New Papers in Fluid Mechanics

Author(s): Linqi Yu, Mustafa Z. Yousif, Young-Woo Lee, Xiaojue Zhu, Meng Zhang, Paraskovia Kolesova, and Hee-Chang Lim

This study developed a mapping generative adversarial network (M-GAN) to predict unavailable parameters: streamwise velocity, temperature, and pressure from available velocity components. Two-dimensional Rayleigh–Bénard flow and turbulent channel flow are used to evaluate M-GAN performance. The results indicate that the proposed model has good capability to map the available parameters to unavailable parameters. Furthermore, M-GAN also has good generalization to predict the parameters from channel flows at different Reynolds numbers.

[Phys. Rev. Fluids 9, 024603] Published Tue Feb 20, 2024

Author(s): Vincent Labarre, Pierre Augier, Giorgio Krstulovic, and Sergey Nazarenko

We analyze direct numerical simulations of stratified turbulence without shear modes, and with or without vortical modes at various Froude and buoyancy Reynolds numbers. It allows us to investigate the effects of vortical modes on the dynamics of stratified flows. A spatiotemporal analysis reveals slow internal gravity waves interacting by triadic resonance instabilities in our strongly stratified flow simulations such as the one in the figure. We observe that removing vortical modes helps to concentrate the energy around the wave frequency, but it is not enough to observe a weak internal gravity wave’s turbulence regime.

[Phys. Rev. Fluids 9, 024604] Published Tue Feb 20, 2024

Author(s): Roshan Samuel and Mahendra K. Verma

Through high-resolution direct numerical simulations, we demonstrate that energy transfers in two-dimensional (2D) thermal convection exhibit Bolgiano-Obukhov scaling. This is in contrast with convection in three dimensions which follows the Kolmogorov-Obukhov phenomenology. This difference arises from the presence of inverse cascade in 2D which leads to a negative kinetic energy flux at small wavenumbers. The magnitude of this flux decreases with wavenumber due to the effect of buoyancy. We also demonstrate that entropy dissipation in the thermal boundary layers displays a scaling law that is observable from the entropy flux curves.

[Phys. Rev. Fluids 9, 023502] Published Fri Feb 16, 2024

Author(s): Hamad El Kahza and Pejman Sanaei

Using the Stokes equation for fluid flow and the advection-diffusion equation for the transport of solids, alongside threshold laws governing erosion and deposition, we present a model aimed at conducting a comprehensive analysis of both erosion and deposition processes within a porous medium composed of axisymmetric channels.

[Phys. Rev. Fluids 9, 024301] Published Fri Feb 16, 2024

Author(s): J. Meibohm, L. Sundberg, B. Mehlig, and K. Gustavsson

Caustic singularities of the spatial distribution of particles in turbulent aerosols enhance collision rates and accelerate coagulation. The rate of caustic formation depends sensitively on the particle inertia. We study caustic formation in a non-Gaussian statistical model to understand why there is a significant difference in formation rates between direct numerical simulations and Gaussian models. In the limit of small inertia, caustics form due to an optimal fluctuation of the Lagrangian fluid-velocity gradients, and we show that the formation rate depends sensitively on the tails of the gradient distribution, explaining the observed mismatch

[Phys. Rev. Fluids 9, 024302] Published Fri Feb 16, 2024

Author(s): S. Tomasi Masoni, A. Mariotti, M. Antognoli, C. Galletti, R. Mauri, M. V. Salvetti, and E. Brunazzi

Understanding flow regimes and mixing in microreactors is crucial for achieving high reaction yields. This study combines simulations and experiments in an X-microreactor up to Reynolds number (Re) 600. For Re > 375, an unsteady periodic regime is observed with a collapsing central vortical structure and symmetric vorticity shedding. Counterrotating vortices form, merge, and recreate the central vortex. Despite increased mixing in this regime, reaction yield remains similar due to reduced reactant residence time. A model predicting reaction yield based on mixing degree and nominal kinetic constant is developed, successfully encompassing all flow regimes and Damköhler numbers (0.1 < Da < 103).

