New Papers in Fluid Mechanics

Irregular dependence on Stokes number, and nonergodic transport, of heavy inertial particles in steady laminar flows

Physical Review Fluids - Tue, 01/09/2024 - 10:00

Author(s): Anu V. S. Nath, Anubhab Roy, S. Ravichandran, and Rama Govindarajan

The dispersion of heavy inertial particles in a cellular flow made of Taylor-Green vortices is found to display non-ergodicity and sensitive dependence on initial particle location. Even more surprising is the sensitive and non-monotonic dependence on Stokes number. The large time dispersion of particles can be ballistic (red), diffusive (green) or trapped (blue), depending on where they have started in the flow. Diffusive particles show chaotic dynamics. Here the mutually exclusive group of initial particle locations form a non-ergodic set (as in the figure), unlike a turbulent flow, which is known to be ergodic.


[Phys. Rev. Fluids 9, 014302] Published Tue Jan 09, 2024

Gravity current escape from a topographic depression

Physical Review Fluids - Tue, 01/09/2024 - 10:00

Author(s): Edward W. G. Skevington and Andrew J. Hogg

Density-driven flows climb out of topographic depressions if they are sufficiently energetic. We investigate the inertial dynamics of these unsteady flows theoretically as fluid climbs from a lower to an upper plateau and then simultaneously propagates away from and drains back into the depression. The volume of fluid that escapes the confinement diminishes with a power-law dependence upon time; the draining flow becomes self-similar, and the self-similarity is of the second kind, featuring an exponent which is a function of the frontal Froude number. The volume continues to decrease even when viscous processes are non-negligible and ultimately none of the fluid escapes from the depression.


[Phys. Rev. Fluids 9, 014802] Published Tue Jan 09, 2024

Pulsation mechanism of a Taylor cone under a single pulse voltage

Physical Review Fluids - Mon, 01/08/2024 - 10:00

Author(s): Jin-bo Cheng, Qi-you Liu, Li-jun Yang, Jun-xue Ren, Hai-bin Tang, Qing-fei Fu, and Luo Xie

Using a needle-plate electro-atomization experimental device, and applying a single pulse disturbance voltage signal, the response of the Taylor cone to a disturbance signal was explored. At the experimental level, the coupling relationship between polarization charge relaxation time and the oscillation period of the Taylor cone was uncovered, revealing the oscillation mechanism of the Taylor cone under this voltage disturbance.


[Phys. Rev. Fluids 9, 013701] Published Mon Jan 08, 2024

Wave motions due to a point source pulsating and advancing at forward speed parallel to a semi-infinite ice sheet

Physical Review Fluids - Fri, 01/05/2024 - 10:00

Author(s): Z. F. Li, G. X. Wu, and Y. Y. Shi

In the Arctic region, with the reduction of ice extent and thickness, a shipping route may become possible. We theoretically derive the wave motions induced by a point source pulsating and advancing at the marginal ice zone. It is found that when a ship navigates along the edge of an ice sheet, the free surface wave pattern has two V-shaped components. The outer V-wave is very similar to the common free surface wave without the ice sheet, while the inner V-wave is mainly due to the reflection of the outer V-wave by the ice sheet.


[Phys. Rev. Fluids 9, 014801] Published Fri Jan 05, 2024

Colloidal diffusiophoresis in crossed electrolyte gradients: Experimental demonstration of an “action-at-a-distance” effect predicted by the Nernst-Planck equations

Physical Review Fluids - Thu, 01/04/2024 - 10:00

Author(s): Ian Williams, Patrick B. Warren, Richard P. Sear, and Joseph L. Keddie

In an externally imposed electrolyte (salt) concentration gradient, charged colloids drift at speeds of order one micrometre per second. This phenomenon is known as diffusiophoresis. In systems with multiple salts and “crossed” salt gradients, a nonlocal component of the electric field associated wi…


[Phys. Rev. Fluids 9, 014201] Published Thu Jan 04, 2024

Attraction of neutrally buoyant deformable particles towards a vortex

Physical Review Fluids - Thu, 01/04/2024 - 10:00

Author(s): Yutaro Fujiki, Hideto Awai, Yutaro Motoori, and Susumu Goto

Deformable elastic particles can accumulate around a vortex center even if the particle is neutrally buoyant. The angle between the deformed particle and the pathline plays important roles in this accumulation process. In this paper, we propose a simple model to explain this interesting accumulation phenomenon.


