Velocity gradient

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

Velocity gradients as a tracer for magnetic fields

Abstract: Strong Alfvenic turbulence develops eddy-like motions perpendicular to the local direction of magnetic fields. This local alignment induces velocity gradients perpendicular to the local direction of the magnetic field. We use this fact to propose a new technique of studying the direction of magnetic fields from observations, the Velocity Gradient Technique. We test our idea by employing the synthetic observations obtained via 3D MHD numerical simulations for different sonic and Alfven Mach numbers. We calculate the velocity gradient, $\mathbf{\Omega}$, using the velocity centroids. We find that $\mathbf{\Omega}$ traces the projected magnetic field best for the synthetic maps obtained with sub-Alfvenic simulations providing good point-wise correspondence between the magnetic field direction and that of $\mathbf{\Omega}$. The reported alignment is much better than the alignment between the density gradients and the magnetic field and we demonstrated that it can be used to find the magnetic field strength using the Chandrasekhar-Fermi method. Our study opens a new way of studying magnetic fields using spectroscopic data.

Normal stresses in concentrated non-Brownian suspensions

Abstract: We present an experimental approach used to measure both normal stress differences and the particle phase contribution to the normal stresses in suspensions of non-Brownian hard spheres. The methodology consists of measuring the radial profile of the normal stress along the velocity gradient direction in a torsional flow between two parallel discs. The values of the first and the second normal stress differences, is obtained. Most of our results compare well with the different experimental and numerical data present in the literature. In particular, our results show that the magnitude of the particle stress tensor component and their dependence on the particle volume fraction used in the suspension model balance proposed by Morris & Boulay (J. Rheol., vol. 43, 1999, p. 1213) are suitable.

Internal Viscosity of the Red Cell and a Blood Viscosity Equation

Abstract: BLOOD is known to be viscous and thixotropic1,2 and its viscosity decreases as the flow velocity, or velocity gradient, increases. While at near zero flow velocities, human blood may exhibit viscosities from 100 to 10,000 times that of water; at high flow velocities, it is only two to ten times that of water3,4. Perhaps the most remarkable aspect of blood is the fact that it remains fluid even at haematocrits of 95–100 per cent. If the red cells were rigid particles, the consistency of blood at high haematocrits would be that of a brick. It therefore seemed5 both obvious and simple that the fluidity of blood and of the packed blood cells can be explained only by the very low internal viscosity of the red cell.

Instability of certain shear flows in nematic liquids

Abstract: We have studied the simple shear flow, in the laminar regime, of a nematic (uniaxial) liquid film between two glass plates with carefully imposed boundary conditions. In the case considered here, the optical axis at rest is normal both to the flow velocity and to the velocity gradient. Using various methods of optical observation, we find the following facts: (a) When the shear rate s is below a certain threshold ${s}_{c}$, the optical axis is unperturbed everywhere. When $sg{s}_{c}$, it becomes distorted. ${s}_{c}$ is inversely proportional to the sample thickness. (b) When a stabilizing field $H$ is applied, ${s}_{c}$ increases. Furthermore, above a certain limiting field ${H}_{L}$, the nature of the instability changes: a pattern of rolls appears, the rolls being parallel to the (average) flow lines. These effects are then explained in terms of the Ericksen-Leslie-Parodi equations describing the couplings between orientation and flow in a nematic fluid. This analysis has led in turn to the prediction and observation of other remarkable effects occurring when the shear rate is modulated (at low frequencies) and when two fields $H$ (magnetic) and $E$ (electric) are applied at right angles.

Mass transfer from small particles suspended in turbulent fluid

Abstract: Small rigid spherical particles are suspended in fluid, and material is being transferred from the surface of each sphere by convection and diffusion. The fluid is in statistically steady turbulent motion maintained by some stirring device. It is assumed that the Peclet number of the flow around a particle is large compared with unity, so that a concentration boundary layer exists at the particle surface, and that the Reynolds number of the flow around the particle is sufficiently small for the velocity distribution near the particle surface to be given by the Stokes equations.The flow around a particle is a superposition of (a) a streaming flow due to a translational motion of the particle relative to the fluid with a velocity proportional to the density difference, and (b) a flow due to the velocity gradient in the ambient fluid. An expression for the mean transfer rate which is asymptotically exact for large Peclet numbers is obtained in terms of statistical parameters of these two superposed flow fields. As a consequence of the partial suppression of convective transfer by particle rotation, the only relevant parameters are the mean translational velocity of the particle in the direction of the ambient vorticity vector and the mean ambient rate of extension in the direction of the ambient vorticity. The former is shown to be zero in common turbulent flow fields, and an expression for the latter in terms of the mean dissipation rate e is obtained from the equilibrium theory of the small-scale components of the turbulence. The final non-dimensional expression for the transfer rate is 0·55(a2e½/κν½)1/3, where a is the particle radius. This is found to agree well with some previously published sets of data for values of less than 102.