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Velocity gradient

About: Velocity gradient is a research topic. Over the lifetime, 3013 publications have been published within this topic receiving 77120 citations.


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TL;DR: In this paper, the linear stability of zonal, parallel shear flow on a beta-plane is discussed, where the localized shear region supports unstable waves, while the far-field can support Rossby waves because of the ambient potential-vorticity gradient.
Abstract: The linear stability of zonal, parallel shear flow on a beta-plane is discussed. While the localized shear region supports unstable waves, the far-field can support Rossby waves because of the ambient potential-vorticity gradient. An infinite zonal flow with a continuous cross-stream velocity gradient is approximated with segments of uniform flow, joined together by segments of uniform potential vorticity. This simplification allows an exact dispersion relation to be found. There are two classes of linearly unstable solutions. One type is trapped to the source of energy and has large growth rates. The second type is weaker instabilities which excite Rossby waves in the far-field: the influence of these weaker instabilities extends far beyond that of the most unstable waves.

40 citations

Journal ArticleDOI
TL;DR: In this article, a 3D direct numerical simulation (DNS) study of the evolution of a self-propagating interface in forced constant-density statistically stationary homogeneous isotropic turbulence was performed by solving Navier-Stokes and level-set equations under a wide range of conditions that cover various ratios of the interface speed SL to the rms turbulent velocity U' and various (50, 100 and 200) turbulent Reynolds numbers Re by analysing computed data, the following issues were addressed: dependence of the speed and thickness of the fully developed statistically planar mean front that
Abstract: A 3D direct numerical simulation (DNS) study of the evolution of a self-propagating interface in forced constant-density statistically stationary homogeneous isotropic turbulence was performed by solving Navier–Stokes and level-set equations under a wide range of conditions that cover various (from 01 to 20) ratios of the interface speed SL to the rms turbulent velocity U' and various (50, 100 and 200) turbulent Reynolds numbers Re By analysing computed data, the following issues were addressed: (i) dependence of the speed and thickness of the fully developed statistically planar mean front that envelops the interface on U'/SL and Re, (ii) dependence of the fully developed mean turbulent flux of a scalar c that characterizes the state of the fluid (c=0 and 1 ahead and behind the interface respectively) on U'/SL and Re, (iii) evolution of the mean front speed, its thickness, and the mean scalar flux during the front development after embedding a planar interface into the forced turbulence and (iv) relation between canonical and canonical and conditioned moments of the velocity, velocity gradient and pressure gradient fields

40 citations

Journal ArticleDOI
TL;DR: In this article, the shape of the mixing layer appears to be strongly affected by the streamwise acceleration and complex lateral confinement due to the side walls and the corners of the junction, leading to a streamwise increase of the mean velocity along the centerline and a decrease of the velocity difference.
Abstract: When two open-channel flows merge in a three-branch subcritical junction, a mixing layer appears at the interface between the two inflows. If the width of the downstream channel is equal to the width of each inlet channel, this mixing layer is accelerated and is curved due to the junction geometry. The present work is dedicated to simplified geometries, considering a flat bed and a $$90^{\circ }$$ angle where two configurations with different momentum ratios are tested. Due to the complex flow pattern in the junction, the so-called Serret–Frenet frame-axis based on the local direction of the velocity must be employed to characterize the flow pattern and the mixing layer as Cartesian and cylindrical frame-axes are not adapted. The analysis reveals that the centerline of the mixing layer, defined as the location of maximum Reynolds stress and velocity gradient, fairly fits the streamline separating at the upstream corner, even though a slight shift of the mixing layer towards the center of curvature is observed. The shape of the mixing layer appears to be strongly affected by the streamwise acceleration and the complex lateral confinement due to the side walls and the corners of the junction, leading to a streamwise increase of the mean velocity along the centerline and a decrease of the velocity difference. This results in a specific streamwise evolution of the mixing layer width, which reaches a plateau in the downstream region of the junction. Finally, the evaluation of the terms in the Reynolds-Averaged-Navier–Stokes equations reveals that the streamwise and normal acceleration and the pressure gradient remain dominant, which is typical of accelerated and rotational flows.

40 citations

Journal ArticleDOI
TL;DR: In this article, the velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to interaction with the surface pressure fluctuations converted from the velocity fluctuations of the oncoming jet.
Abstract: Local measurement of the mass-transfer rate and velocity gradient when an axisymmetric jet impinges on a flat plate was carried out using an electrochemical technique. Local measurement of the surface pressure on the flat plate was carried out separately using piezoelectric pressure transducers. The stagnation-point mass-transfer coefficient reaches a maximum when the flat plate is placed at 6 nozzle diameters from a convergent nozzle. It has been confirmed that the mass transfer to the flat plate for a high Schmidt number is greatly enhanced owing to the velocity-gradient disturbances in the wall region of the boundary layer, while the momentum transfer is insensitive to such disturbances. The relative intensity of the velocity-gradient fluctuations on the wall has an extremely large value at and near to the stagnation point, and decreases downstream, approaching a large constant value.These velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to the interaction with the surface-pressure fluctuations converted from the velocity fluctuations of the oncoming jet.The three co-ordinate dimensions of large-scale eddies are calculated from the auto- and spatial correlations of the surface-pressure fluctuations. It is considered that such large-scale eddies play an important role in the production of a velocity-gradient disturbance in the wall region of the boundary layer from the velocity turbulence of the approaching jet.

40 citations

Journal ArticleDOI
TL;DR: In this paper, the stress autocorrelation functions, provided independently by these two techniques, agree remarkably well with each other over the period of time investigated, i.e. the length of the non-equilibrium segments (0·76 ps).
Abstract: The shear viscosity of n-butane has been computed by the method of molecular dynamics (MD), by both the Green-Kubo technique and a non-equilibrium method employing a weak fictitious external field inducing a symmetric velocity gradient (pure deformational flow). In this last case, non-equilibrium segments, starting from equilibrium configurations, were averaged, the subtraction technique being used to measure the signal out of noise. The stress autocorrelation functions, provided independently by these two techniques, agree remarkably well with each other over the period of time investigated, i.e. the length of the non-equilibrium segments (0·76 ps). This function presents a long positive tail at two definitely different thermodynamical states: a dense liquid and a supercritical fluid. This tail, which is difficult to interpret, extends over 3·0 and 1·5ps respectively and gives a non-negligible contribution to the viscosity coefficient. In this particular case where the correlation time is relatively long...

40 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202318
202233
2021127
2020116
2019134
201892