[Phys. Rev. Fluids 9, 014301] Published Thu Jan 04, 2024

Off-lattice Boltzmann simulation of conjugate heat transfer for natural convection in two-dimensional cavities

Physical Review E - Wed, 01/03/2024 - 10:00

Author(s): Kuldeep Tolia, Sai Ravi Gupta Polasanapalli, and Kameswararao Anupindi

This study addresses the inadequacy of isothermal wall conditions in predicting accurate flow features and thermal effects in multicomponent systems. A finite-difference characteristic-based off-lattice Boltzmann method (OLBM) with a source term-based conjugate heat transfer (CHT) model is utilized …


[Phys. Rev. E 109, 015101] Published Wed Jan 03, 2024

Self-diffusiophoresis with bulk reaction

Physical Review Fluids - Wed, 01/03/2024 - 10:00

Author(s): Rodolfo Brandão, Gunnar G. Peng, David Saintillan, and Ehud Yariv

Catalytic motors, which self-propel in a liquid due to an inhomogeneous surface reaction, constitute an important illustration of active matter in a non-biological context. Prevailing models of the associated self-diffusiophoretic transport assume a chemical reaction at the boundary of the swimmer. We here address the more realistic scenario where that reaction is balanced by a homogeneous reaction in the bulk. The associated diffusive transport of solute, described by two Damköhler numbers, exhibits a boundary-layer topology which is not encountered in the prevailing models.


[Phys. Rev. Fluids 9, 014001] Published Wed Jan 03, 2024

Asymmetric Kelvin-Helmholtz instabilities in stratified shear flows

Physical Review Fluids - Tue, 01/02/2024 - 10:00

Author(s): Adam J. K. Yang, Mary-Louise Timmermans, and Gregory A. Lawrence

This study elucidates the regime of fluid instabilities that can arise in a stratified shear flow when density and velocity interfaces are not aligned - a common occurrence in various geophysical flows. Through a combination of linear stability analysis and direct numerical simulations, we unveil a hybrid mode characterized by features of both Kelvin-Helmholtz and Holmboe instabilities. By quantifying the crucial role of asymmetry, our findings contribute to a refined understanding of the dynamics and mixing in these stratified shear flows.


[Phys. Rev. Fluids 9, 014501] Published Tue Jan 02, 2024

Effect of layer thickness for the bounce of a particle settling through a density transition layer

Physical Review E - Thu, 12/28/2023 - 10:00

Author(s): Shuhong Wang, Jiadong Wang, and Jian Deng

We study numerically a spherical particle settling through a density transition layer at moderate Reynolds numbers Reu=69∼259 for the upper fluid. We investigate how the transition layer thickness affects the particle's bouncing behavior as it crosses the interface. The previous intuitive understand…


[Phys. Rev. E 108, 065108] Published Thu Dec 28, 2023

Vorticity wave interaction, Krein collision, and exceptional points in shear flow instabilities

Physical Review E - Thu, 12/28/2023 - 10:00

Author(s): Cong Meng and Zhibin Guo

We relate the model of vorticity wave interaction to Krein collision, PT-symmetry breaking, and the formation of exceptional points in shear flow instabilities. We show that the dynamical system of coupled vorticity waves is a pseudo-Hermitian system with nonreciprocal coupling terms. Krein signatur…


[Phys. Rev. E 108, 065109] Published Thu Dec 28, 2023

Dynamic wetting experiments with nitrogen in a quasi-capillary tube

Physical Review Fluids - Thu, 12/28/2023 - 10:00

Author(s): Domenico Fiorini, Alessia Simonini, Johan Steelant, David Seveno, and Miguel Alfonso Mendez

We perform experiments with liquid nitrogen’s gas-liquid interface oscillations in a U-shaped quartz tube and investigate the wetting dynamics in inertia-dominated conditions. The experiments reveal a linear relationship between dynamic contact angle evolution and Capillary number in advancing conditions while the contact angle remains near equilibrium in receding conditions. An equivalent contact angle, derived from a model, shows the overall independence of the capillary pressure from the actual contact angle evolution. Theoretical analysis indicates viscous forces dominate in small tubes, while gravity and inertial forces govern larger tube oscillations.


[Phys. Rev. Fluids 8, 124004] Published Thu Dec 28, 2023

Unified description over time of heterogeneous condensation with quenched disorder

Physical Review E - Wed, 12/27/2023 - 10:00

Author(s): Ruddy Urbina and Wenceslao González-Viñas

We report experimental results on breath figures (BFs) observed on substrates with quenched disorder. The evolution of BFs is found to be primarily influenced by global parameters associated with boundary conditions. We investigate classical statistical measures and explore topological properties us…


[Phys. Rev. E 108, 065107] Published Wed Dec 27, 2023

Unsteady granular chute flows at high inertial numbers

Physical Review Fluids - Tue, 12/26/2023 - 10:00

Author(s): Satyabrata Patro, Anurag Tripathi, Sumit Kumar, and Anubhav Majumdar

High speed granular flows flowing over inclined surfaces are studied using discrete element method and continuum simulations. Significant slip velocity at the base and strong oscillations in the layer height are observed. The popular inertial-number-based JFP rheological model fails to capture the transient flow dynamics at high inertial numbers. Accounting for the layer dilatancy effect and the presence of normal stress difference is essential to accurately predict the average flow behavior. A modified rheological model recently proposed by the authors, accounting for all these effects describes the evolution of the high-speed granular flow very well for both low as well as high inertial numbers.


