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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, an experimental study of the flow pattern in the wall region of an SMX static mixer was performed by using electrochemical shear rate sensors, which were related to the local shear stress and the fluctuating rate of the velocity gradient.
Abstract: An experimental study of the flow pattern in the wall region of an SMX static mixer was performed by using electrochemical shear rate sensors. Electrochemical signals were related to the local shear stress and the fluctuating rate of the velocity gradient. Signal fluctuation analysis allowed one to define the flow regimes inside the static mixers. The laminar flow was characterized by a time-constant evolution of the velocity gradient, while the turbulent flow corresponded to the stabilization of the fluctuating rate of the velocity gradient for high values of pore Reynolds number, defined by considering the static mixer as a porous medium and by using a capillary model. The transition from a laminar flow to an intermediate flow occurred at a pore Reynolds number of about 200. The turbulent flow was observed at a pore Reynolds number of between 1500 and 3000.

26 citations

Journal Article
TL;DR: Primary flow may significantly influence cellular aggregation, platelet activation and endothelial cell mechanotransduction measurements, and this analysis methodology can also be extended to other experimental systems with complex non-linear flows.

26 citations

Journal ArticleDOI
TL;DR: In this paper, a flume experiment was performed using the pioneer plant Scirpus mariqueter and suspended sediment, and a statistical method was used to analyse the average velocity and the turbulence intensity.
Abstract: Vegetation in tidal flats can alter flow dynamics by increasing the velocity gradient and attenuating the wave energy. In this study, a flume experiment was performed using the pioneer plant Scirpus mariqueter and suspended sediment. Two cases are analysed: current-only and current-wave conditions with a regular wave. A statistical method is used to analyse the average velocity and the turbulence intensity. Results demonstrate that the plants can cause a velocity decrease in the vegetation region and an increase in the turbulence intensity below the top of the canopy. The combined effect of waves and vegetation on turbulence dramatically increases the flow velocity above the average water depth as well as the turbulence intensity profiles. In this study, the attenuation efficiency of the wave height is 0.0448 m -1 , which is identical to results using artificial plants with the same relative submerged depth. The drag force in current-wave conditions is almost twice of that observed in current-only conditions. The spectral analysis shows that only waves can influence high-frequency motion. In addition, an increase is observed in the bottom shear stress, mean grain size, and suspended concentration of the sediment during current-wave conditions.

26 citations

Journal ArticleDOI
TL;DR: In this paper, a modified set of transport equations that satisfy causality were derived from the underlying Boltzmann equation and applied to two problems: particle diffusion and viscous transport.
Abstract: It is well known that the standard transport equations violate causality when gradients are large or when temporal variations are rapid. We derive a modified set of transport equations that satisfy causality. These equations are obtained from the underlying Boltzmann equation. We use a simple model for particle collisions which enables us to derive moment equations non-perturbatively, i.e. without making the usual assumption that the distribution function deviates only slightly from its equilibrium value. We apply the model to two problems: particle diffusion and viscous transport. In both cases we show that signals propagate at a finite speed and therefore that the formalism obeys causality. When the velocity gradient is large on the scale of a mean free path, the viscous shear stress is suppressed relative to the prediction of the standard diffusion approximation. The shear stress reaches a maximum at a finite value of the shear amplitude and then decreases as the velocity gradient increases. In the case of a steady Keplerian accretion disk with hydrodynamic turbulent viscosity, the stress-limit translates to an upper bound on the Shakura-Sunyaev $\alpha$-parameter, namely $\alpha<0.07$. The limit on $\alpha$ is much stronger in narrow boundary layers where the velocity shear is larger than Keplerian.

26 citations

Journal ArticleDOI
TL;DR: In this article, the average behavior at different times of an ensemble of bead-spring chains, contained in a fluid particle that is moving around in a random velocity field obtained from direct numerical simulation of turbulent flow of a Newtonian fluid in a channel was studied.
Abstract: FENE-P bead–spring chains unravel in the presence of large enough velocity gradients. In a turbulent flow, this can result in intermittent added stresses and exchanges of energy between the chains and the fluid, whose magnitudes depend on the degree of unravelling and on the orientations of the bead–spring chains. These effects are studied by calculating the average behaviour at different times of an ensemble of chains, contained in a fluid particle that is moving around in a random velocity field obtained from direct numerical simulation of turbulent flow of a Newtonian fluid in a channel. The results are used to evaluate theoretical explanations of drag reduction observed in very dilute solutions of polymers.In regions of the flow in which the energy exchange with the fluid is positive, the possibility arises that turbulence can be produced by mechanisms other than the interaction of Reynolds stresses and the mean velocity gradient field. Of particular interest, from the viewpoint of understanding polymer drag reduction, is the finding that the exchange is negative in velocity fields representative of the wall vortices that are large producers of turbulence. One can, therefore, postulate that polymers cause drag reduction by selectively changing the structures of eddies that produce Reynolds stresses. The intermittent appearance of large added shear stresses is consistent with the experimental finding of a stress deficit, whereby the total local shear stress is greater than the sum of the Reynolds stress and the time-averaged shear stress calculated from the time-averaged velocity gradient and the viscosity of the solvent.

26 citations


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