Velocity gradient

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

Gas-flow measurements in a jet flame using cross-correlation of high-speed-particle images

Abstract: Temporal variations of a two-dimensional distribution of velocities in a nitrogen jet and a methane-jet flame are measured by cross-correlation particle-image velocimetry (PIV). Two different approaches to investigating the turbulence characteristics are demonstrated. One gives the distribution of ensemble-averaged velocities and turbulence intensities by means of repetitive PIV measurements using a double-pulse laser for a longer period. The other provides the detailed motion of velocity profiles for a shorter duration, allowing one to analyse the characteristic scale of turbulence using a high-power continuous laser. The accuracy of measurements of the time-averaged velocity and turbulence intensity is quantitatively assessed on the basis of the agreement with the results from hot-wire-anemometry (HWA) measurement. This indicates the feasibility of the PIV measurement, which may supply information about turbulence characteristics. From the measured results for a jet and a jetting flame, it is shown that the velocity gradient in the shear layer in the reacting zone is increased due to the local acceleration caused by buoyancy, resulting in higher turbulence intensities than those in a non-reacting jet. Also, from the change in the distribution of velocity vectors with time, it is clear that the turbulence eddies are carried downstream along the gas motion with little transformation. The time scale of turbulence at each location in the flow is obtained from the autocorrelation function of the velocity fluctuations. Furthermore, this can afford an estimate of the turbulence length scale if one assumes that the Taylor hypothesis is valid and multiplies the time scale and the time-average velocity. It is shown that the characteristic length scales of a flaming jet are about 1.5 times greater than those of a non-flaming jet. The effects of combustion on the turbulence in a flaming jet are discussed in detail on the basis of these experimental results.

A new approach to determine optically thick H2 cooling and its effect on primordial star formation

Abstract: We present a new method for estimating the H2 cooling rate in the optically thick regime in simulations of primordial star formation. Our new approach is based on the TreeCol algorithm, which projects matter distributions onto a spherical grid to create maps of column densities for each fluid element in the computational domain. We have improved this algorithm by using the relative gas velocities, to weight the individual matter contributions with the relative spectral line overlaps, in order to properly account for the Doppler effect. We compare our new method to the widely used Sobolev approximation, which yields an estimate for the column density based on the local velocity gradient and the thermal velocity. This approach generally underestimates the photon escape probability, because it neglects the density gradient and the actual shape of the cloud. We present a correction factor for the true line overlap in the Sobolev approximation and a new method based on local quantities, which fits the exact results reasonably well during the collapse of the cloud, with the error in the cooling rates always being less than 10%. Analytical fitting formulae fail at determining the photon escape probability after formation of the first protostar (error of 40%) because they are based on the assumption of spherical symmetry and therefore break down once a protostellar accretion disc has formed. Our method yields lower temperatures and hence promotes fragmentation for densities above 10^{10}/ccm at a distance of 200AU from the first protostar. Since the overall accretion rates are hardly affected by the cooling implementation, we expect Pop III stars to have lower masses in our simulations, compared to the results of previous simulations that used the Sobolev approximation.

Influence of nozzle geometry on polystyrene degradation in convergent flow

Abstract: Polymer degradation is readily observed in flows where the extensional component surpasses the rotational component of the velocity gradient. This type of flow is conveniently obtained by pushing a liquid into a convergent channel across an orifice. Kinetics of chain scission is sensitive to subtle modification of the coil conformation, which in turn depends on the details of the pervading flow field. By changing the orifice diameter and the conical angle of the inlet, it is possible to modify the spatial distribution of the velocity gradient, and hence, the residence time of a fluid element in the high strain-rate region. Degradation yields, measured under π-conditions in decalin by Gel Permeation Chromatography, showed a strong dependence on the fluid velocity at the orifice, but not on the magnitude of the strain-rate. This result is contrary to the common belief that assumes viscous friction, proportional to the strain-rate, is the determining factor for the scission rate of a bond under stress. Rather, experimental findings tend to indicate that the driving force for chain scission was provided by the energy accumulated in the coil during the flow-induced deformation process. The sharp propensity for mid-chain scission was maintained regardless of the nozzle geometry.

A Continuous Adjoint Method for the Minimization of Losses in Cascade Viscous Flows

Abstract: A continuous adjoint formulation for the minimization of viscous losses in laminar cascade flows is presented. The losses are expressed in terms of entropy generation due to the boundary layer formation and development. The minimization of the entropy difference between the inlet to and outlet from the flow domain results from the minimization of a field integral, expressed in terms of the velocity gradient. For the latter, appropriate field adjoint equations along with boundary conditions are derived, leading to sensitivity derivatives depending only upon wall boundary terms. The Lagrange multiplier penalty method is used to handle geometrical constraints related to the minimum allowed thickness of the designed cascade airfoils. For the sake of comparison, a discrete adjoint method was also programmed and used for the solution of the same problem, in which the total pressure losses, instead of the entropy increase, was used as the objective function.

Effect of the orientation of the magnetic field on the flow of a magnetorheological fluid. I. Plane channel

Abstract: Shear flow and Poiseuille flow of magnetorheological fluids in a plane channel under an inclined magnetic field are studied. The proposed theoretical model is based on stability analysis of chain-like clusters of dipolar magnetic particles. Hydrodynamic and magnetic torque acting on aggregates balances each other such that some misalignment between orientations of chains and the field takes place. Two magnetic field directions symmetric relative to the velocity gradient influence the aggregate behavior in different ways. In one case, the aggregates tend to turn along the flow whereas in the other case, they tend to turn transverse to the flow. The predicted peak value of the yield stress arises for θ0=−π/4 (where θ0 is the clockwise angle between the velocity gradient and the direction of the field) and is 1.8 times the value for θ0=0. If we consider Poiseuille flow between two parallel plates, the velocity gradient, being positive on one side and negative on the other, the stress is no longer symmetric r...