Journal of Fluid Mechanics

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ISSN / EISSN : 0022-1120 / 1469-7645
Published by: Cambridge University Press (CUP) (10.1017)
Total articles ≅ 26,391
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Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.524

Abstract:
Direct numerical simulations of free round jets at a Reynolds number ( is reduced, large mean strain rates in the transition region of laminar-inflow jets significantly enhance velocity fluctuations (non-dimensionalized by local mean velocity) and scalar mixing, whereas the effects are minimal in jets from turbulent inflow.
Islam Benouaguef, Naga Musunuri, Edison C. Amah, Denis Blackmore, Ian S. Fischer,
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.500

Abstract:
The aim of this paper is to experimentally and analytically study the solutocapillary flow induced in a waterbody due to the presence of a solute source on its surface and the mixing induced by this flow of the solutes and gases dissolved at and near the surface into the waterbody. According to the analytic solution, the induced flow is analogous to a doublet flow in the sense that the flow is directed towards (or away from) the source within a conical region with its vertex at the source, and outside the conical region, the flow is away from (or towards) the source. Our particle image velocimetry and planar laser-induced fluorescence data show that the actual flow is far more complex than the analytic solution because of the influence of other factors which are not accounted for in the analytic model. There is an approximate agreement, but only for the intermediate distances from the source needle. In experiments, the flow changes direction into the waterbody when the solute-induced surface tension gradient driving the flow becomes comparable to the surface tension gradients on the surface due to factors such as contamination and temperature gradients. This causes the mixing of gases dissolved near the surface into the waterbody. Also, as the solute gradient at the surface gives rise to the force that drives the flow, when the solute diffusion coefficient is smaller the flow persists longer because the gradient is maintained for a longer distance.
Sina Heydari,
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.551

Abstract:
Fish schools are ubiquitous in marine life. Although flow interactions are thought to be beneficial for schooling, their exact effects on the speed, energetics and stability of the group remain elusive. Recent numerical simulations and experimental models suggest that flow interactions stabilize in-tandem formations of flapping foils. Here, we employ a minimal vortex sheet model that captures salient features of the flow interactions among flapping swimmers, and we study the free swimming of a pair of in-line swimmers driven with identical heaving or pitching motions. We find that, independent of the flapping mode, heaving or pitching, the follower passively stabilizes at discrete locations in the wake of the leader, consistent with the heaving foil experiments, but pitching swimmers exhibit tighter and more cohesive formations. Further, in comparison to swimming alone, pitching motions increase the energetic efficiency of the group while heaving motions result in a slight increase in the swimming speed. A deeper analysis of the wake of a single swimmer sheds light on the hydrodynamic mechanisms underlying pairwise formations. These results recapitulate that flow interactions provide a passive mechanism that promotes school cohesion, and afford novel insights into the role of the flapping mode in controlling the emergent properties of the school.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.424

Abstract:
Cavitating flows include vapour structures with a wide range of different length scales, from micro-bubbles to large cavities. The correct estimation of small-scale cavities can be as important as that of large-scale structures, because cavitation inception as well as the resulting noise, erosion and strong vibrations occur at small time and length scales. In this study, a multi-scale cavitating flow around a sharp-edged bluff body is investigated. For numerical analysis, while popular homogeneous mixture models are practical options for large-scale applications, they are normally limited in the representation of small-scale cavities. Therefore, a hybrid cavitation model is developed by coupling a mixture model with a Lagrangian bubble model. The Lagrangian model is based on a four-way coupling approach, which includes new submodels, to consider various small-scale phenomena in cavitation dynamics. Additionally, the coupling of the mixture and the Lagrangian models is based on an improved algorithm that is compatible with the flow physics. The numerical analysis provides a detailed description of the multi-scale dynamics of cavities as well as the interactions between vapour structures of various scales and the continuous flow. The results, among others, show that small-scale cavities not only are important at the inception and collapse steps, but also influence the development of large-scale structures. Furthermore, a comparison of the results with those from experiment shows considerable improvements in both predicting the large cavities and capturing the small-scale structures using the hybrid model. More accurate results (compared with the traditional mixture model) can be achieved even with a lower mesh resolution.
, José-Agustín Arguedas-Leiva, , ,
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.543

Abstract:
Rotations of spheroidal particles immersed in turbulent flows reflect the combined effects of fluid strain and vorticity, as well as the time history of these quantities along the particle's trajectory. Conversely, particle rotation statistics in turbulence provide a way to characterise the Lagrangian properties of velocity gradients. Particle rotations are also important for a range of environmental and industrial processes where particles of various shapes and sizes are immersed in a turbulent flow. In this study, we investigate the rotations of inertialess spheroidal particles that follow Lagrangian fluid trajectories. We perform direct numerical simulations (DNS) of homogeneous isotropic turbulence and investigate the dynamics of different particle shapes at different scales in turbulence using a filtering approach. We find that the mean-square particle angular velocity is nearly independent of particle shape across all scales from the Kolmogorov scale to the integral scale. The particle shape does determine the relative split between different modes of rotation (spinning vs tumbling), but this split is also almost independent of the filter scale suggesting a Lagrangian scale-invariance in velocity gradients. We show how the split between spinning and tumbling can be quantitatively related to the particle's alignment with respect to the fluid vorticity.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.541

