SPHERE WAKES IN STILL SURROUNDINGS AT INTERMEDIATE REYNOLDS-NUMBERS

The wakes of spheres in a still environment were studied for sphere Reynolds numbers Re in the range 30-4000. The experiments consisted of lowed spheres in quiescent baths of glycerin and water mixtures. Measurements included dye traces illuminated by a laser light sheet for visualization and laser velocimetry for streamwise velocities. The recirculation region on the downstream side of the sphere was stable and symmetric for Re < 200, stable and unsymmetric for 200 less-than-or-equal-to Re less-than-or-equal-to 280, and unstable with vortex shedding f or Re greater-than-or-equal-to 280. Three wake regions were identified: a fast-decay region that was observed only when vortex shedding was present, followed in succession by turbulent and laminar wake regions. Vortex shedding increased the distance to the onset of the turbulent wake region by an order of magnitude due to the presence of the fast-decay wake region. 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 a local wake Reynolds number Re(w) almost-equal-to 10. Within the turbulent wake region, turbulence intensities along the axis were roughly 85% for Re(w) > 70; however, as the onset of the laminar wake region was approached, turbulence intensities along the axis were proportional to Re(w)7/4, which is consistent with scaling proposed earlier for the final decay period of axisymmetric wakes.