Measurement of wake properties of a sphere in freestream turbulence

Experimental investigations were carried out for flow past a sphere in a closed circuit wind tunnel. The sphere was made of polymer material and had a diameter of 102 mm. Freestream turbulence was generated by placing an orificed perforated plate at the entrance of the test section. The Reynolds number of the flow, based on the diameter of the sphere (d), was set at 3.3 × 104, 5 × 104 and 6.6 × 104 by varying the mean flow velocity. The sphere was placed at 20D (=7.5d) downstream from the entrance of the test section, where D = 37.5 mm is the size of the holes in the perforated plate. Velocity profile of the flow was measured using a hot wire probe at 10D (=3.75d), 20D (=7.5d) and 30D (=11.25d) downstream of the sphere (center). Experiments were carried out without and with the perforated plate in place in order to study the influence of freestream turbulence on parameters such as Kolmogorov length, dissipation rate, integral length, and the skewness and flatness factors. The presence of freestream turbulence was found to reduce the vortex shedding process behind the sphere, that is, large organized motions were suppressed by the freestream turbulence.