New Papers in Fluid Mechanics

Author(s): J. Simon Kern, Valerio Lupi, and Dan S. Henningson

Unsteady flows in curved pipes are ubiquitous in science and engineering but their stability characteristics are not well understood in most cases of practical interest. We study the linear stability of pulsatile flow in the archetypal configuration of a toroidal pipe, which appears e.g. in aortic blood flow. The Floquet stability analysis of the harmonically forced system reveals that the curvature leads to nonlinear interactions in the baseflow andconsiderable stabilization that can be orders of magnitude larger than in the corresponding planar case. The figure shows a typical snapshot of the streamwise velocity field in a torus subject to a pulsating pressure gradient.

[Phys. Rev. Fluids 9, 043906] Published Tue Apr 09, 2024

Author(s): Runfeng Zhou, Mathew M. Swisher, Akshay Deshmukh, Chengzhen Sun, John H. Lienhard, and Nicolas G. Hadjiconstantinou

We develop a model that describes the permeance of simple fluids as well as small hydrocarbon molecules through nanoporous, atomically thin membranes. The model is in agreement with molecular dynamics simulations for a wide range of pore sizes, including pores approaching the steric exclusion limit, as needed for understanding separation processes using such membranes.

[Phys. Rev. Fluids 9, 044202] Published Tue Apr 09, 2024

Author(s): Hai-Tao Zhu, Long Chen, and Ming-Jiu Ni

This numerical simulation investigates the vertical convection of liquid metal with varying magnetic fields and wall conductivities. The applied horizontal magnetic field alters plume dynamics and topology, leading to a more coherent large-scale flow structure but weakening convection through Joule dissipation. This competition between rectification and magnetic damping determines the magnetic field’s impact on heat transfer, with the quasi-two-dimensional state being the threshold. Our analysis demonstrates that while the plume area remains constant, condensation of coherent structures enhances horizontal heat transport per unit area, significantly improving overall heat transfer.

[Phys. Rev. Fluids 9, 043701] Published Mon Apr 08, 2024

Author(s): Zhibo Li, Clément Bielinski, Anke Lindner, Olivia du Roure, and Blaise Delmotte

We combine experiments and numerical simulations to investigate the interaction between a rigid fiber and a triangular obstacle in a microfluidic channel. We find different dynamics depending on the initial position and orientation of the fiber. We show that these dynamics are dictated by the fiber configuration in the vicinity of the obstacle. Some dynamics induce a cross-stream migration which grows with the fiber length. Our findings could in the future be used to design and optimize microfluidic sorting devices to sort rigid fibers by length.

[Phys. Rev. Fluids 9, 044302] Published Mon Apr 08, 2024

Author(s): Yan Yan Liu, Sjouke W. Schekman, Mo Xiao Li, Tian Jian Lu, and Tongbeum Kim

A flow progression from laminar flow slippage to rotational vortices in cavitied microchannels is shown to cause a reversal of flow resistance, i.e. a reduced flow resistance at low Reynolds numbers compared to an unmodified microchannel but a comparatively higher flow resistance at high Reynolds numbers. Furthermore, an earlier transition of initial laminar flow to turbulent flow is suggested to be triggered by instabilities generated along shear layers, formed between the mainstream flow and rotational vortices in each cavity that modifies the sidewalls of microchannels.

[Phys. Rev. Fluids 9, 044201] Published Fri Apr 05, 2024

Author(s): Shiyu Lyu, Jiaqing Kou, and Nikolaus A. Adams

In this work, we developed a novel framework for incorporating the near-wall non-overlapping domain decomposition (NDD) method with the machine learning technique. It allows the solution to be calculated with a Robin-type (slip) wall boundary condition on a relatively coarse mesh and then be corrected in the near-wall region by solving the thin boundary-layer equations on a fine subgrid. Through an estimated turbulent viscosity profile provided by a neural network, the proposed method can be easily extended to different turbulence models and achieve commendable accuracy for the test cases of turbulent wall-bounded flows at various Reynolds numbers.

