Physical Review Fluids

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): 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

Author(s): Ehud Yariv, Rodolfo Brandão, David K. Wood, Hannah Szafraniec, John M. Higgins, Parisa Bazazi, Philip Pearce, and Howard A. Stone

We show that particle roughness can significantly modify viscous dissipation in the limit of small particle-wall separation, with the corrugation amplitude comparable with the separation. In particular, a lubrication analysis provides the rectilinear and angular velocities of the two-dimensional particle as functions of the instantaneous angular configuration. The time-averaged rectilinear velocity is a geometric quantity, obtained without the need to address any time dynamics, with the result that the particle may either translate while rotating or become “locked” in a specific phase and translate without rotation.

[Phys. Rev. Fluids 9, L032301] Published Mon Mar 11, 2024

Author(s): Xinyu Si and Lei Fang

We find that the impact of active matter on Lagrangian coherent structures (LCSs) was much more significant compared to localized random noise with similar energy. This is because the perturbation generated by active matter could couple with the background flow and further deform the LCSs. In addition, rotational elliptical regions of the flow were much more susceptible to active matter perturbation than the hyperbolic regions. Lastly, we revealed that the LCSs could be decently altered even at a small number density of active matter.

[Phys. Rev. Fluids 9, 033101] Published Fri Mar 08, 2024

Author(s): Tao Liu, Benoît Semin, Ramiro Godoy-Diana, and José Eduardo Wesfreid

The self-regenerating nature of coherent structures is a key feature that sustains turbulence in wall-bounded shear flows. Theoretical and numerical works have examined the smallest flow region that can sustain these processes, but experimental studies have been scarce due to the technical difficulty of measuring three-dimensional velocity fields. This study reports experimental results that quantify two basic physical mechanisms: the lift-up effect and the production of wall-normal vorticity. These processes are quantified for the first time by a novel local analysis of the three-dimensional velocity field.

[Phys. Rev. Fluids 9, 033901] Published Fri Mar 08, 2024

Author(s): Yan Zhang, Ji-Yan Qiao, Wan-Long Ren, Xu-Hui Zhang, Peng Li, and Xiao-Bing Lu

We investigate the concentration instabilities that arise in vertical dense particulate flows during hydraulic conveying, where the widely used two-fluid model is employed. Results show that the system exhibits instability across a broad spectrum of controlling parameters. We also obtain a fully nonlinear transient numerical solution for the system by using the finite difference method. It is found that small disturbances transform into saturation waves with finite amplitudes when the nonlinear effect becomes dominant, which corresponds to plug flow or slug flow.

[Phys. Rev. Fluids 9, 034303] Published Fri Mar 08, 2024

Author(s): Adrien Lefauve and Miles M. P. Couchman

Our understanding of fluid turbulence has traditionally relied on a few canonical laboratory experiments. In this paper, we present a relatively new canonical experiment, the stratified inclined duct, whose density stratification allows for the study of coherent and intermittent states at higher Reynolds numbers than in unstratified flows. Applying a novel data-driven technique to a large experimental database of shadowgraph visualizations, we automatically identify distinct turbulent states and transitions between them, paving the way for the reduced-order modeling of stratified turbulence.

[Phys. Rev. Fluids 9, 034603] Published Fri Mar 08, 2024

Author(s): Gopal Verma, Ashwini Kumar, Sapna Soni, Kapil Yadav, and Wei Li

We introduce a pump-probe laser setup to explore the interaction of light momentum and fluid mechanics in a two-layer liquid system. Creating a nanometric bulge in the upper layer reveals a transient bulge on the liquid-liquid interface propelled by viscous stress towards the higher refractive index liquid. Noninvasive measurements and numerical simulations validate our findings, unraveling the intricate interplay between light momentum theories (Minkowski and Abraham) and fluid mechanics. The transient deformation height serves as a precise indicator of surface tension and viscosity, enabling nanoscale manipulation with potential applications in sensors, actuators, and optical devices.

[Phys. Rev. Fluids 9, 034801] Published Fri Mar 08, 2024

Author(s): Jianyi Zheng, Yu Guan, Liangliang Xu, Xi Xia, Larry K. B. Li, and Fei Qi

We explore the vortical interactions in a swirling combustion system via the construction of time-varying weighted spatial turbulence networks whose node strength distribution is derived from the Biot-Savart law. We find widespread evidence of scale-free topology in the vortical networks, with the most coherent flow structures acting as the primary network hubs. Crucially, we find that even after the onset of thermoacoustic instability, the scale-free topology can persist continuously in time, contrary to some suggestions from the literature. This discovery could have important implications for the design of flow controllers that rely on destroying the primary hubs of vortical networks.

