induced bubble generates a high-speed jet. When the jet impacts the elastic membrane, significant membrane deformation occurs, accompanied by secondary cavitation. To analyze the mechanism of the secondary cavitation, fluid acceleration is introduced through PIV experiments to define the dimensionless inertial force. Secondary cavitation is triggered when the dimensionless inertial force surpasses the dimensionless pressure difference.

[Phys. Rev. Fluids 9, 093604] Published Thu Sep 19, 2024

Author(s): Z. Taebel, M. L. McAllister, A. Scotti, M. Onorato, and T. S. van den Bremer

Experimental efforts to corroborate the theoretical Kolmogorov-Zakharov spectrum of surface gravity wave turbulence (WT) have encountered obstacles in the form of finite-size effects and intermittency. We investigated whether the consequences of these dynamics could be outcompeted by rigorously enforcing the assumption of WT in an idealized environment, in which we supply isotropic forcing and random-phased waves. We find that even under these conditions the wave field is modulated by dissipation and intermittency, which we study with higher-order statistics. Nevertheless, we do observe evidence of a wave-driven energy cascade beneath the strongly nonlinear and dissipative effects.

[Phys. Rev. Fluids 9, 094803] Published Thu Sep 19, 2024

Author(s): Kyle I. McKee, Bauyrzhan K. Primkulov, Kotaro Hashimoto, Yoshiyuki Tagawa, and John W. M. Bush

This study elucidates the origin of traveling waves observed on the lower surface of a levitating droplet rolling on a rotating cylindrical drum. The research begins with a simplified model of the lubrication flow beneath the droplet and examines the linear stability of this base state to Tollmien–Schlichting-type perturbations. By solving the Orr-Sommerfeld equation perturbatively, the study predicts the wavelength and phase velocity of the most unstable mode, yielding good agreement with experimental observations.

[Phys. Rev. Fluids 9, 093603] Published Tue Sep 17, 2024

Author(s): Huiyong Feng, Haibo Huang, Jian Hou, Chao Li, and Bei Wei

Polymer flooding is a popular method for enhancing oil recovery in the field of oil extraction. Experimental studies have shown that during injection, polymers experience significant viscosity loss due to shear degradation. To address the issues, a method of encapsulating polymers by synthesizing micro-nano capsules is proposed. The variation of viscosity of capsule suspension during the process of capsule rupture and polymer release are explored in details and an available law that relates suspension viscosity is established.

[Phys. Rev. Fluids 9, 093602] Published Mon Sep 16, 2024

Author(s): Basheer A. Khan, Shai Arogeti, and Alexander Yakhot

This figure displays the iso-surfaces of a turbulent puff’s azimuthal velocity. We have demonstrated that azimuthal motion is the primary cause of triggering the onset of turbulence, a chaotic state. Puffs abruptly break out of a chaotic state at Reynolds numbers Re < 1870, then decay exponentially. We found that the decay rate is entirely consistent with the cubic expression Sreenivasan used in 1979, but only after adding a constant.

[Phys. Rev. Fluids 9, 093903] Published Mon Sep 16, 2024

Author(s): P. Pirdavari, H. Tran, Z. He, and M. Y. Pack

This paper describes how the drainage of capillary tubes in the presence of surfactants drive spontaneous thin film climbing events which are limited by the competition among advection, diffusion, and adsorption/desorption kinetics.

[Phys. Rev. Fluids 9, 094005] Published Mon Sep 16, 2024

Author(s): Palas Kumar Farsoiya, Stéphane Popinet, Howard A. Stone, and Luc Deike

We present an open-source Direct Numerical Simulation framework to analyze surfactant-laden flows. With adaptive mesh refinement and parallelization, this tool enables researchers to explore the effects of surfactants on interfacial flows, particularly their impact on rising bubbles. The simulations show that surfactants slow down bubbles and alter their trajectory. Such numerical frameworks on the solutal Marangoni effect are crucial for understanding and predicting the behavior of multiphase flows in natural and industrial processes.

[Phys. Rev. Fluids 9, 094004] Published Fri Sep 13, 2024

Author(s): Sukhdev Mouraya, Nandita Pan, and Supratik Banerjee

A universal energy cascade is studied for incompressible ferrofluid turbulence by means of exact relations. Under weak external magnetic field, kinetic and total energy cascades occur at similar rates. Upon increasing the strength of the external magnetic field, the total energy cascade becomes nonstationary and occurs at a rate different from that of the kinetic energy cascade. However, the scale independent nature of the cascade remains universal.

[Phys. Rev. Fluids 9, 094604] Published Fri Sep 13, 2024

Author(s): A. Mohamed, A. Delache, F. S. Godeferd, J. Liu, M. Oberlack, and Y. Wang

This study investigates the focusing of inertial waves (IW) generated by an axisymmetric torus oscillating in a rotating fluid. A full range of vertical kinetic energy propagation angles at the focal point was explored using direct numerical simulations (DNS). A systematic comparison was made between linear DNS and nonlinear DNS. It was found that there is an optimal angle that maximizes energy transfer from the torus to the focal zone. In addition, triadic IW resonances were identified as a source of turbulence and a large central vertical vortex was also identified in agreement with the theory of Davidson et al. (2006).

[Phys. Rev. Fluids 9, 094605] Published Fri Sep 13, 2024

Author(s): Diederik Beckers and Jeff D. Eldredge

This study uses deep reinforcement learning to design airfoil pitch control for minimizing lift variations in disturbed flows. Tested in both classical unsteady and nonlinear viscous flow environments, the reinforcement learning controller, enhanced with wake information from pressure sensors and memory of past observations, matches or exceeds the performance of traditional linear controllers. The findings highlight the potential of reinforcement learning for improved aerodynamic control during random disturbances.

[Phys. Rev. Fluids 9, 093902] Published Thu Sep 12, 2024

Author(s): Vincent Gourmandie, Juliette Pierre, Valentin Leroy, and Caroline Derec

Only under certain conditions does a drop falling onto a bath entrap an air bubble. We propose a phenomenological law that describes these bubbling conditions in terms of Froude, Weber, and capillary numbers.

[Phys. Rev. Fluids 9, 094002] Published Thu Sep 12, 2024

Author(s): Chunheng Zhao, Taehun Lee, and Andreas Carlson

Through numerical simulations we reveal the three phase flow as a newtonian droplet comes in contact with an immiscible viscoelastic liquid film. The droplet dynamics becomes insensitive to the film height when the viscoelastic effects dominate. A viscoelastic ridge forms at the moving contact line, which evolves with a power-law dependence on time.

[Phys. Rev. Fluids 9, 094003] Published Thu Sep 12, 2024

Author(s): Raphael Stuhlmeier, Conor Heffernan, Alberto Alberello, and Emilian Părău

Modulational instability is the major energy transfer mechanism between ocean surface waves in deep water. In this work we explore the effects of nonuniform damping, such as that encountered by waves propagating through sea ice, on this important instability. We relax common assumptions about narrow spectral width but are nevertheless able to capture the dynamics of the unstable triad of waves using dynamical systems techniques. We elucidate the differences between uniform and nonuniform damping and explore the consequences for subsequent spectral broadening.

[Phys. Rev. Fluids 9, 094802] Published Thu Sep 12, 2024