Physical Review Fluids

Author(s): Jingwen Fu, Wenwei Liu, Xing Jin, and Yun Huang

A numerical study on the inertial migration of particle suspension in a circular pipe with thermal effect is performed by means of the Lattice Boltzmann method coupled with the discrete element method (LBM-DEM). The particle position and heat transfer for single particle as well as particle suspensions are discussed. Then, we extend the work to varied temperature conditions. It is shown that the variation of the circumferential equilibrium position can be well regressed by the Richardson number. A nonmonotonic variation of the radial equilibrium position as well as the Nusselt number is discovered, which is attributed to the particle crowding effect.

[Phys. Rev. Fluids 9, 064302] Published Fri Jun 07, 2024

Author(s): Taketo Ariki

Passive scalar turbulence in the viscous-convective range is investigated via a self-consistent closure theory. Without relying on any empirical parameter the theory successfully explained the scalar-variance spectrum proportional to the inverse wavenumber k−1 from the scalar’s deformation timescale dominated by the Kolmogorov-scale eddy, which agrees with the physical viewpoint of Batchelor (1959). High Schmidt number (Sc) calculations up to Sc=100000 suggest that a clear Batchelor spectrum may appear in kη≳1 for Sc≳10000 where η is the Kolmogorov length.

[Phys. Rev. Fluids 9, 064603] Published Fri Jun 07, 2024

Author(s): Qiang Zhong and Daniel B. Quinn

Hydrofoils with symmetric oscillations can generate asymmetric vortex wakes. This surprising asymmetry has been widely reproduced, but a simple metric to predict its onset has remained elusive. Here, using a combination of vortex modeling and water channel experiments, we show that vortex wake deflection is well-predicted by the “relative dipole angle”. In addition to offering a predictive physics-based metric, our results show that a hydrofoil’s wake can converge much more slowly than previously thought (200+ oscillation cycles), and that the wake’s asymmetry is more than a memory of the hydrofoil’s initial condition - it is an instability inherent to the vortex street.

[Phys. Rev. Fluids 9, 064702] Published Fri Jun 07, 2024

Author(s): Olivier Vincent, Théo Tassin, Erik J. Huber, and Abraham D. Stroock

We study water transport in bi-disperse porous structures inspired by xylem tissue in vascular plants (arrays of microchannels interconnected by a nanoporous layer). With various experiments (high pressure-driven flow, spontaneous imbibition, transpiration-driven flow at negative pressure), we show that transport rates can be tuned by varying the shape of the microchannels. Even with a fixed shape, spontaneous imbibition behaves very differently depending on sample preparation (air-filled vs. evacuated), because of a dramatic change of transport mechanism in the microchannels. We provide analytical (effective medium) approaches and numerical simulations to rationalize these observations.

[Phys. Rev. Fluids 9, 064202] Published Thu Jun 06, 2024

Author(s): Giulio Dacome, Robin Mörsch, Marios Kotsonis, and Woutijn J. Baars

With the goal of performing opposition control of large-scale drag-producing turbulence structures, we present a successful control strategy that attenuates large-scale velocity fluctuations in a turbulent boundary layer. Our control architecture consists of a wall-embedded sensor that feeds information to a real-time controller, which selectively operates a jet actuator. We quantify the performance of this single-input/single-output system with spectral statistics and direct skin-friction measurements. Additionally, we link the changes in skin-friction drag to changes in the statistical integral quantities to gauge the correlation between control output and skin-friction variation.

[Phys. Rev. Fluids 9, 064602] Published Thu Jun 06, 2024

Author(s): Ivan Kharsansky Atallah, Luc Pastur, Romain Monchaux, and Laurent Zimmer

An experimental study is conducted on a thin symmetric airfoil at stall. Below a critical Reynolds number, the flow exhibits low-frequency oscillations (LFOs) characterized by a broadband peak in the aerodynamic force spectrum. Beyond this threshold, the LFOs are replaced by intermittent random switches between two states of either high or low lift (attached or detached flow). The states are explored randomly in time for a fixed angle of attack, contrary to the classical hysteresis often observed in airfoil flows at stall, where both states are absorbing. We model this using a continuous Markov chain and extreme value theory, a framework that can determine the system bifurcation points.

[Phys. Rev. Fluids 9, 063902] Published Wed Jun 05, 2024

Author(s): Rodion Stepanov, Peter Frick, Vladimir Dulin, and Dmitriy Markovich

Our experimental research demonstrates that helicity in turbulent flows undergoes a direct spectral transfer from large to small scales. Tomographic particle image velocimetry provides insights into the spatial and spectral segregation of turbulent flows with different helicity signs. We show that helicity generation and decay along the jet dramatically depends on the inflow swirl. Notably, we provide direct experimental evidence of the helicity cascade, discovering that swirls of the same sign can impart turbulent helicity of the opposite sign, challenging conventional assumptions. These findings offer valuable benchmarks for numerical simulations using different turbulent closure methods.

