Homogeneous, isotropic flow in grid generated turbulence

Detailed grid generated turbulent analysis has been completed using a three -dimensional hot-wire anemometer and traversing mechanism to identify a hom ogeneous, isotropic flow region downstream of a square mesh. The three-dime nsional fluctuating velocity measurements were recorded along the centerlin e of a wind tunnel test section and spatially over the entire wind tunnel c ross section downstream of the square mesh. Turbulent intensities for vario us grid sizes and Reynolds numbers ranged from a minimum of 0.2 percent to a maximum of 2.2 percent in each of the three principal velocity directions . Spatial homogeneity and isotropy were determined for several turbulent fl ow conditions and downstream positions using the method of covariances. Cov ariances, in theory, should approach zero asymptotically; however, in pract ice, this was not achievable. A subjective judgment is required to determin e downstream location where the variance of the three covariances reaches a value close to zero. The average standard deviation provides an estimate f or defining the limit or subjective threshold needed to determine the onset of homogeneous, isotropic flow. Implementing this threshold, a quantitativ e method was developed for predicting the streamwise location for the onset of the homogeneous, isotropic flow region downstream of a 25.4 mm square g rid as a function of Reynolds number. A comparison of skewness, determined from one-dimensional hot wire anemometer measurements, and covariances, det ermined from three dimensional hot wire anemometer measurements, indicates a need for caution when relying solely on one-dimensional measurements for determination of turbulence isotropy. The comprehensive three-dimensional c haracterization also provides an improved understanding of spatial distribu tion of fundamental turbulence quantities generated by the grid within a lo w-speed wind tunnel. [S0098-2202(00)02501-3].