[Phys. Rev. Fluids 8, 124303] Published Tue Dec 26, 2023

Freezing of sessile droplet and frost halo formation

Physical Review Fluids - Fri, 12/22/2023 - 10:00

Author(s): Sivanandan Kavuri, George Karapetsas, Chander Shekhar Sharma, and Kirti Chandra Sahu

Freezing droplets are ubiquitous in many practical applications, like in aircraft and wind turbine icing, spray technologies, food and pharmaceutical engineering, and natural phenomena such as raindrop formation. In the context of sessile droplets, the freezing process unveils fascinating physics, featuring a frost halo on the substrate, the evolution of the liquid-ice interface, and the formation of a cusp-like morphology at the tip of the droplet. Our study explores the novel theoretical aspects of frost halo formation, revealing its intricate link to the inherent evaporation process during the initial freezing phases.


[Phys. Rev. Fluids 8, 124003] Published Fri Dec 22, 2023

Hydrodynamic interactions change the buckling threshold of parallel flexible sheets in shear flow

Physical Review Fluids - Fri, 12/22/2023 - 10:00

Author(s): Hugo Perrin, Heng Li, and Lorenzo Botto

Viscous-flow induced buckling modifies the morphology of sheet-like particles suspended in liquids. Experiments reveal that a pair of sheets can bend at a shear rate ten times lower than the buckling threshold defined for a single sheet. Using simulations and modeling, we demonstrate that this softening is due to hydrodynamic interactions. Our study suggests that the morphology of sheet-like particles in suspensions and the resulting rheology is not a purely material property, but also depends on particle concentration and microstructure.


[Phys. Rev. Fluids 8, 124103] Published Fri Dec 22, 2023

Numerical comparison of two-dimensional Navier-Stokes flows on the whole plane and the periodic domain

Physical Review Fluids - Fri, 12/22/2023 - 10:00

Author(s): Koji Ohkitani

A direct numerical comparison is made of two-dimensional Navier-Stokes flows between two different boundary conditions, namely the whole plane and a periodic domain. We study their differences after adjusting the Reynolds number. The norms generally decay faster on the periodic domain than on the whole space. In the case of a simple vortex merger the difference is appreciable, whereas the difference is small in the case of turbulence. This gives support to studying finite-energy turbulence under periodic boundary conditions.


[Phys. Rev. Fluids 8, 124607] Published Fri Dec 22, 2023

<i>A priori</i> screening of data-enabled turbulence models

Physical Review Fluids - Thu, 12/21/2023 - 10:00

Author(s): Peng E. S. Chen, Yuanwei Bin, Xiang I. A. Yang, Yipeng Shi, Mahdi Abkar, and George I. Park

A posteriori validation and verification of black box machine learned turbulence models is time consuming and is not always fruitful. We discuss a theoretical framework that allows a priori screening of machine-learned models that are based on feed-forward neural networks. It requires no knowledge of the weights and bias and only knowledge of the activation function. The method tells one whether a machine learned model preserves basic calibrations like the law of the wall.


[Phys. Rev. Fluids 8, 124606] Published Thu Dec 21, 2023

Amplitude of water pouring sound

Physical Review Fluids - Thu, 12/21/2023 - 10:00

Author(s): Mouad Boudina, Joonoh Kim, and Ho-Young Kim

The familiar pouring sound we all hear when preparing tea or coffee has been a rare topic of study so far, despite its importance in several applications. We experimentally find that the sound amplitude increases with the jet corrugation, indicating that thin jets are louder than thick ones for the same given height. When pouring from a high distance, the jet breaks up into impacting drops, and the amplitude increases with the jet length and diameter. Results show that the jet corrugation relates to the volume of entrained air, hence the pouring sound can enter as a practical method to measure water aeration rates.


[Phys. Rev. Fluids 8, L122002] Published Thu Dec 21, 2023

Prebifurcation enhancement of imbibition-drainage hysteresis cycles

Physical Review Fluids - Wed, 12/20/2023 - 10:00

Author(s): Ido Lavi, Lauren Rose, Ramon Planet, Jaume Casademunt, Stéphane Santucci, and Jordi Ortín

When flowing through disordered media, fluid-fluid interfaces are subjected to sharp capillary jumps that shape displacements at large scales. The intricacies of this phenomenon are hard to decipher due to the inherent stiffness and non-locality of the problem. Here we use an “imperfect” Hele-Shaw cell to elucidate the response to a single topographical defect as a function of the imposed flow rate, keeping to stable Saffman-Taylor conditions. Through imbibition-drainage experiments and numerical integration of a robust two-dimensional model we showcase the profound impact of the proximity to the fingering instability.


[Phys. Rev. Fluids 8, 124002] Published Wed Dec 20, 2023

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