Latest papers in fluid mechanics

Author(s): Q. Nguyen and D. V. Papavassiliou

Since flow regions of high helicity are associated with low dissipation of kinetic energy, helicity can be used to mark the flow structures that are major contributors to turbulent scalar dispersion. Employing a Lagrangian approach to compare the helicity between scalar tracers and fluid particles in the same flow field shows that flow structures that contribute to the transport of low Schmidt number markers are different than those for fluid particles. This is quite important in anisotropic wall turbulence, where the interplay between molecular and turbulent transport is critical.

[Phys. Rev. Fluids 5, 062601(R)] Published Tue Jun 23, 2020

Author(s): Marian Albers and Wolfgang Schröder

Drag reduction in turbulent boundary layer flow via swept transversal surface waves is investigated by large-eddy simulations. For partially downstream traveling waves, a decreased friction drag reduction and positively contributing pressure drag are determined while the opposite occurs for partially upstream traveling waves. It is also shown that an amplification of the outer layer turbulence can coexist with a drag-reduced near-wall state.

[Phys. Rev. Fluids 5, 064611] Published Tue Jun 23, 2020

Author(s): Bérengère Podvin, Stéphanie Pellerin, Yann Fraigneau, Antoine Evrard, and Olivier Cadot

The global wake dynamics of an Ahmed body are correctly captured by a proper-orthogonal-decomposition (POD)-based low-dimensional model. The POD modes include the quasisteady wake deviation with intermittent switches, both vortex shedding modes and bubble pumping mechanisms.

[Phys. Rev. Fluids 5, 064612] Published Tue Jun 23, 2020

Author(s): O. Soulard, J. Griffond, B.-J. Gréa, and G. Viciconte

Density variations modify the way pressure transports information over large distances. As a result, the long-range correlations of the velocity field behave differently in constant- and variable-density flows. This raises the question of the permanence of large eddies in variable-density turbulence. Does this principle also hold in this context? If yes, how is it expressed? An exploration of these issues is presented.

[Phys. Rev. Fluids 5, 064613] Published Tue Jun 23, 2020

Physics of Fluids, Volume 32, Issue 6, June 2020.
Hydraulic system is widely used as a power source in deep-sea operation equipment. The sealing performance and the relative movement of the parts in hydraulic components are significantly affected by the structural deformation at the clearance fit and the viscosity increase of the hydraulic fluid medium, which are both caused by high seawater pressure. In this paper, the deformation formula at the clearance fit in the deep-sea environment is deduced, which indicates that the deformed height of the fit clearance decreases linearly in the axial direction. A minimum clearance design criterion is proposed, and it is found that the smaller the difference between the bulk modulus of the matching parts and the fit radius are, the smaller the variation of the height of the fit clearance is under various working conditions and the smaller the required minimum initial height is. The leakage flow rate formula at the clearance fit in the deep-sea environment is deduced as well, which introduces modified factors to consider the effects of structural deformation, viscosity increase, and eccentricity. The calculation result shows that under the condition of 11 000 m deep sea, the leakage flow rate calculated by the classic formula is about five times larger than that of the modified formula. Multi-parameter fluid–solid-interaction simulations are carried out to verify the correctness of the deduced deformation and leakage flow rate formulas. The leakage flow rate of the situation with inclination involved is also analyzed through numerical simulations.

