The structure of turbulent entraining flow in an annulus with a rotating screen

Experiments on mixed-layer growth in an annulus with rotating screen are analyzed. Horizontal and vertical profiles of the mean velocities and turbulent intensities are obtained from velocity measurements with hot-film anemometer. When the rotating screen covers the entire horizontal area, as in conventional annulus experiments, it was found that a turbulent Ekman layer of thickness two tenths of the typical mixed-layer depth exists just below the rotating screen; its termination prohibits the downward transport of the externally applied surface stress to the rest of the mixed layer. The measured turbulent intensities in a narrow region near the outer wall are an order of magnitude greater than those in the central region. A large secondary circulation is found to be important to the momentum transfer in the mixed layer. The mean flow is found to be stable inertially in most of the annulus area except in the narrow region near the outer wall.