[Phys. Rev. Fluids 9, L062601] Published Wed Jun 05, 2024

Author(s): Ludovico Nista, Heinz Pitsch, Christoph D. K. Schumann, Mathis Bode, Temistocle Grenga, Jonathan F. MacArt, and Antonio Attili

We compare supervised super-resolution convolutional neural networks (CNNs) against generative adversarial networks (GANs)-based architectures in the ability to reconstruct turbulent flow fields. GANs demonstrated superior in-sample performance but faced challenges with out-of-sample flows. Incorporating a partially unsupervised adversarial training step with large eddy simulation inputs and dynamic upsampling selection improved GANs’ out-of-sample robustness, capturing small-scale features and turbulence statistics better than standard supervised CNNs. The study recommends integrating discriminator-based training to enhance super-resolution CNNs’ reconstruction capabilities.

[Phys. Rev. Fluids 9, 064601] Published Tue Jun 04, 2024

Author(s): A. Caquas, L. R. Pastur, and A. Genty

Have you ever tried spinning your water bottle to empty it more quickly? This experiment, familiar to the general public, has rarely been studied in the scientific literature, which focuses mainly on the non-rotational case. We show that this popular experiment is surprisingly complex. Our study reveals the presence of three flow regimes, which have a direct impact on the efficiency of the draining process.

[Phys. Rev. Fluids 9, 064701] Published Tue Jun 04, 2024

Author(s): V. Dzanic, C. S. From, and E. Sauret

This study explores, for the first time, the impact of plasticity on inertialess viscoelastic instabilities in strong elongational flows. Through detailed numerical simulations, it reveals how elastoviscoplastic effects induce complex and dynamic flow behaviors, leading to new flow states. Crucially, our findings reveal that plasticity can laminarize and suppress these instabilities, offering new strategies for controlling the instability mechanism.

[Phys. Rev. Fluids 9, 063301] Published Mon Jun 03, 2024

Author(s): Laurent Talon, Andreas Andersen Hennig, Alex Hansen, and Alberto Rosso

We consider different boundary conditions for imposing flow of yield stress fluids in porous media. In contrast to Newtonian fluids in porous media, imposing pressure or a given flow profile at the boundary leads to significantly different flow fields. In particular, we show that imposing a flow profile leads to a merging tree structure whose properties are governed by the dynamics of a directed polymer in a random medium.

[Phys. Rev. Fluids 9, 063302] Published Mon Jun 03, 2024

Author(s): David A. Cook and Joseph W. Nichols

Hypersonic boundary layers are susceptible to flow instabilities that cause laminar flow to transition to turbulence, significantly increasing aerodynamic drag and wall heating. We focus on how these instabilities are triggered by the environment by applying a control systems theory technique called “input-output analysis” that relies in part upon solving the Navier-Stokes equations in reverse, tracing instabilities back to their origins. In the complex interactions between atmospheric disturbances, shock waves created near the nose cone of a hypersonic vehicle, and boundary layer instabilities, we find two physical processes strongly connected to the bluntness of the nose cone tip.

[Phys. Rev. Fluids 9, 063901] Published Mon Jun 03, 2024

Author(s): Ran Holtzman, Marco Dentz, Marcel Moura, Mykyta V. Chubynsky, Ramon Planet, and Jordi Ortín

Fluid-fluid displacement is often irreversible—exhibiting hysteresis where reversal of the driving force (e.g. external pressure) does not reverse the fluids’ configuration. This irreversibility is linked to energy dissipation, a key to efficient design of engineering operations such as subsurface cleanup or energy storage. Here, we analyze (analytically, numerically, and experimentally) a novel model system that exposes a striking phenomenon: emergence of hysteresis and dissipation in a system made of individually “reversible” (non-hysteretic) entities, due to their spatial interactions mediated by interfacial tension.

[Phys. Rev. Fluids 9, 064001] Published Mon Jun 03, 2024

Author(s): J. P. Martínez Cordeiro and N. R. Aluru

Most of the literature on flow through nanoporous two-dimensional membranes has focused on static membranes, yet various studies have shown the relevance of fluid-structure interactions – particularly dynamic coupling – on flow through nanopores. Herein, we use Molecular Dynamics (MD) simulations to study the effects of rigid in-plane harmonic pore oscillations on water flow through nanoporous graphene. First, we repurpose a used technique as a framework to isolate the physical mechanisms caused by the dynamic pore from the injected heat. We show that dynamic opening/closing of flow routes inside the pore enhances flow by increasing axial velocity and decreasing water density inside the pore.

