The persistence of drag reduction following the injection of microbubbles into a turbulent boundary layer

Data from two series of experiments has been used to investigate the way in which the reduction in wall shear stress, due to microbubble injection, changes with distance downstream of the injection region. Based on a nondimensional parameter, defined in the text, and related to the downstream distance, the free stream velocity and gas injection rate, the shear stress is found to relax exponentially to its original value. Good agreement between the two sets of data is obtained.