Experiments in highly sheared, nearly homogeneous turbulence.

Abstract

Nearly homogeneous, uniformly sheared, turbulent flow was generated in a high speed wind tunnel at shear rates ranging from 436 to 705 s$\sp{-1}$ and Mach numbers below 0.2. The shear rates are substantially higher than those generated in the past, and comparable to those in the inner region of turbulent boundary layers. Measurements were obtained using a three-beam, two-component, laser-Doppler velocimetry (LDV) system in back-scatter mode. The system employs a 100mW Argon-Ion laser, which is connected to the measuring probe via fibre optics. Although the signal was filtered by the data acquisition system, additional computer programs were required to remove extraneous noise, which was defined as data samples beyond 2.5 standard deviations from the mean. LDV measurements show that the turbulence structure attains a self-similar state with approximately constant dimensionless stresses and exponential kinetic energy growth. The main difference from realizations at lower shear rates (comparable to those in outer boundary layers) is a marked decrease in the dimensionless Reynolds shear stress, $-K\sb $, which attained an average value of 0.11 over all present experiments, compared to the value 0.15 obtained previously in uniform shear flow and outer boundary layers. The Reynolds shear stress correlation coefficient averaged over all the present shear flows was 0.35, and was consistently lower than the value of 0.45 which has been found in outer boundary layers and uniform shear flows at lower shear rates. It was concluded that although the partition of turbulent kinetic energy among its components most likely depends on the proximity of a wall, the rate of production of turbulence in shear flow depends mainly on the rate of mean shear, irrespectively of the shear generation mechanism. (Abstract shortened by UMI.)