[Phys. Rev. Fluids 9, 064201] Published Mon Jun 03, 2024

Author(s): Ianto Cannon, Stefano Olivieri, and Marco E. Rosti

We use immersed boundary methods to simulate finite-size spheres and fibers in turbulent flows across a range of Taylor Reynolds numbers (12.8<Reλ<442) and solid mass fractions (0≤M≤1). Both particle shapes act as a “spectral shortcut” to the flow, with fibers extending this effect further into the dissipative range. Spheres enhance dissipation in two-dimensional sheets, while fibers enhance dissipation in structures with dimension between one and two. However, the particles’ effect on the anomalous dissipation tends to vanish as Reλ→∞. These findings have implications for microplastics in oceans, volcanic ash clouds, and sandstorms.

[Phys. Rev. Fluids 9, 064301] Published Mon Jun 03, 2024

Author(s): Johannes N. Hillestad, Srikar Yadala, Ingrid Neunaber, Leon Li, R. Jason Hearst, and Nicholas A. Worth

The “anomalous” peaks in experimentally obtained power spectral density plots in the wake of wind turbines are investigated with time-resolved volumetric measurements. To promote early tip vortex interaction, blades with different angles-of-attack are used on the same rotor. Using an advanced volumetric technique to obtain the velocity field in the wake, the tip vortex interaction is visualized and quantified. The captured tip vortices corroborate the findings from power spectral density plots at different downstream locations that only one vortex is dominant, demonstrating that a difference in initial vortex strength can result in vortical energy being distributed at unexpected frequencies.

[Phys. Rev. Fluids 9, L052701] Published Fri May 31, 2024

Author(s): T. Zemach

We investigate high-Reynolds-number polydisperse gravity currents propagating along a channel of general cross-section into a linearly stratified ambient fluid. We formulate and solve numerically the shallow water equations and present typical height and velocity profiles of the current and particle mass concentration. Two dimensionless parameters, Stratification (S) and particle buoyancy (Π), are relevant. Increasing S decreases the current velocity propagation, but as Π increases, the current propagates faster. For a specific S, Π dependence, an equilibrium occurs for a significant time and the system behaves like a system without particles propagating into the ambient of constant density.

[Phys. Rev. Fluids 9, 054105] Published Tue May 28, 2024

Author(s): Alessio Roccon, Francesco Zonta, and Alfredo Soldati

The transportation of oil through pipelines and channels is a highly energy-intensive operation, primarily due to the significant viscosity of the oil and the consequent high friction. Among the various friction reduction methods utilized in this domain, the water-lubricated approach has emerged as particularly promising. Our investigation focuses on assessing the efficacy of this technique through direct numerical simulations of turbulent channel flow. In this setup, we introduce two thin water layers near the walls, which serve to lubricate the flow of oil within the core.

[Phys. Rev. Fluids 9, 054611] Published Tue May 28, 2024

Author(s): Saini Jatin Rao, Siddhant Jain, and Saptarshi Basu

Often considered a childhood pastime, soap bubbles emerged as a captivating domain for rigorous scientific inquiry for generations. While blowing soap bubbles is familiar to everyone, the underlying physics of inflating them remains unanswered. In our investigation, we visualize the previously unexplored internal airflow experimentally, revealing a toroidal vortical flow that resembles a bound vortex ring. The air enters the bubble as a round jet, emerging from the nozzle opening and impinges on the expanding concave interior to form this toroidal vortex. We also predict several scaling laws for the inflation rate and dynamics of this confined vortical flow by varying the source pressure.

[Phys. Rev. Fluids 9, L051602] Published Tue May 28, 2024

Author(s): Loïc Chagot, Simona Migliozzi, and Panagiota Angeli

In this Letter, we explore the influence of colloids at liquid-liquid interfaces on droplet pinch-off dynamics in microfluidic devices. We uncover a significant deviation in droplet formation time compared to pure systems, similarly to surfactant-laden systems. Yet notably, colloids exert minimal impact on droplet size, indicating potential nonlinear effects. The dynamics of neck thinning without colloids agree with the classic pendant drop scaling laws, while particle presence replaces traditional viscous and inertial-viscous regimes with a single power law, suggesting an elastic behavior driven by soft particle interactions.

[Phys. Rev. Fluids 9, L052201] Published Tue May 28, 2024