New Papers in Fluid Mechanics

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

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

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

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

Author(s): Diego Berzi

The case is made that the kinetic theory of granular gases provides the long-sought universal framework to predict the flow of realistic particles from dilute to very dense conditions. In so doing, the popular inertial rheology and its nonlocal extension to deal with heterogeneities based on the granular fluidity concept are derived as special limits of the kinetic theory.

[Phys. Rev. Fluids 9, 034304] Published Wed Mar 20, 2024

Author(s): Akihito Kiyama, Sharon Wang, Jisoo Yuk, and Sunghwan Jung

Our study demonstrates how a child’s rubber popper, when snapped underwater, creates fascinating cavitation bubble formations. By using high-speed imaging, we explore the fluid mechanics behind the formation and collapse of these cavitation bubbles due to pressure changes. Interestingly, the cavitation bubble forms a toroidal shape rather than a spherical one, with a lifespan shorter than that of a spherical bubble with the same outer radius. This research illuminates the intricate interplay between bubble dynamics within a thin gap and material elasticity.

[Phys. Rev. Fluids 9, 030501] Published Tue Mar 19, 2024

Author(s): Michele Pellegrino and Berk Hess

Viscosity and contact line friction are the main channels of energy dissipation in wetting dynamics. Experiments and theoretical models haven’t singled out a universal scaling between the two. We perform molecular dynamics simulations of droplets spreading over hydrophilic surfaces, aiming to disentangle the effect of viscosity on friction. The viscosity of the fluid is tuned by changing the chemical composition of the liquid, mimicking real-world experiments. It is found that contact line friction does not scale linearly with viscosity. The trend is explained by accounting for molecular depletion in the near-wall region.

[Phys. Rev. Fluids 9, 034002] Published Tue Mar 19, 2024

Author(s): A. Bongarzone and F. Gallaire

In this work, we employ a physics-inspired mathematical model based on successive linear eigenmode projections to solve the relaxation dynamics of liquid oscillations in a U-shaped tube and subjected to a phenomenological nonlinear contact line model. Each projection induces a rapid loss of total energy in the motion and contributes to its nonlinear damping. The present approach not only describes well the transient stick-slip dynamics, but it also captures the global stick-slip-to-stick transition and the residual exponentially decaying bulk motion following the arrest of the contact line. This study offers a further contribution to rationalizing the impact of contact angle hysteresis.

[Phys. Rev. Fluids 9, 034401] Published Tue Mar 19, 2024

Author(s): M. Di Renzo, C. T. Williams, and S. Pirozzoli

This work investigates the interaction of turbulent hypersonic flows with a 15° compression corner using direct numerical simulations. Different values of stagnation enthalpy, ranging approximately from 600kJ to 4MJ, are considered. Vibrational excitation of the gas molecules is also included in the model for the higher stagnation enthalpy cases. The analysis presented in the manuscript illustrates the effects of the flow thermodynamic state on quantities that determine the main mechanical and thermal stresses generated on a hypersonic vehicle, namely the shear stress, heat flux, and pressure fluctuations at the wall.

[Phys. Rev. Fluids 9, 033401] Published Mon Mar 18, 2024

Author(s): Boon Siong Neo and Eric S. G. Shaqfeh

The stresslet in a dilute suspension of rigid spheres in a viscoelastic (Oldroyd-B) fluid is studied under imposed shear and uniaxial extensional flow. We observe that, due to its hyperbolic nature, the polymer constitutive equation can be directly evaluated along streamlines of the flow. Specifically, evaluating along streamlines on the particle surface, in the limit of the Newtonian flow fields, produces an analytical scaling which we evaluate and test against numerical results. This approach also provides physical insight into the mechanism driving the observed trends.

[Phys. Rev. Fluids 9, 033301] Published Fri Mar 15, 2024

Author(s): Letian Chen, Zhanqi Tang, Ziye Fan, and Nan Jiang

This study reports on the outer-layer self-similarity of the turbulent boundary layer at low to moderate Reynolds number range from the perspective of the turbulent/non-turbulent (T/NT) interface. From the new perspective, the exponential self-similarity of the mean velocity is discovered in the outer region. For further consideration, we propose the energy-superposition effect based on the outer-layer structures to reveal the similarity of the turbulent kinetic energy in the outer layer. The present work reveals that the interface-based perspective provides an alternative scheme to understand the outer layer of the turbulent boundary layer.

[Phys. Rev. Fluids 9, 034607] Published Fri Mar 15, 2024

Author(s): Xiangnan Yu, Marco Dentz, HongGuang Sun, and Yong Zhang

This study investigates how microscale physics impacts anomalous particle dispersion in disordered media. To this end, disordered media with random sorption and random flow properties are considered. To quantify anomalous large-scale particle transport, a continuous time random walk model is developed that represents both disorder mechanisms. While random advection and sorption may give rise to similar large-scale transport behaviors, they can be clearly distinguished in their response to uniform injection conditions. These findings highlight the importance of microscale physics for the interpretation and prediction of anomalous dispersion phenomena in disordered media.

