Velocity gradient

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

A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific

Abstract: A new approach for constraining the seismic shear velocity structure above the core-mantle boundary is introduced, whereby SH-SKS differential travel times, amplitude ratios of SV/SKS, and Sdiff waveshapes are simultaneously modeled. This procedure is applied to the lower mantle beneath the central Pacific using da.ta from numerous deep-focus southwest Pacific earthquakes recorded in North America. We analyze 90 broadband and 248 digitized analog recordings for this source-receiver geometry. SH-SKS times are highly variable and up to 10 s larger than standard reference model predictions, indicating the presence of laterally varying low shear velocities in the study area. The travel times, however, do not constrain the depth extent or velocity gradient of the low-velocity region. SV/SKS amplitude ratios and SH waveforms are sensitive to the radial shear velocity profile, and when analyzed simultaneously with SH-SKS times, rnveal up to 3% shear velocity reductions restricted to the lowermost 190±50 km of the mantle. Our preferred model for the central-eastern Pacific region (Ml) has a strong negative gradient (with 0.5% reduction in velocity relative to the preliminary reference Earth model (PREM) at 2700 km depth and 3% reduction at 2891 km depth) and slight velocity reductions from 2000 to 2700 km depth (0–0.5% lower than PREM). Significant small-scale (100–500 km) shear velocity heterogeneity (0.5%–1%) is required to explain scatter in the differential times and amplitude ratios.

Statistical theory for the stochastic Burgers equation in the inviscid limit

Abstract: A statistical theory is developed for the stochastic Burgers equation in the inviscid limit. Master equations for the probability density functions of velocity, velocity difference, and velocity gradient are derived. No closure assumptions are made. Instead, closure is achieved through a dimension reduction process; namely, the unclosed terms are expressed in terms of statistical quantities for the singular structures of the velocity field, here the shocks. Master equations for the environment of the shocks are further expressed in terms of the statistics of singular structures on the shocks, namely, the points of shock generation and collisions. The scaling laws of the structure functions are derived through the analysis of the master equations. Rigorous bounds on the decay of the tail probabilities for the velocity gradient are obtained using realizability constraints. We also establish that the probability density function Q() of the velocity gradient decays as jj 7=2 as !1 . c 2000 John Wiley & Sons, Inc.

Effect of microbubbles on the structure of turbulence in a turbulent boundary layer

Abstract: The time-averaged velocity and turbulence intensity distributions were measured by a laser Doppler velocimeter in a turbulent boundary layer filled with microbubbles. The void fraction distribution was also measured using a fiber-optic probe. The velocity decreased in the region below 100 wall units with an increase in bubble density. This led to a decrease in the velocity gradient at the wall, which was consistent with a decrease in shearing stress on the wall. The turbulence intensity in the buffer layer increased at a low microbubble density, and then began to decrease with an increasing microbubble density. Based on the present measurements, the mechanism of turbulence reduction by microbubbles is discussed and a model is proposed.

Velocity gradients at the wall for flow around a cylinder for Reynolds numbers between 60 and 360

Abstract: This paper shows how electrochemical techniques can be used in studies of flow around solid objects to measure the velocity gradient at the solid boundary. The method holds the advantages that it is not necessary to calibrate and that the test element is easy to fabricate. A study of the distribution of the wall velocity gradient around a cylinder of 1 in. diameter indicates that boundary-layer theory correctly predicts the measurements between the front stagnation point and the separation for Reynolds number, R , greater than 150. The wall velocity gradients in the wake are much smaller than in the front part of the cylinder and they reveal a minimum which is quite close to the separation point.