[Phys. Rev. Fluids 9, 044603] Published Fri Apr 05, 2024

Author(s): Mohammad Hossein Doranehgard, Nader Karimi, Iman Borazjani, and Larry K. B. Li

We numerically explore the two-dimensional, incompressible, isothermal flow through a wavy channel, with a focus on how the channel geometry affects the routes to chaos at Reynolds numbers between 150 and 1000. We find that (i) the period-doubling route arises in a symmetric channel, (ii) the Ruelle…

[Phys. Rev. E 109, 045103] Published Thu Apr 04, 2024

Author(s): Zhiying Liu (刘志颖), Qianghui Xu (许强辉), Junyu Yang (杨君宇), Kai H. Luo (罗开红), and Lin Shi (史琳)

The existing hybrid-scale micro-continuum approach faces difficulties in numerical diffusion issues at the gas-liquid interface and the solid boundary. This article proposes a multiscale compressive Continuum Species Transfer (MC-CST) scheme and a concentration extrapolation algorithm to improve the accuracy of two-phase reactive flow simulations. Furthermore, a case study simulating calcite dissolution in a porous medium is presented to underscore the importance of multiscale fluid-rock interactions for an in-depth comprehension of the dissolution regime.

[Phys. Rev. Fluids 9, 043801] Published Thu Apr 04, 2024

Author(s): Michail Chatzimanolakis, Pascal Weber, and Petros Koumoutsakos

We present the automated discovery of control strategies for drag reduction in cylinder flows. Reinforcement Learning algorithms discover control strategies for two-dimensional configurations that generalize to three dimensional flows. We discuss the physical processes involved in the drag reduction mechanisms along with their generalization capabilities. This work demonstrates a practical approach to handling the computationally intensive task of deploying Reinforcement Leaning for bluff body flow control problems: namely train in 2D and control in 3D.

[Phys. Rev. Fluids 9, 043902] Published Thu Apr 04, 2024

Author(s): Jordi Casacuberta, Stefan Hickel, and Marios Kotsonis

A physical mechanism has been identified through which a small surface add-on to a swept (angled-back) aircraft wing can delay the development of turbulent disorganized airflow. To reduce the aerodynamic drag of an aicraft, it is important to maximize the extent of smooth laminar airflow around its wings. The mechanism, termed “reverse lift-up effect”, dampens out the critical instability waves leading to turbulent airflow over swept aircraft wings under certain conditions. This may be achieved through particular design of a surface pattern on the aicraft wings. Overall, the present findings aim to contribute to novel design methodologies for future generations of laminar aicraft wings.

[Phys. Rev. Fluids 9, 043903] Published Thu Apr 04, 2024

Author(s): Kevin Cherng, Sanjiva Lele, and Daniel Livescu

We systematically examine how heat conduction between two fluids at different temperatures, large contrasts in transport properties, and sudden changes in transport properties can affect the fully compressible Rayleigh-Taylor instability (RTI) using direct numerical simulations. These variations cause departures from the classical self-similar development of the RTI, along with misalignment between regions of mixing and regions of most intense turbulent activity. Under certain conditions, dynamical quantities such as vorticity and dissipation appear to depend only on the transport properties and not on past flow history.

[Phys. Rev. Fluids 9, 043904] Published Thu Apr 04, 2024

Author(s): Frank G. Jacobitz and Kai Schneider

Taylor’s hypothesis of frozen flow is revisited in homogeneous turbulent shear flow by examining the cancellation properties of Eulerian and convective accelerations at different spatial flow scales. The anti-alignment in the joint pdf given in the figure shows that the Eulerian and convective accelerations cancel for small scales of the turbulent motion, indicating that Taylor’s hypothesis holds. This is not so for the large-scale motion of homogeneous turbulent shear flow.

[Phys. Rev. Fluids 9, 044602] Published Wed Apr 03, 2024

Author(s): Makoto Iima

We theoretically studied the optimal control, frequency lock-in, and phase lock-in phenomena due to the spatially localized periodic forcing in flow past an inclined plate. Although frequency lock-in is evident in many fluid phenomena, especially fluid-structure interactions, not many researchers ha…

[Phys. Rev. E 109, 045102] Published Tue Apr 02, 2024

Author(s): Mandeep Saini, Youssef Saade, Daniel Fuster, and Detlef Lohse

Cavitation bubbles are present in a plethora of industrial and medical applications, and their understanding proves crucial to the development and tuning processes. In this study, we perform three dimensional direct numerical simulations (DNS) of multiple cavitation bubbles driven by pressure waves. Similarly observed in previously conducted experiments, it is found that these bubbles can exhibit an asymmetry in the direction of wave propagation. We show that this asymmetry is a consequence of the force induced by the wave on the bubbles, due to the finite speed of sound in the liquid medium.

