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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 authors consider the statistical properties of small-scale velocity fluctuations in a turbulent channel flow at moderately high Reynolds number using the Johns Hopkins University Turbulence Database.
Abstract: In a channel flow, the velocity fluctuations are inhomogeneous and anisotropic. Yet, the small-scale properties of the flow are expected to behave in an isotropic manner in the very-large-Reynolds-number limit. We consider the statistical properties of small-scale velocity fluctuations in a turbulent channel flow at moderately high Reynolds number ( ), using the Johns Hopkins University Turbulence Database. Away from the wall, in the logarithmic layer, the skewness of the normal derivative of the streamwise velocity fluctuation is approximately constant, of order 1, while the Reynolds number based on the Taylor scale is . This defines a small-scale anisotropy that is stronger than in turbulent homogeneous shear flows at comparable values of . In contrast, the vorticity–strain correlations that characterize homogeneous isotropic turbulence are nearly unchanged in channel flow even though they do vary with distance from the wall with an exponent that can be inferred from the local dissipation. Our results demonstrate that the statistical properties of the fluctuating velocity gradient in turbulent channel flow are characterized, on one hand, by observables that are insensitive to the anisotropy, and behave as in homogeneous isotropic flows, and on the other hand by quantities that are much more sensitive to the anisotropy. How this seemingly contradictory situation emerges from the simultaneous action of the flux of energy to small scales and the transport of momentum away from the wall remains to be elucidated.
23 citations
TL;DR: In this article, a cone-and-plate device (CP-D) was used to obtain transient data on short glass fiber-filled polymeric fluids, and the Lipscomb model coupled with the Folgar-Tucker model was fit to the stress growth behavior measured using both the PP geometry and CP-D resulting in different parameters.
Abstract: The conventional method for obtaining transient rheological data on short glass fiber-filled polymeric fluids is to use the parallel disk (PP) geometry in a rotational rheometer. Using the PP geometry large transient stress overshoot behavior was observed during the startup of flow measurements on a 30 wt % short glass fiber-filled polybutylene terephthalate. A contributing factor to this behavior is believed to be induced fiber collisions caused by the inhomogeneous velocity field (radial varying velocity gradient). A novel approach was taken in which a “donut” shaped sample was used in a cone-and-plate device (CP-D) to maintain a sufficient gap to fiber length ratio. The magnitude of the first normal stress difference was reduced by 70%, and the time to reach steady state was reduced by 100 strain units. The Lipscomb model coupled with the Folgar–Tucker model for the evolution of fiber orientation was fit to the stress growth behavior measured using both the PP geometry and CP-D resulting in different parameters. In addition, the fitted model parameters were found to depend on the initial fiber orientation. It is believed that the CP-D allows for an accurate determination of the stress growth behavior and eventually will allow one to obtain unambiguous model parameters.
23 citations
TL;DR: In this paper, the lattice Boltzmann method was used for simulating the solute transport in rough fractures, and it was found that the film transport results in high solute dispersion, which is a better explanation for a long tailing in the breakthrough curve, rather than the diffusive exchange of solutes between mobile and immobile fluids.
Abstract: The purposes of this study are to investigate how fracture roughness affects solute transport and is to find a better transport mechanism causing a long breakthrough tailing. To achieve the purposes, the lattice Boltzmann method was used for simulating the solute transport in rough fractures. Rougher fracture yields a large velocity gradient across the aperture, especially at high average fluid velocity (Re>20). Consequently, solute transports fast in the middle of the aperture, and a significant fraction of solute are delayed and transport slowly along the fracture wall, which can be described as the “film” transport. This “film” transport results in high solute dispersion, and was found to be a better explanation for a long tailing in the breakthrough curve, rather than the most common explanation for a long tailing, the diffusive exchange of solutes between mobile and immobile fluids. As long as fluid velocity is kept small, discrepancy between tracer and hydraulic apertures may not be significant, regardless of fracture roughness.
23 citations
TL;DR: The theoretical analysis of dispersion with or without stress-induced diffusion and chemical reaction during laminar flow of viscoelastic fluids in capillaries is given in this paper.
Abstract: The theoretical analysis of dispersion with or without stress-induced diffusion and stress-induced chemical reaction during laminar flow of viscoelastic fluids in capillaries is given. The results are applied to diffusion and degradation in polymers under nonhomogeneous velocity gradient fields. It is shown that the concentration profiles during dispersion are strongly dependent on axial and radial positions in the presence of stress-induced diffusion. Stress-induced degradation rate of macromolecules during capillary flow decreases if the macromolecules are subject to stress-induced diffusion.
23 citations
TL;DR: In this article, the authors explore the ability of velocity gradients to measure the magnetization of interstellar medium and demonstrate that the distribution of velocity gradient orientations provides a reliable estimation of the magnetisation of the media.
Abstract: Recent developments of the Velocity Gradient Technique (VGT) show that the velocity gradients provide a reliable tracing of magnetic field direction in turbulent plasmas. In this paper, we explore the ability of velocity gradients to measure the magnetization of interstellar medium. We demonstrate that the distribution of velocity gradient orientations provides a reliable estimation of the magnetization of the media. In particular, we determine the relation between Alfvenic Mach number $M_A$ in the range of $M_A \in [0.2,1.7]$ and properties of the velocity gradient distribution, namely, with the dispersion of velocity gradient orientation as well as with the peak to base ratio of the amplitudes. We apply our technique for a selected GALFA-HI region and find the results consistent with the expected behavior of $M_A$. Using 3D MHD simulations we successfully compare the results with our new measure of magnetization that is based on the dispersion of starlight polarization. We demonstrate that, combined with the velocity dispersion along the line of sight direction, our technique is capable to delivering the magnetic field strength. The new technique opens a way to measure magnetization using other gradient measures such as synchrotron intensity gradients (SIGs) and synchrotron polarization gradients (SPGs).
23 citations