Use of laser Doppler anemometry in the investigation of roughness particle induced transition on suction surfaces

This paper examines the effect of isolated surface roughness elements on the development of suction stabilised laminar boundary layers. Laser Doppler anemometry was used to enable detailed flow measurements to be taken within the boundary layer over porous suction surfaces, using several techniques which are described and illustrated in the paper. These ranged from pseudo-visualisation of the induced turbulent wedge by close scans parallel to the surface using RMS velocity to identify the turbulent region, to full three component velocity surveys. In addition quick and precise methods were developed to identify the critical Reynolds number at which disturbances were induced by a given particle. Experimental data are presented to illustrate the principal findings. These showed that in the presence of boundary layer suction, disturbances were initiated at a constant critical Reynolds number, based on particle height and velocity at the top of the particle, the same as that found in the un-sucked case. Suction downstream of the particle was not found to delay the formation of a turbulent wedge, but did result in a progressive reduction in the included angle of the wedge, leading to complete stabilisation of the disturbance at very high suction levels. Suction upstream of the particle was found only to reduce the thickness of the boundary layer at the particle, resulting in the initiation of disturbances at reduced freestream velocities. However the critical value of roughness Reynolds number remained constant.