[Phys. Rev. Fluids 9, 034502] Published Wed Mar 13, 2024

Author(s): Jiajun Hu, Zhen Lu, and Yue Yang

A neural-network interpolation (NNI) is proposed to improve the prediction of preferential concentration in particle-laden turbulence. The NNI uses the particle position and velocity on neighboring grid points to estimate the fluid velocity at the particle position. To evaluate the NNI, we simulate a two-dimensional homogeneous isotropic turbulence subjected to high-wavenumber forcing. The NNI recovers the effect of small-scale motion on particle distribution from the low-resolution field, adding high-wavenumber energy to the turbulence field. Consequently, the NNI improves the prediction accuracy of the preferential concentration on coarse grids.

[Phys. Rev. Fluids 9, 034606] Published Wed Mar 13, 2024

Author(s): Silvia Ceccacci, Sophie A. W. Calabretto, Christian Thomas, and James P. Denier

Surface slip, characterized by a slip length, λ, suppresses flow separation induced by Gaussian-shaped deformations on a flat plate. Bumps generate more intense regions of reversed flow than gaps, requiring larger λ to inhibit separation. The study extends to double-bump configurations, where the distance between bumps establishes either a stabilizing effect or nonlinear oscillatory flow. However, increasing λ ultimately suppresses this phenomenon and eliminates all pockets of separated flow.

[Phys. Rev. Fluids 9, 033902] Published Tue Mar 12, 2024

Author(s): Clive Ellegaard and Mogens T. Levinsen

While bouncing walking silicone oil droplets (walkers) do show many quantumlike phenomena, the original, most intriguing, double-slit experiment with walkers has been shown to be an overinterpretation of data. Several experiments and numerical simulations have proven that for at least some parameter…

[Phys. Rev. E 109, 035101] Published Mon Mar 11, 2024

Author(s): Xiangyi Liu, Fenghui Lin, Zhiye Zhao, Nansheng Liu, and Xiyun Lu

The elastic-plastic Richtmyer-Meshkov instability of multiple interfaces is investigated by numerical simulation using a multimaterial solid mechanics algorithm based on an Eulerian framework. This Richtmyer-Meshkov instability problem is realized by a copper layer that is flanked by vacuum and a co…

[Phys. Rev. E 109, 035102] Published Mon Mar 11, 2024

Author(s): Carlos Corral-Casas, Yichong Chen, Matthew K. Borg, and Livio Gibelli

This molecular dynamics study investigates the effect of fluid density, confinement, and surface roughness on velocity slip in nanochannels. The key finding is the linearity between the fluid-wall friction coefficient and the peak density at the wall regardless of the wall curvature: tighter confinements attenuate fluid layering at the boundary, reducing the interfacial friction and promoting slip, while higher densities have the opposite effect. Additionally, smoother surfaces characterized by lower accommodation reduce friction via the Smoluchowski factor. These results shed light on the atomistic mechanisms of slip in dense fluids and highlight the importance of fluid-fluid interactions.

[Phys. Rev. Fluids 9, 034201] Published Mon Mar 11, 2024

Author(s): Chandra Shekhar Lohani, Suraj Kumar Nayak, and Kannabiran Seshasayanan

This work studies the effect of confinement on fastly rotating turbulent flows, leading to a dimensional transition in the presence of large-scale friction. Performing a linear stability analysis under very high rotation, the threshold between the two- and three-dimensional regime is determined. We discuss about two instability mechanisms: centrifugal and parametric. The quantification of the instability length scale is found to scale as the square root of the Rossby number. Spatial and temporal co-relation of strain rate tensor with the growth of perturbations is observed for parametric type instability. Finally, these instabilities were also studied with the oscillatory Kolmogorov flow.

[Phys. Rev. Fluids 9, 034604] Published Mon Mar 11, 2024

Author(s): Yusuf Patel, Vincent Mons, Olivier Marquet, and Georgios Rigas

In this work, we bridge the gap between data assimilation using Physics-Informed Neural Networks (PINNs) and variational methods (based on a classical discretization of the flow equations), when used to reconstruct mean flow from accurate sparse pointwise mean velocity measurements. Tested on the turbulent periodic hill flow (Reynolds number of 5600), we propose the use of Spalart-Allmaras turbulence model augmented PINNs for turbulent mean flow reconstruction. Importantly, we demonstrate how these turbulence model augmented PINNs can reconstruct mean flow more accurately than the equivalent variational data assimilation, using the same sparse velocity measurements and physics constraints.

[Phys. Rev. Fluids 9, 034605] Published Mon Mar 11, 2024

Author(s): Xiaohang Shi, Qiulin Qu, Peiqing Liu, Yunlong Zheng, and Hao Guo

When a flat plate impacts water at small deadrise angles, its rotational motion is dominated by the asymmetrical air cushioning effect underneath the plate. In the regime of impact velocity and deadrise angle, four typical motion patterns are found: pitching-down, fluctuating-pitching-down, pitching-up-down, and pitching-up. These motions are mainly dictated by two physical processes of asymmetrical air cushioning effect: keel compression and edge compression. In keel compression, the air underneath the keel is compressed and produces a pitching down moment; in the following edge compression stage, the air underneath the edge is heavily compressed and produces a strong pitching up moment.

[Phys. Rev. Fluids 9, 034802] Published Mon Mar 11, 2024