Physics of Fluids, Volume 32, Issue 6, June 2020.
We have performed direct numerical simulations of flow past a tapered circular cylinder during the early transition to three dimensions for two successive taper ratios (TR) of 20 and 12.5. Our results indicate the random occurrence of vortex splits and dislocations as the topology of the shedding signature. In particular, we observe oblique cellular shedding with multiple spanwise patterns and oppositely oriented oblique cells in the shed structure. Unlike flow imposed shear, the vortex formation length becomes sensitive to the taper ratio, which removes oblique frequency waves noticed for lower shear rate. The local Strouhal frequency (Stz) at the higher TR case exhibits a decreasing trend with remarkably smaller finite jumps at the cell boundaries and is found close to uniform cylinder flow. The wavelet analysis reveals the narrowing of the spectrum at a lower TR. A higher TR case shows a distinctly regular and evenly spaced spectrum which does not reach the maximum Stz, making it a rare event. The present results show that tapering causes the appearance of a secondary motion, which completely reverses at the downstream cylinder wake. Our numerical calculations show that pressure has an indirect role in the growth of the secondary instabilities, where isobars align along with the taper profile. The geometrically induced shear promotes greater mixing in the near wake, and we found that the maximum cross-stream velocity never exceeds 10% of the mean flow even with the steepest TR. The streamwise growth of the defect layer is slower for increasing TR and reaches an early saturation. Although the velocity deficit is higher at the steepest TR, it causes a delay in the momentum recovery along the streamwise direction. The shape factor for the lower TR case shows a delay in the laminar–turbulent transition. Finally, our global stability analysis results employing dynamic mode decomposition revealed a nonlinear dynamical system with spanwise dissipation of the dynamic modes.

Physics of Fluids, Volume 32, Issue 6, June 2020.
The application of artificial roughness to mitigate tip vortex cavitation inception is analyzed through numerical and experimental investigations carried out on an elliptical foil. Different roughness configurations and sizes are tested, and effects on cavitation inception, drag, and lift are studied. Implicit Large Eddy Simulation is employed to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. Two different approaches including using a rough wall function and resolving the flow around roughness elements are evaluated. New experiments, performed in the cavitation tunnel at Kongsberg Hydrodynamic Research Center, for the rough foil are presented. The vortical structures and vorticity magnitude distributions are employed to demonstrate how different roughness patterns and configurations contribute to the vortex roll-up and consequently on the tip vortex strength. It is found that the application of roughness on the leading edge, tip region, and trailing edge of the suction side is acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The boundary layer characteristics show that roughness separation line is the reason for a more even distribution of vorticity over the tip compared to the smooth foil condition, leading to a reduction in vortex strength. For the optimum roughness pattern, both the numerical results and experimental measurements show a decrease in the tip vortex cavitation inception as large as 33% compared to the smooth foil condition with a drag force increase observed to be less than 2%.

Physics of Fluids, Volume 32, Issue 6, June 2020.
In this study, we derive a spatially averaged two-fluid model for heat transport in moderately dense gas–particle flows. In the context of multiphase turbulence modeling, closure models for the unresolved terms in the filtered transport equations in the presence of mesoscale heterogeneous particle clusters are postulated. In analogy to the drift velocity correction for the resolved gas–particle drag force, we propose to approximate the filtered interphase heat transfer by the resolved heat transfer corrected by a drift temperature. This drift temperature represents the gas-phase temperature fluctuations seen by the particles and can be expressed as a correlation between the solid volume fraction variations and the gas-phase temperature fluctuations, i.e., the turbulent internal energy. Therefore, transport equations for the turbulent internal energies of the phases are derived, where a cluster-induced turbulence production term arises in the gas-phase. Except for the interphase exchange terms, we find that closure models based on single phase turbulence modeling can be applied to the unresolved terms in the transport equations for both the filtered and turbulent internal energies. The interphase exchange terms can be expressed by the variances of the temperatures scaled by correlation coefficients. A dynamic adjustment of the correlation coefficients by using test-filters in coarse-grid simulations is proposed. In an a priori study, the developed closure models show good agreement with the predictions obtained by filtering fine-grid, two-fluid model simulation data of Geldart type A and B particles in three-dimensional wall-bounded fluidized beds.

Author(s): Xiaohui Deng, Xiaoyu Wei, Xiaoping Wang, and Ping Sheng

We have obtained analytically the complete set of hydrodynamic modes (HMs) for a two-dimensional (2D) fluid confined within a channel with the Navier slip boundary condition at the hydrodynamic boundary. The HMs are orthogonal to each other and hence each represents an independent degree of freedom....

