Self-similar decay of the drag wake of a dimpled sphere

The drag wake of a dimpled sphere is studied experimentally using stereo particle image velocimetry at Re=5×104 to a downstream distance of ∼200 diameters. The wake growth and velocity decay are analyzed and compared with self-similar decay laws. The measured exponents for the mean axial velocity field differ from the classical mean-field self-similar solution. This is consistent with other recent experimental observations of the near wake of axisymmetric bodies but is the first to show a full decade of scaling with nonclassical exponents, indicating the scaling is not merely a near-wake effect. The data also indicate that deviations from the classical solution may be due to the failure of the assumption that velocity fluctuations are in balance with production from the mean axial flow. Relaxing this assumption yields a family of self-similar decay solutions, one of which is consistent with the measured wake decay. The observed solution preserves the velocity fluctuation Reynolds number with downstream distance. The experimental data also show that individual wake realizations bear little resemblance to the mean flow, suggesting that mean flow solutions may have limited practical value.