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Outer-layer self-similarity of the turbulent boundary layer based on the turbulent/non-turbulent interface

Fri, 03/15/2024 - 10:00

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

Impact of microscale physics in continuous time random walks for hydrodynamic dispersion in disordered media

Wed, 03/13/2024 - 10:00

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

Improving prediction of preferential concentration in particle-laden turbulence using the neural-network interpolation

Wed, 03/13/2024 - 10:00

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

Dynamics and control of separated flow over small-scale surface deformations with slip

Tue, 03/12/2024 - 10:00

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

Density and confinement effects on fluid velocity slip

Mon, 03/11/2024 - 10:00

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

Effect of confinement on the transition from two- to three-dimensional fast-rotating turbulent flows

Mon, 03/11/2024 - 10:00

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

Turbulence model augmented physics-informed neural networks for mean-flow reconstruction

Mon, 03/11/2024 - 10:00

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

Motion response induced by air cushioning effect during the water impact of a plate at small deadrise angles

Mon, 03/11/2024 - 10:00

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

Hydrodynamic interactions between rough surfaces

Mon, 03/11/2024 - 10:00

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

Interaction between swarming active matter and flow: The impact on Lagrangian coherent structures

Fri, 03/08/2024 - 10:00

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

Lift-up and streak waviness drive the self-sustained process in wall-bounded transition to turbulence

Fri, 03/08/2024 - 10:00

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

Linear stability and numerical analysis of vertical dense particulate flows in hydraulic conveying

Fri, 03/08/2024 - 10:00

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

Data-driven classification of sheared stratified turbulence from experimental shadowgraphs

Fri, 03/08/2024 - 10:00

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

Probing interplay of light momentum and fluid mechanics in two-layer liquids

Fri, 03/08/2024 - 10:00

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

Scale-free topology of vortical networks in a turbulent thermoacoustic system

Thu, 03/07/2024 - 10:00

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

Turbulent momentum and kinetic energy transfer of channel flow over three-dimensional wavy walls

Thu, 03/07/2024 - 10:00

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

Analysis of coupled energy and helicity spectra in stratified turbulence: Theory and balloon measurements

Wed, 03/06/2024 - 10:00

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

Turbulence enhancement in body force opposed flows

Tue, 03/05/2024 - 10:00

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

Transported filtered density function in self-adaptive turbulence eddy simulation

Mon, 03/04/2024 - 10:00

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

Parametric study of the dispersion of inertial ellipsoidal particles in a wave-current flow

Mon, 03/04/2024 - 10:00

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