Experimental study of the stability of heated laminar boundary layers in water

Abstract

Linear sinusoidal disturbances are introduced into the boundary layer over a heated flat plate of uniform surface temperature using a vibrating ribbon. The experiment is performed in a low turbulence water tunnel with free-stream turbulence intensities of 0[sdot ]1–0[sdot ]2%. Measurements are made using temperature-compensated hot-film anemometry. Neutral-stability characteristics obtained for the unheated case agree favourably with previous results obtained both in water and in air. Neutral-stability and spatial disturbance-growth-rate characteristics measured for wall temperatures up to 8°F above the free-stream temperature verify trends established by parallel-flow solutions of the disturbance momentum and energy equations. Disturbance growth rates and the band of amplified disturbance frequencies both decrease as wall heating is increased. The experimentally observed increase in the minimum critical Reynolds number Recmin with increased wall heating agrees with the trend predicted by theory. However, effects of non-parallel flow act to reduce the measured values of Recmin by about 120 units compared with predicted parallel-flow values independent of the level of wall heating.