Hot-wire anemometry of transitional boundary layers exposed to different freestream turbulence intensities

Abstract

Transitional boundary layer flows on a flat plate, subjected to different freestream turbulence intensities, have been investigated experimentally in details. The inlet freestream turbulence levels were varied from 1.5 to 4.4 per cent using different circular bar grids of various sizes installed upstream in a proper distance from the plate leading edge. A single hot-wire probe was used for the measurements of instantaneous flow velocities within the boundary layer. Longitudinal and vertical movements of the hot-wire probe were performed by a two-component traversing device, which was itself controlled by a computerized system. Experiments were conducted in an open circuit wind tunnel of suction type. For each level of freestream turbulence, boundary-layer surveys of the mean longitudinal velocity and root mean square of the velocity fluctuations were obtained at several streamwise locations. Experimental results were used to determine the variations of streamwise skin-friction coefficient and the boundary-layer shape factor versus the local Reynolds number. Evaluation of the final results showed that increasing the freestream turbulence intensity causes the transitional region to shrink and to shift up-stream. Current experimental results together with reliable data presented by other researchers led to propose a new correlation capable of predicting the onset of transition for a wide range of freestream turbulence intensities.