Reynolds number and pressure gradient history effects on rough-wall turbulent boundary layers*

In this study, we examine the effects of Reynolds number for different pressure gradient histories imposed on a rough-wall boundary layer. Measurements are carried out at the University of Southampton’s 1.2m x 1m cross-section, 12m long boundary layer wind tunnel with the freestream speed varied from 10m/s to 35m/s. A NACA0012 wing of chord 1.25m is mounted at the centreline of the test section (at 0.5 m above the bottom wall) and is used to exert a non-equilibrium pressure gradient history on the rough wall turbulent boundary layer developing on the bottom wall of a wind tunnel. Mean static pressure measurements were obtained in the rough-wall over a large streamwise distance to record the pressure gradient histories. Hot-wire measurements are taken in the boundary layer over the rough-wall at a location of one-chord length downstream of the trailing edge of the aerofoil. At this location, the local pressure gradient is almost zero regardless of the angle of attack of the wing in the freestream and this gives us data for different pressure gradient histories at the same Reynolds number and vice-versa. Using this data, mean and turbulence statistics as well as spectra will be presented allowing comparison of both pressure gradient history and Reynolds number effects.