[Phys. Rev. Fluids 9, 043602] Published Tue Apr 02, 2024

Author(s): Jihoo Moon, Ehsan Mahravan, and Daegyoum Kim

This work investigates the interaction of a viscoelastic solid and a nearby collapsing bubble, using numerical simulations. The Deborah number, which quantifies the relative time scales of solid deformation and bubble dynamics, is found to effectively characterize the bubble-solid interaction. Analyses on the temporal distribution of energy components and the imbalance in pressure distribution surrounding the bubble reveal the rationale behind changes in bubble behaviors with respect to the viscosity and elasticity of the solid. Furthermore, the solid deformation induced by an expanding and collapsing bubble and the shape of the crater formed by a bubble jet are examined.

[Phys. Rev. Fluids 9, 043603] Published Tue Apr 02, 2024

Author(s): Antoine Jouin, Nicola Ciola, Stefania Cherubini, and Jean Christophe Robinet

A three-dimensional boundary layer is the seat of a spatially amplified instability, the crossflow instability, which is encountered for example on swept wings. Beyond a certain amplitude, the flow destabilizes again through a secondary instability. The objective of this work is to show that by using a discrete spatial Floquet method (Block method) from a certain transversally periodic elementary pattern, there can exist a large-scale modulation linked to amplification mechanisms involving detuned modes.

[Phys. Rev. Fluids 9, 043901] Published Tue Apr 02, 2024

Author(s): Alicia Aguilar-Corona, Micheline Abbas, Matthieu Mercier, and Laurent Lacaze

The concept of apparent coefficient of restitution is extended, to describe the wall-bouncing of a cylinder falling in a viscous fluid. When the Stokes number (St) is increased, the coefficient of restitution increases from 0 to 1 over two decades of St, with a critical Stc=75±25 (compared to Stc≈10 for spheres). While rationalizing results from experiments and numerical simulations, we evidence the relevance of lumping the complex details of physical phenomena involved during contact into a simple concept based on the contact apparent roughness and elasticity. The increase of dissipation associated with the contact elasticity is well captured by a model based on solid contact time scale.

[Phys. Rev. Fluids 9, 044301] Published Tue Apr 02, 2024

Author(s): Yongyun Hwang

Two possible and physically meaningful scalings for near-wall streamwise turbulence intensity are derived using velocity spectra. First, if viscous wall effect on the wall-attached inactive motions from the log and outer layers is negligible enough so that the related spectrum does not diminish with Reynolds number, the near-wall turbulence intensity grows indefinitely and scales as ln Reτ. Whereas if it is strong enough so that the near-wall turbulence intensity remains finite at all Reynolds numbers, it scales as 1/ln Reτ. The existing measurement data favors the latter scaling, but the issue will be settled by the availability of such data at higher Reynolds numbers.

[Phys. Rev. Fluids 9, 044601] Published Tue Apr 02, 2024

Author(s): Vladyslav Pushenko and Jörg Schumacher

The impact of turbulent mixing on an ensemble of initially monodisperse water droplets is studied in a turbulent bulk that serves as a simplified setup for the interior of a turbulent ice-free cloud. A mixing model was implemented that summarizes the balance equations of water vapor mixing ratio and…

[Phys. Rev. E 109, 045101] Published Mon Apr 01, 2024

Author(s): Tristan Gilet

The dynamics of a liquid drop attached to a solid and subjected to arbitrary accelerations can be qualitatively reproduced with a low-dimensional model in which the drop is replaced by a cuboid. The cuboid deforms, vibrates and slides similarly to a drop, while being much simpler to describe mathematically.

[Phys. Rev. Fluids 9, 043601] Published Mon Apr 01, 2024