The structure of sphere wakes at intermediate Reynolds numbers in still and turbulent environments

The structure of sphere wakes was studied for sphere Reynolds numbers in the range of 30-4000 in still environments and for sphere Reynolds numbers in the range of 130-1600 in turbulent environments. Measurements included dye traces illuminated by a laser light sheet for flow visualization and laser velocimetry for streamwise and crosstream velocities. Classical similarity results for turbulent and laminar wakes were used to interpret and correlate the measurements.;There are three wake regions identified in still environments: a fast-decay wake region that was observed only when vortex shedding was present (Re280), followed in succession by turbulent and laminar wake regions. Mean velocities within the turbulent and laminar wake regions scaled according to classical similarity theories, with transition between these regions at conditions where their estimates of mean streamwise velocities along the axis were the same: this occurred at local wake Reynolds numbers, Re;The structure of sphere wakes were measured in the turbulent environments with streamwise integral scales much larger than the sphere diameter and Kolmogorov length scales less than the sphere diameter. At these conditions, some phenomena observed for sphere wakes at comparable Reynolds numbers in nonturbulent environments were suppressed; instead, while the wakes were turbulent, their mean streamwise velocities scaled like self-preserving laminar wakes but with enhanced viscosities due to turbulence. Effective turbulent viscosities were relatively independent of position and the ratios of integral length scales and Kolmogorov length scales to sphere diameters, however, they progressively increased with sphere Reynolds numbers. Low and high Reynolds number regimes were observed, separated by a transition regime involving sphere Reynolds numbers in the range of 300-600 which was associated with the onset of vortex shedding in the near wake region. Wake mixing properties within the laminar-like turbulent wake region were affected by both the sphere Reynolds number and the ambient turbulence intensity in the low Reynolds number regime; in contrast, wake mixing properties mainly varied with the sphere Reynolds number in the high Reynolds number regime where wake turbulence was dominated by wake generated turbulence. Observed Strouhal numbers associated with eddy shedding were generally lower than the values observed earlier in turbulence-free environments.