Latest papers in fluid mechanics

Modal and nonmodal stability analysis of turbulent stratified channel flows

Physical Review Fluids - Mon, 01/29/2024 - 10:00

Author(s): Donato Variale, Enza Parente, Jean Christophe Robinet, and Stefania Cherubini

This work aims at studying the modal and nonmodal stability of stably stratified turbulent channel flow, assessing the influence of stratification at fixed friction Reynolds number. When increasing the stratification, the energy gain for streamwise-independent perturbations increases by two orders of magnitude, and the spanwise wavenumber for which the energy gain peaks reaches values comparable to those reported in the direct numerical simulations. Moreover, for nonzero values of the streamwise and spanwise wavenumbers, α and β, the energy gain curve has two peaks, one for shorter target times and α>β, leading to a center-channel temperature peak, and another for α<β at larger target times.


[Phys. Rev. Fluids 9, 013904] Published Mon Jan 29, 2024

Sound waves, diffusive transport, and wall slip in nanoconfined compressible fluids

Physical Review Fluids - Mon, 01/29/2024 - 10:00

Author(s): Hannes Holey, Peter Gumbsch, and Lars Pastewka

In this work we compare long wavelength fluctuations in nanoconfined simple fluids from molecular dynamics simulations with continuum descriptions. Quasi-two-dimensional descriptions of these confined fluids give rise to attenuation behavior of hydrodynamic fluctuations distinct from the bulk. Interactions with the walls introduce additional dissipation and can be related to wall slip and an anomalous dispersion relation for sound.


[Phys. Rev. Fluids 9, 014203] Published Mon Jan 29, 2024

Study of the wall pressure variations on the stall inception of a thick cambered profile at high Reynolds number

Physical Review Fluids - Fri, 01/26/2024 - 10:00

Author(s): Caroline Braud, Bérengère Podvin, and Julien Deparday

Understanding the flow physics near stall at high chord-based Reynolds numbers (> 106) remains a challenge for both experimental and numerical approaches. Generally statistical bi-dimensionality of the flow is assumed a priori. In the present study, the wall pressure at two chords provides a description of the flow evolution with the angle of incidence. Our results indicate that flow separation at high Reynolds numbers is an inherently local, three-dimensional and unsteady process that occurs in a continuous manner. However, as it can be represented with mainly two proper orthogonal decomposition modes, our results also suggest that a low-order approach may offer a viable modeling route.


[Phys. Rev. Fluids 9, 014605] Published Fri Jan 26, 2024

Reassessing the transport properties of fluids: A symbolic regression approach

Physical Review E - Thu, 01/25/2024 - 10:00

Author(s): Dimitrios Angelis, Filippos Sofos, and Theodoros E. Karakasidis

The viscosity and thermal conductivity coefficients of the Lennard-Jones fluid are extracted through symbolic regression (SR) techniques from data derived from simulations at the atomic scale. This data-oriented approach provides closed form relations that achieve fine accuracy when compared to well…


[Phys. Rev. E 109, 015105] Published Thu Jan 25, 2024

Hydrodynamic bound states of rotating microcylinders in a confining geometry

Physical Review Fluids - Thu, 01/25/2024 - 10:00

Author(s): Hanliang Guo, Yi Man, and Hai Zhu

Locomotion is a combination of translation and rotation. With suitable boundary conditions, rotation can induce translation. The induced translations are more intriguing when multiple objects are rotating together. In this paper, we study theoretically the hydrodynamic bound states of two active cylinders rotating inside cylindrical confinement filled with viscous fluid. We find that the active cylinder pair can fall into four distinct non-trivial hydrodynamic bound states depending on their initial positions. Our findings may be helpful in understanding bacterial flagella rotating within a confining geometry.


[Phys. Rev. Fluids 9, 014102] Published Thu Jan 25, 2024

Vortex-induced vibration of a flexible pipe under oscillatory sheared flow

Physical Review Fluids - Thu, 01/25/2024 - 10:00

Author(s): Xuepeng Fu, Shixiao Fu, Mengmeng Zhang, Haojie Ren, Bing Zhao, and Yuwang Xu

An experimental apparatus for studying vortex-induced vibration (VIV) of a flexible pipe was designed and constructed in an ocean basin. A VIV test of a tensioned flexible pipe in oscillatory sheared flow (OSF) was performed based on this rotation rig. One end of the test pipe is fixed, and one end is forced to oscillate harmonically with various amplitudes and periods. The results show that VIV under OSF exhibits amplitude modulation and hysteresis phenomena. A critical Keulegan-Carpenter (KC) number is proposed to describe the occurrence of modulated VIV under OSF. The experimental results provide benchmark data for future VIV prediction research.