[Phys. Rev. Fluids 9, 024202] Published Thu Feb 15, 2024

Author(s): Francisco Alcántara-Ávila, Luis Miguel García-Raffi, Sergio Hoyas, and Martin Oberlack

The symmetry-based turbulence theory has been used to derive new scaling laws for the streamwise velocity and temperature moments of arbitrary order. For this, it has been applied to an incompressible turbulent channel flow driven by a pressure gradient with a passive scalar equation coupled in. To …

[Phys. Rev. E 109, 025104] Published Wed Feb 14, 2024

Author(s): Sankalp Nambiar and J. S. Wettlaufer

We study the fluid mediated hydrodynamics of an orientable microscopic swimmer that is near a deformable boundary, and that can intrinsically execute random orientation changes. Accounting for swimmer reorientations via orientation tumbles or active Brownian rotations on time scales comparable to the boundary deformations, we find that a pusher-type swimmer can rotate away from the interface, while its attraction towards the interface is enhanced. Depending on the viscosity of the fluids on either side of the interface, the swimmer can experience a stronger migration towards the interface at short times, and away from the interface in the long-time limit.

[Phys. Rev. Fluids 9, 023102] Published Wed Feb 14, 2024

Author(s): Ryuta X. Suzuki, Shoji Seya, Takahiko Ban, Manoranjan Mishra, and Yuichiro Nagatsu

We experimentally investigate displacement of a more viscous liquid by a less viscous one in a Hele-Shaw cell using an aqueous two phase system, where phase separation occurs in the growing liquid-liquid interfacial region, by varying the injection flow rate and the phase separation rate. We show that the degree of the interfacial phase separation scales as a unique function of the ratio of the flow and phase separation rates and it decreases with the ratio. These results demonstrate that the interfacial phase separation is arrested by the imposed flow and determined by competition between the flow and phase separation rates. The arresting effect and the mechanism are numerically verified.

[Phys. Rev. Fluids 9, 024003] Published Wed Feb 14, 2024

Author(s): Dayi Wang, Ziao Wang, Juntao Chang, Lianjie Yue, Gang Wang, and Hao Chen

To better understand the three-dimensional topology of the interaction between the shock train and the background wave, the steady and dynamic characteristics of a shock train were investigated using wind-tunnel experiments and numerical simulation. A 14° wedge placed at the bottom and sidewalls was…

[Phys. Rev. E 109, 025103] Published Tue Feb 13, 2024

Author(s): Mohammad Mohaghar, Angelica A. Connor, Shuai Wu, Ruike Renee Zhao, and Donald R. Webster

This study quantifies the effects of breaking the symmetry of magnetic-responsive material appendage motion. Asymmetry is achieved through distinct shape changes and asymmetric cycle periods. The appendage with an asymmetric joint and asymmetric cycle demonstrates significantly faster downward motion (and enhances swimming efficiency) by reducing the vorticity strength and viscous energy dissipation in the surrounding fluid. The study provides insights into the induced flow and opens avenues for bio-inspired aquatic robots made from magnetic-responsive soft materials with the potential for fostering underwater propulsion and exploration in aquatic environments.

[Phys. Rev. Fluids 9, 023101] Published Tue Feb 13, 2024

Author(s): Rajnandan Borthakur and Uddipta Ghosh

Electrophoresis often coexists with imposed background flows in many applications. When particles carry nonuniform surface charge, and the fluid itself is complex, the particles may follow fascinating trajectories, as shown in this work. Indeed, the combined action of a background flow and complex fluidic medium may cause particles to undergo cross-stream migration. However, the very nature of its trajectory and the extent of its migration depends on whether the imposed flow or the electrophoretic propulsion dominates the motion. The insights provided here may be exploited for improving electrophoretic separation and sorting of particles based on their size and surface charge.

[Phys. Rev. Fluids 9, 023302] Published Tue Feb 13, 2024

Author(s): Ilya Barmak, Davide Picchi, Alexander Gelfgat, and Neima Brauner

A rigorous numerical solution for steady laminar flows of shear-thinning fluids in rectangular ducts is presented for the first time. We derive universal scaling laws for the effective viscosity that depends on the dimensionless rheological parameters and the effective channel size that is a function of the aspect ratio of the duct. These allow us to generalize the classical formula for the friction factor of Newtonian flows (12/Re) to Carreau fluids flowing in rectangular ducts of any aspect ratio, where the Reynolds number is based on the effective channel size and the effective viscosity.

