Survey and new measurements of turbulent structure near the wall

A survey of recent measurements of turbulent structure near the wall which are applicable to the phenomenon of drag reduction by polymer additives is presented. Knowledge of the vorticity in the wall region provides a framework for understanding turbulent structure and the effects of polymer additives. New measurements using a small, calibrated streamwise vorticity probe are described. Near the wall where the turbulent production and dissipation are a maximum the streamwise vorticity is a maximum and is highly intermittent. New measurements of the Reynolds stress have also been made with a very small, computer calibrated hot-wire array. Intermittent small scale structures producing rapid oscillations of the hot-wire signals are detected near the wall which are too small to be resolved by the array. Their spatial scale is much less than the Kolmogoroff length. It is suggested that they are associated with streamwise vorticity.