[Phys. Rev. Fluids 9, 014604] Published Thu Jan 25, 2024

Linear stability of ultrathin spherical coatings

Physical Review Fluids - Wed, 01/24/2024 - 10:00

Author(s): D. Moreno-Boza and A. Sevilla

This work investigates the linear stability of an ultrathin non-wetting liquid film on a spherical substrate in the limit of negligible inertia. The interplay between destabilizing van der Waals forces, with an exact potential description, and stabilizing surface tension gives rise to a discrete set of unstable temporal modes. Also, the performance of classical lubrication models has been assessed against the complete Stokes equations of motion.


[Phys. Rev. Fluids 9, 014004] Published Wed Jan 24, 2024

Koopman-based model predictive control with morphing surface: Regulating the flutter response of a foil with an active flap

Physical Review Fluids - Wed, 01/24/2024 - 10:00

Author(s): Tso-Kang Wang and Kourosh Shoele

In this work, we demonstrated how model predictive control (MPC) can effectively manipulate a highly nonlinear foil-and-flap system to follow designated lift trajectories to sub-5% error. The surrogate model is built through a data-driven method merging fluidic and structural information which can be readily deployed to numerous fluid-structure interaction systems. The rapid optimization procedure utilized to generate the control signal can also be used for estimating ambient environmental change


[Phys. Rev. Fluids 9, 014702] Published Wed Jan 24, 2024

Quantifying and predicting near-wall global intermittency in stably stratified channel flow

Physical Review Fluids - Wed, 01/24/2024 - 10:00

Author(s): Haoyang Cen, Artem Korobenko, and Qi Zhou

In this study, we explore the transition from fully developed turbulence to global intermittency in stably stratified channel flow using direct numerical simulations across varying friction Reynolds and shear Richardson numbers. We quantify intermittency by measuring the volume fraction of turbulent patches and find that intermittency can originate from either near-wall or mid-channel regions, depending on the above parameters. The study identifies a critical value of the Nusselt number (approximately 3.0) below which near-wall intermittency consistently occurs. The critical friction Richardson number for intermittency is found to be proportional to the squared Reynolds number.


[Phys. Rev. Fluids 9, 014803] Published Wed Jan 24, 2024

Multiobjective optimization for flapping foil hydrodynamics with a multitask and multifidelity approach

Physical Review E - Tue, 01/23/2024 - 10:00

Author(s): Zhangyuan Wang, Dehan Yuan, Chenglong Wu, Xu Chen, Ruipeng Li, Weicheng Cui, and Dixia Fan

We develop a multitask and multifidelity Gaussian process (MMGP) model to accurately predict and optimize the multiobjective performance of a flapping foil while minimizing the cost of high-fidelity data. Through a comparison of three kernels, we have selected and applied the spectral mixture kernel…


[Phys. Rev. E 109, 015103] Published Tue Jan 23, 2024

Fingering stabilization and adhesion force in the lifting flow with a fluid annulus

Physical Review E - Tue, 01/23/2024 - 10:00

Author(s): Írio M. Coutinho, Pedro H. A. Anjos, Rafael M. Oliveira, and José A. Miranda

The lifting Hele-Shaw cell flow commonly involves the stretching of a viscous oil droplet surrounded by air, in the confined space between two parallel plates. As the upper plate is lifted, viscous fingering instabilities emerge at the air-oil interface. Such an interfacial instability phenomenon is…


[Phys. Rev. E 109, 015104] Published Tue Jan 23, 2024

Cavity dynamics and vibrations of a flexible hydrofoil in the cavitating flow

Physical Review Fluids - Tue, 01/23/2024 - 10:00

Author(s): Yunqing Liu, Qin Wu, Hanzhe Zhang, and Biao Huang

This work reveals the mechanism of cavitation-structure interaction by comparing experimental results between a stainless-steel hydrofoil and a flexible hydrofoil, encompassing hydrodynamic loads, cavitation structures, vibrations, and deformations. The hydrodynamic load induces nose-up twisting deformation, which subsequently increases the angle of attack, promoting cavitation inception. Additionally, the dynamics of cloud cavities induced by the shock wave mechanism are clarified based on high-speed images.


[Phys. Rev. Fluids 9, 014304] Published Tue Jan 23, 2024

Space-local Navier–Stokes turbulence

Physical Review Fluids - Tue, 01/23/2024 - 10:00

Author(s): Ryo Araki, Wouter J. T. Bos, and Susumu Goto

How local is turbulence? This question has been extensively examined in scale space. In this study, we investigate the locality in physical space by artificially truncating the spatially nonlocal nonlinear interactions and see how the energy cascade is affected by the truncation in three-dimensional turbulence.


[Phys. Rev. Fluids 9, 014603] Published Tue Jan 23, 2024

Self-organization of autophoretic suspensions in confined shear flows

Physical Review Fluids - Mon, 01/22/2024 - 10:00

Author(s): Prathmesh Vinze and Sebastien Michelin

Phoretic particles swim and interact exploiting their dual chemical and hydrodynamic footprint on their environment, resulting in complex collective behavior spontaneously but also in response to external forcing. Here, based on a kinetic model for dilute suspensions, we numerically characterize their response to shear in a confined environment and identify three different regimes depending on the relative magnitude of shear forcing, chemotaxis, self-propulsion and confinement. The particles, in turn, exert microscopic stresses on their surroundings, resulting in a complex rheological response tightly linked to the self-organization regime.