[Phys. Rev. Fluids 9, 023303] Published Tue Feb 13, 2024

Author(s): Ronald du Puits

Roughness at a surface hotter or colder than its environment may significantly enhance the convective heat transfer coefficient. This enhancement results from modification of the near-wall flow field. Our work demonstrates how roughness elements deform the temperature field compared to a smooth surface, and, under which conditions the heat transfer coefficient is enhanced.

[Phys. Rev. Fluids 9, 023501] Published Tue Feb 13, 2024

Author(s): Garima Singh and Naveen Tiwari

The stability of a traveling wave on the outside of a heated solid vertical cylinder is considered. The axisymmetric traveling wave becomes unstable and leads to the formation of asymmetric droplets over the cylinder. Eigenvalue analysis indicates that at smaller wavenumbers, the gravity mode exists, and a thermocapillary mode also exists but at larger wavenumbers. Patterns generated using nonlinear analysis show that the selected mode is governed by the geometric confinement in the azimuthal direction. Interesting nonlinear patterns are obtained due to an interesting interplay between the gravity mode and thermocapillary mode.

[Phys. Rev. Fluids 9, 024002] Published Tue Feb 13, 2024

Author(s): Vitaliy Kaminker

Turbulent power is introduced in to Jupiter’s magnetosphere near the cen- trifugal equator of the plasma disc. Turbulent fluctuations are generated within the plasma fluid. Turbulent energy then travels out of the plasma disc via shear Alfvén waves, dissipating out of the system along the way.

[Phys. Rev. Fluids 9, 024602] Published Tue Feb 13, 2024

Author(s): Alireza Mashayekhi, Coralie Vazquez, Hongying Zhao, Michael Gattrell, James F. Gilchrist, and John M. Frostad

In prior work, it was observed that some bitumen droplets coalesced faster when colliding in shear than colliding head-on. Inspired by this observation, we aimed to reproduce the same behavior in a simpler system composed of pure oil, water, and surfactants/particles. Using a cantilevered-capillary force apparatus we observed this phenomenon for droplets stabilized by cellulose nanocrystals and coined the term “shear-triggered coalescence” to describe it.

[Phys. Rev. Fluids 9, 023602] Published Thu Feb 08, 2024

Author(s): Vira Dhaliwal, Christian Pedersen, Kheireddin Kadri, Guillaume Miquelard-Garnier, Cyrille Sollogoub, Jorge Peixinho, Thomas Salez, and Andreas Carlson

Liquid nanofilms are subject to rupture due to intermolecular forces triggered by surface perturbations arising from thermal fluctuations. When a shear stress is imposed at the free surface it becomes stable in the direction of shear, but perturbations can still grow in the perpendicular direction to the shear.

[Phys. Rev. Fluids 9, 024201] Published Thu Feb 08, 2024

Author(s): Daniel Morón and Marc Avila

Pulsatile pipe flow, or the flow in a pipe with a mean and one or more harmonic bulk velocity components, is a benchmark to study unsteady driven flows in industrial and biological applications. We study the transitional regime of pulsatile pipe flow at moderate-to-high amplitudes and intermediate pulsation frequencies. We show that, as in steady driven pipe flow, the first long-lived turbulent structures are localized. We combine direct numerical simulations, causal analysis and turbulence modeling to describe the behavior of these turbulent patches in pulsatile pipe flow, and to determine the physical mechanisms by which they survive the pulsation.

[Phys. Rev. Fluids 9, 024601] Published Thu Feb 08, 2024

Author(s): Yu Zhao and Haitao Xu

We propose a focused laser heating method to study wave properties of flowing soap films. The laser-induced thermocapillary flows lead to symmetric disturbances in film thickness. The shock waves originating from the propagating symmetric elastic waves, which have remained elusive despite considerable experimental efforts, are thus stimulated on flowing soap films with low surfactant concentrations, or appreciable elasticities. Interestingly, on soap films with high surfactant concentrations, or vanishingly small elasticities, the laser-induced disturbances in film thickness remain unchanged and flow with the film without propagating, creating “laser-engraving” on free liquid films.

[Phys. Rev. Fluids 9, L022001] Published Thu Feb 08, 2024