Experimental study of the effects of surface roughness on laminar boundary layer stability in water

Experimentally measured stability characteristics are presented for a laminar boundary layer developing in water over a flat plate with both smooth and rough surfaces. Three different grades of sandpaper are used to provide different levels of distributed roughness. The experiment is performed in the CWRU low turbulence water tunnel with free stream turbulence intensities of 0.1% to 0.2%. Linear sinusoidal disturbances are introduced into the boundary layer using a vibrating ribbon. The mean flow measurements indicate Blasius profiles for the finest roughness tested. With coarser roughness, the profile is best approximated by a Blasius profile up to Re(x) = 120,000 but then progresses toward a turbulent profile. The coarsest roughness triggers transition under all test conditions. Neutral stability and disturbance growth rate characteristics measured for the finest roughness indicate only very minor departures from the results for a smooth surface. For the coarser roughness case, disturbance growth is greatest at frequencies below the Tollmien-Schlichting band. Results of detailed measurements of mean velocity, disturbance velocities and spectra are presented and analyzed.