[Phys. Rev. Fluids 9, 014202] Published Mon Jan 22, 2024

Low-order planar pressure reconstruction of stalled airfoils using particle image velocimetry data

Physical Review Fluids - Fri, 01/19/2024 - 10:00

Author(s): D. W. Carter and B. Ganapathisubramani

Structures in turbulent flow are largely responsible for variations in the forces experienced by the body of interest. In this work, the impact of specific structures in the velocity field on the resulting pressure field are revealed using a data-driven framework. The body of interest is a stalled NACA 0012 airfoil obtained from high-resolution large-scale time-resolved particle image velocimetry.


[Phys. Rev. Fluids 9, 014602] Published Fri Jan 19, 2024

Conformations, correlations, and instabilities of a flexible fiber in an active fluid

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Scott Weady, David B. Stein, Alexandra Zidovska, and Michael J. Shelley

Many biological systems rely on interactions between active processes and passive, deformable structures to properly function. An important example is chromatin in the cell nucleus, where ATP-powered processes, such as transcription or DNA repair, act on the chromatin fiber and influence its motion. Motivated by this system, in this study we develop and analyze a model of a flexible fiber in an active suspension as an analog to a chromatin fiber in an active environment - the nucleoplasm. Interactions between the suspension and the fiber lead to a novel bend instability, and nonlinear simulations identify coherent motions and conformations of the fiber over long timescales.


[Phys. Rev. Fluids 9, 013102] Published Thu Jan 18, 2024

Activity-induced asymmetric dispersion in confined channels with constriction

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Armin Maleki, Malihe Ghodrat, and Ignacio Pagonabarraga

Our Brownian dynamics model uncovers response mechanisms of microorganisms such as Escherichia coli to shear flows and constrictions, shedding light on anomalous accumulation of active particles in confined channels. Our findings highlight the butterfly-like attractors that trap particles past the constriction and characterize relevant dynamical regimes of active particle accumulation. We also explore the impact of particle size, channel geometry, and fluid velocity on trapping probability and accumulation strength. This intriguing phenomenon could be utilized as geometrical sift for mixture of active particles with different sizes/velocities and lead to innovations in particle manipulation.


[Phys. Rev. Fluids 9, 013103] Published Thu Jan 18, 2024

Onset of Lagrangian chaos: From fractal power spectrum to the absolutely continuous one

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Rafil V. Sagitov, Igor I. Wertgeim, and Michael A. Zaks

We study fluid motions excited by a spatially periodic force in a plane region with periodic boundary conditions; mean drift in both directions across the domain is nonzero. Under weak force, the flow is stationary; Fourier spectra of velocity for advected tracer particles are singular continuous (fractal). At higher amplitudes of the force, the flow pattern periodically oscillates, ensuring the onset of Lagrangian chaos in tracer dynamics. We illustrate the transformation of the power spectrum from the fractal object to the conventional smooth curve (subplots (i-iv)), and describe the accompanying changes in the pattern of autocorrelation for the tracer velocity.


[Phys. Rev. Fluids 9, 014401] Published Thu Jan 18, 2024

Role of vorticity distribution in the rise and fall of lift during a transverse gust encounter

Physical Review Fluids - Thu, 01/18/2024 - 10:00

Author(s): Antonios Gementzopoulos, Girguis Sedky, and Anya Jones

In this paper, we experimentally investigate the flow fields and lift transients associated with transverse gust encounters. We highlight the key differences between inviscid and viscous gust encounters and discuss how contrasting shed vorticity distributions lead to dissimilar lift behavior during the gust exit. Our findings contribute to a refined understanding of the utility of inviscid models in the prediction of loads during atmospheric gust encounters.


[Phys. Rev. Fluids 9, 014701] Published Thu Jan 18, 2024

Absolute and convective instability of a round jet emerging into an ambient medium of different viscosity

Physical Review Fluids - Wed, 01/17/2024 - 10:00

Author(s): Jinwei Yang and Vinod Srinivasan

The case of a round jet of one liquid emerging into an ambient medium of a different viscosity is relevant to many industrial mixing situations. A linear spatiotemporal analysis of such a configuration shows that for sufficiently high viscosity contrast, the flow is absolutely unstable. Both axisymmetric and helical modes are unstable, with the axisymmetric mode being slightly dominant. The absolute/convective instability transition boundary defined in terms of viscosity ratio and jet Reynolds number is compared to the results of recent experiments and good agreement is found.


[Phys. Rev. Fluids 9, 013903] Published Wed Jan 17, 2024

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