Velocity gradient

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

Near-Wake Turbulent Flow Structure and Mixing Length Downstream of a Fractal Tree

Abstract: In order to study the turbulence structure behind a multiscale tree-like element in a boundary layer, detailed particle image velocimetry measurements are carried out in the near-wake of a fractal-like tree. The tree is a pre-fractal with five generations, each consisting of three branches and a scale-reduction factor of 1/2 between consecutive generations. Detailed mean velocity and turbulence stress profiles are documented, as well as their downstream development. Scatter plots of mean velocity gradient (transverse shear in the wake) and Reynolds shear stress exhibit a good linear relation at all locations in the flow. Therefore, in the transverse direction of the wake evolution, the data support the Boussinesq eddy-viscosity concept. The measured mixing length increases with streamwise distance, in agreement with classic wake expansion rates. Conversely, the measured eddy viscosity and mixing length in the transverse direction decrease with increasing elevation, which differs from the behaviours measured in the vertical direction in traditional boundary layers or in canopy flows studied before. In order to find an appropriate single length scale to describe the wake evolution behind a multiscale object, two models are proposed, based on the notion of superposition of scales. One approach is based on the radial spectrum of the object while the second is based on its length-scale distribution evaluated using fractal geometry tools. Both proposed models agree well with the measured mixing length. The results suggest that information about multiscale clustering of branches must be incorporated into models of the mixing length for flows through single or sparse canopies of multiscale trees.

Direct Measurements of Gas Bulk Flows in the Intracluster Medium of the Centaurus Cluster with the Chandra Satellite

Abstract: We present the analysis of the velocity structure of the intracluster gas near the core of Abell 3526 obtained with two off-center Chandra observations, specifically designed to eliminate errors due to spatial variations of the instrumental gain. We detected a significant velocity gradient along the northeast-southwest direction, roughly perpendicular to the direction of the incoming subgroup Cen 45, in agreement with previous ASCA SIS measurements. The presence of gas bulk velocities is observed both with and without the inclusion of the Fe K line complex in the spectral fittings. The configuration and magnitude of the velocity gradient is consistent with near transonic circulatory motion, either bulk or eddylike. The velocity difference obtained using the best calibrated central regions of ACIS-S3 is found to be (2.4 ± 1.0) × 103 km s-1 for rectangular regions 24 × 3' roughly diametrically opposed around the cluster's core. There are also indications of a high-velocity zone toward the southern region with similar magnitudes. The detection of velocity gradients is significant at >99.4% confidence, and simulations show that intrachip gain fluctuations >1800 km s-1 are required to explain the velocity gradient by chance. The measurements suggest that >1% of the total merger energy can still be bulk kinetic 0.4 Gyr after the merging event. This is the first direct confirmation of velocity gradients in the intracluster gas with independent instruments and indicates that strong departure from hydrostatic equilibrium is possible even for cool clusters that do not show obvious signs of merging.

Forced oscillations of an enclosed rotating fluid

Abstract: The initial value problem related to axisymmetric forced oscillations of a rigidly rotating inviscid fluid enclosed in a finite circular cylinder is examined in linear approximation with the aid of the Laplace transform technique. An impulsive starting motion is considered. The solution consists of a ‘periodic’ motion which oscillates with the forcing frequency, together with a doubly infinite set of inertial modes whose presence is determined by the initial conditions and whose frequencies form a dense set in the range (−2ω, 2ω), where ω is the angular velocity. The nature of the periodic or ‘steady-state’ part of the solution is strongly dependent on the precise value of the forcing frequency α (α > 0) when α ≤ 2ω. In particular the system will resonate if α equals any one value of the dense set of resonant frequencies. It is shown that no internal sets of discontinuities in velocity or velocity gradient are present in the inviscid flow for finite times. Effects of viscosity on the inviscid solution are also discussed, and it is argued that when the inertial modes decay the steady-state flow will contain pseudo-random patterns of internal shear layers for some values of α < 3ω. It seems possible that these shear layers may be interpreted as owing their existence indirectly to viscosity.

Numerical studies of asymmetric adiabatic accretion flow - The effect of velocity gradients

Abstract: A numerical study of the time variation of the angular momentum and mass capture rates for a central object accreting from a uniform medium with a velocity gradient transverse to the direction of the mean flow is presented, covering a range of velocity asymmetries and Mach numbers in the incident flow. It is found that the mass accretion rate in a given evolutionary sequence varies in an irregular manner, with the matter accreting onto the central object from either a continuously moving accretion wake or from an accretion disk. The implications of the results from the study of short-term fluctuations observed in the pulse period and luminosity of X-ray pulsars are discussed.