[Phys. Rev. E 101, 063104] Published Mon Jun 22, 2020

Author(s): Antoine Monier, Axel Huerre, Christophe Josserand, and Thomas Séon

Water flowing down a freezing plane builds a static layer of ice on which it keeps flowing. After a rapid diffusive ice growth, the system reaches a steady state resulting from the balance between the heat flow in the water and in the ice. This gives the ice a surprising linear shape.

[Phys. Rev. Fluids 5, 062301(R)] Published Mon Jun 22, 2020

Author(s): G. Boffetta, M. Borgnino, and S. Musacchio

Rayleigh-Taylor mixing in a porous medium is studied numerically in two and three dimensions. Density plumes are found to grow in a non-self-similar way, with length increasing faster than width. The evolution of the mixing layer is found to be quantitatively different in two and three dimensions.

[Phys. Rev. Fluids 5, 062501(R)] Published Mon Jun 22, 2020

Author(s): Pooyan Tirandazi and Carlos H. Hidrovo

Engineered surface textures can manipulate boundary layers affecting fluid drag. We study periodic, infinitely long spanwise grooves on a laminar boundary layer over a plate for 1000 < ReL < 25000. Below a certain width-to-depth aspect ratio (AR), a primary vortex inside each groove causes the freestream to “slip over”, reducing skin friction. Increasing AR poses a tradeoff in drag reduction due to pressure drag from groove vertical walls. Overall, transverse grooves for laminar flow can reduce total drag up to 10% compared to a flat plate, despite increasing the wetted surface area.

[Phys. Rev. Fluids 5, 064102] Published Mon Jun 22, 2020

Author(s): Pravat Rajbanshi and Animangsu Ghatak

Examining fluid flow inside a monolithic, three-dimensionally oriented, microfluidic channel is experimentally challenging because it does not permit a laser sheet to be effectively applied; tracking microscopic tracer particles is difficult; and monolithic construction prevents access to any material inside. We overcome these problems with an approach which captures the flow both along the channel axis and in a plane perpendicular to it, yet lets us capture the flow pattern over a wide Reynolds number range. Our observations corroborate results from flow simulations.

[Phys. Rev. Fluids 5, 064502] Published Mon Jun 22, 2020

Physics of Fluids, Volume 32, Issue 6, June 2020.
The role of an electric field on dielectric liquids confined in a differentially heated cavity is considered in microgravity conditions. In the purpose of sustaining permanent thermally induced electrohydrodynamic flows, a new electrode arrangement is proposed, different from the typical electrode configurations usually investigated in the literature. The aim is to generate a non-uniform distribution of the imposed electric field and to gain benefit from the existing temperature gradient to generate angular momentum. Scaling analysis and a numerical study are developed in order to investigate dielectrophoretic-induced convective heat transfers. The results show that a significant enhancement of heat transfers is made possible from the use of a non-intense non-uniform electric field with no need for giving rise to unstable regimes.

Physics of Fluids, Volume 32, Issue 6, June 2020.
In recent years, increasing attention has been paid to the ecological role of groyne fields as habitats for aquatic vegetation; however, knowledge on interactions between vegetation and recirculating flow is still lacking, especially vegetation effects on large-scale coherent structures in the mixing layer, which control the mass exchange between the side-cavity and the main channel. In this paper, the hydrodynamics of the mixing layer in straight open channels without sediments in the flow, with consecutive groyne fields, of different vegetation densities, is investigated both experimentally through particle image velocimetry and numerically through large eddy simulation. The results show that the presence of plants rearranges the circulation systems in the groyne field, namely, from double gyres to a single gyre. With an increase in the vegetation density, the exchange coefficient between the cavity and the main channel gradually decreases. Note that the exchange rate is calculated from a newly proposed exchange layer, which is located away from the groyne tip. Based on the analysis of the Kelvin−Helmholtz eddies along the shear layer, a phenomenological model is proposed for the evolution of coherent structures and the variations in flow hydrodynamics associated with these eddies. Compared to the non-vegetation case, the presence of vegetation could suppress the evolution of coherent eddies in the mixing layer, with a consequent effect on the flow hydrodynamics around the interface.