[Phys. Rev. Fluids 9, 033202] Published Thu Mar 07, 2024

Author(s): Enwei Zhang, Zhan Wang, and Qingquan Liu

This study focuses on the turbulent channel flow over three-dimensional wavy walls. Through temporal-spatial averaging decomposition, the momentum flux and kinetic energy transfers by mean, time-averaged, dispersive, and turbulent motions are revealed. We find a notable correlation between dispersive shear stress and vorticity enhancement. Another finding is that the dispersion-turbulence exchange significantly contributes to turbulent kinetic energy production.

[Phys. Rev. Fluids 9, 034602] Published Thu Mar 07, 2024

Author(s): Niklas Dusch, Victor Avsarkisov, Michael Gerding, Claudia Stolle, and Jens Faber

In this study, we evaluate the effect of kinetic helicity on the slope of the vertical spectrum of kinetic energy in stratified turbulence. Our theoretical approach allows us to define energy-dominated, helicity-dominated, and joint dual cascade regimes in turbulent flows at various stratification rates. Some of them are verified with the balloon measurements from the Troposphere and Lower Stratosphere. To summarize, one of the conclusions of this work states that domination of helicity flattens the spectrum while an increase in the stratification makes it steeper.

[Phys. Rev. Fluids 9, 033801] Published Wed Mar 06, 2024

Author(s): S. Jackson and S. He

Idealized nonuniform body force profiles are used to explain the root cause of turbulence enhancement in various physical flows encountered within such fields as mixed convection, magnetohydrodynamics, and flow control. A recent theory used to explain laminarization is extended to include turbulence enhancement and it is demonstrated that turbulence enhancement can be explained by an increased “apparent Reynolds number”.

[Phys. Rev. Fluids 9, 034601] Published Tue Mar 05, 2024

Author(s): Yuxuan Chen, Tianwei Yang, Hua Zhou, Xingsi Han, and Zhuyin Ren

Theoretical and numerical exploration of the transported Filtered Density Function (FDF) in the framework of Self-Adaptive Turbulence Eddy Simulation (SATES) was conducted, encompassing fundamental definitions and a model for the scalar mixing timescale. To address the model inconsistency in terms of the scalar mixing timescale between RANS and LES modes, a novel model was proposed. Subsequently, a posteriori testing showcased the merits of this novel approach, highlighting its potential to be employed in SATES-FDF simulations of turbulent reacting flows.

[Phys. Rev. Fluids 9, 033201] Published Mon Mar 04, 2024

Author(s): Laura K. C. Sunberg, Michelle H. DiBenedetto, Nicholas T. Ouellette, and Jeffrey R. Koseff

The extent to which particles such as larvae, seagrass pollen, and microplastics are dispersed by waves and currents has many ecological impacts. Here, we systematically examine the effect of a comprehensive set of parameters on the dispersion of ellipsoidal particles in a wave-current flow using a numerical computation approach. Our results show that all of the parameters considered have some effect on the particle dispersion, but that the settling-wave timescale ratio has the greatest effect.

[Phys. Rev. Fluids 9, 034302] Published Mon Mar 04, 2024

Author(s): Moutassem El Rafei and Ben Thornber

We investigate compressible turbulent mixing evolving in spherical implosions with differing initial conditions using high-resolution implicit large eddy simulations. We examine in detail temporal and spatial turbulent transport budgets including density self-correlation, turbulent mass flux, and turbulent kinetic energy. This analysis provides improved understanding of the mixing process initiated by Richtmyer-Meshkov and Rayleigh-Taylor instabilities including quantification of contributions to asymmetries in the mixing layer and numerical dissipation.

[Phys. Rev. Fluids 9, 034501] Published Mon Mar 04, 2024

Author(s): P. Pico, L. Kahouadji, S. Shin, J. Chergui, D. Juric, and O. K. Matar

In this investigation, we dive into the phenomenon of drop encapsulation in elongated bubbles travelling through liquid-filled capillary channels in the presence of surface-active material. Our numerical results reveal that complex interactions between surfactant parameters, Marangoni stresses, viscosity, and inertia are responsible for dramatically altering the pinch-off times, along with the number, size, and velocity of the encapsulated drops. We summarize these interactions in three distinct encapsulation morphological regimes, providing a structured overview of the underlying dynamics.

[Phys. Rev. Fluids 9, 034001] Published Fri Mar 01, 2024

Author(s): Jinchi Li, Ping Wang, and Xiaojing Zheng

The Saffman force is one of the key factors for particle transport and hence the interaction among phases in two-phase wall-bounded turbulence. This numerical work finds that the accumulation of particles near the wall and preferential concentration in low-speed streaks are suppressed by the lift force, leading to destruction of the conditional hairpin vortices and decreasing velocity fluctuations near the wall. However, in the outer layer, the particle-turbulence interaction is increased by the lift force because of higher particle concentration.

[Phys. Rev. Fluids 9, 034301] Published Fri Mar 01, 2024