Abstract:
Dip coating is a common technique used to cover a solid surface with a thin liquid film, the thickness of which was successfully predicted by the theory developed in the 1940s by Landau & Levich (Acta Physicochem. URSS, vol. 17, 1942, pp. 141–153) and Derjaguin (Acta Physicochem. URSS, vol. 20, 1943, pp. 349–352). In this work, we present an extension of their theory to the case where the dipping bath contains two immiscible liquids, one lighter than the other, resulting in the entrainment of two thin films on the substrate. We report how the thicknesses of the coated films depend on the capillary number, on the ratios of the properties of the two liquids and on the relative thickness of the upper fluid layer in the bath. We also show that the liquid/liquid and liquid/gas interfaces evolve independently from each other as if only one liquid were coated, except for a very small region where their separation falls quickly to its asymptotic value and the shear stresses at the two interfaces peak. Interestingly, we find that the final coated thicknesses are determined by the values of these maximum shear stresses.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.529

Abstract:
Liquid foam exhibits surprisingly high viscosity, higher than each of its phases. This dissipation enhancement has been rationalized by invoking either a geometrical confinement of the shear in the liquid phase, or the influence of the interface viscosity. However, a precise localization of the dissipation, and its mechanism, at the bubble scale is still lacking. With this aim, we simultaneously monitored the evolution of the local flow velocity, film thickness and surface tension of a five-film assembly, induced by different controlled deformations. These measurements allow us to build local constitutive relations for this foam elementary building block. We first show that, for our millimetric foam films, the main part of the film has a purely elastic, reversible behaviour, thus ruling out the interface viscosity in explaining the observed dissipation. We then highlight a generic frustration at the menisci, controlling the interface transfer between neighbour films and resulting in the localization of a bulk shear flow close to the menisci. A model accounting for surfactant transport in these small sheared regions is developed. It is in good agreement with the experiment, and demonstrates that most of the dissipation is localized in these domains. The length of these sheared regions, determined by the physico-chemical properties of the solution, sets a transition between a large bubble regime, in which the films are mainly stretched and compressed, and a small bubble regime, in which they are sheared. Finally, we discuss the parameter range where a model of foam viscosity could be built on the basis of these local results.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.557

Abstract:
The streamwise vortex instability of boundary layers caused by wall roughness in the form of surface undulations is investigated. The instability is characterised by a roughness parameter needed for instability.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.517

Abstract:
Polarized vortical structures (i.e. with axial flow, thus coiled vortex lines) are generic to turbulent flows – hence the importance of their dynamics, interactions and cascade. Direct numerical simulations of two anti-parallel polarized vortex tubes are performed for vortex Reynolds numbers . Therefore, polarization can significantly alter the dynamics of vortex reconnection as well as turbulence cascade.
Journal of Fluid Mechanics, Volume 922; doi:10.1017/jfm.2021.518

Abstract:
We investigate deterministic and stochastic bifurcations in electroconvecitve flows of a dielectric liquid confined between two parallel plates subjected to a strong unipolar injection by direct numerical simulations. A long-standing discrepancy of linear instability criteria between the experiment and theory exists in this flow. We here test the hypothesis that the discrepancy may be related to the inhomogeneity in ion-exchange membranes used in experiments, contrasted by the homogeneous ion injection assumed in theoretical and numerical analyses. To study this effect, we consider stochastic boundary conditions around linear criticality and first bifurcations in this flow. For a complete presentation of flow bifurcations, deterministic bifurcation analysis (without stochasticity) is first performed to investigate primary bifurcations in this flow by progressively increasing the strength of electric field. Lyapunov spectrum and dimension are calculated and probed to characterise the chaotic motion therein. Our results confirm the high dimensionality of chaos in electroconvective flows and reveal for the first time that its chaos is extensive in a range of finite-sized systems. We then conduct stochastic bifurcation analyses by considering random perturbations in the boundary conditions of charge density and electric potential. Owing to the subcritical nature of electroconvective flows, the linear instability criteria under stochastic boundaries are closer to the experimental values than former theoretical and numerical results (assuming the homogeneous charge injection) for different levels of stochasticity, which confirms the hypothesis aforementioned. Furthermore, stochasticity can also enhance the efficiency of ionic transport.
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