Physics of Fluids, Volume 32, Issue 6, June 2020.
Heat transfer enhancement due to flapping flags in a heated duct flow is studied using three-dimensional (3D) fully coupled fluid–structure–thermal simulations. Following prior work, which was limited to two-dimensional models, we examine the mechanisms and the heat transfer performance for a more realistic, 3D model of a flag in a rectangular duct heat exchanger. We then examine the role of the flag aspect-ratio and spanwise confinement, which are key design parameters for this device. We find that the narrow flags do not exhibit sufficiently energetic flapping to generate any meaningful heat transfer enhancement. We also find that the wide flags significantly increase heat flux and an increase in the width of the flag can further increase the thermal enhancement factor.

Physics of Fluids, Volume 32, Issue 6, June 2020.
A study of optimal temporal and spatial disturbance growths is carried out for three-dimensional viscous incompressible fluid flows with slippery walls. The non-modal temporal stability analysis is performed under the framework of normal velocity and normal vorticity formulations. A Chebyshev spectral collocation method is used to solve the governing equations numerically. For a free surface flow over a slippery inclined plane, the maximum temporal energy amplification intensifies with the effect of wall slip for the spanwise perturbation, but it attenuates with the wall slip when perturbation considers both streamwise and spanwise wavenumbers. It is found that the boundary for the regime of transient growth appears far ahead of the boundary for the regime of exponential growth, which raises a question on the critical Reynolds number for the shear mode predicted from the eigenvalue analysis. Furthermore, the eigenvalue analysis or the modal stability analysis reveals that the unstable region for the shear mode decays rapidly in the presence of wall slip, which is followed by the successive amplification of the critical Reynolds number for the shear mode and ensures the stabilizing effect of slip length on the shear mode. On the other hand, for a channel flow with slippery bounding walls, the maximum spatial energy amplification intensifies with the effect of wall slip in the absence of angular frequency, but it reduces with the wall slip if the angular frequency is present in the disturbance. Furthermore, the maximum spatial energy disturbance growth can be achieved if the disturbance excludes the angular frequency. Furthermore, it is observed that the angular frequency plays an essential role in the pattern formation of optimal response. In addition, the pseudo-resonance phenomenon occurs due to external temporal and spatially harmonic forcings, where the pseudo-resonance peak is much higher than the resonance peak.

Author(s): Zhengfeng Fan and Ming Dong

A compressible Rayleigh-Taylor (RT) unstable flow with a diffuse interface where the density varies smoothly supports a multiplicity of unstable eigenmodes that grow exponentially in time. This paper studies two relevant problems in a two-dimensional (2-D) RT flow by use of multimode decomposition. ...

[Phys. Rev. E 101, 063103] Published Fri Jun 19, 2020

Author(s): Florian Zaussinger, Peter Haun, Peter S. B. Szabo, Vadim Travnikov, Mustafa Al Kawwas, and Christoph Egbers

The GeoFlow experiment on the International Space Station was the first experiment to investigate rotating convection in a radial gravitylike force field. This led to the first observation of the existence of global columnar cells in a microgravity environment. The flow structures are identified by a machine learning algorithm and complemented by numerical simulations.

[Phys. Rev. Fluids 5, 063502] Published Fri Jun 19, 2020

Author(s): Basile Poujol, Adrian van Kan, and Alexandros Alexakis

The transition from forward to inverse energy cascade in turbulent flows in thin layers, varying the functional form of the forcing and the thickness of the layer, is investigated. As the forcing function becomes more three-dimensional, the inverse cascade is suppressed and the critical height hc, where the transition occurs, is decreased.

[Phys. Rev. Fluids 5, 064610] Published Fri Jun 19, 2020