Experimental Investigations on Bypass Transitional Flows under Variable Streamwise Pressure Gradients using Hot Wire Anemometry

Among many factors affecting transition from laminar to turbulent flow, the most dominant ones can be referred to streamwise pressure gradient and freestream turbulence intensity. In the present research work characteristics of transitional region within boundary layer flow over a flat plate, exposed to variable streamwise pressure gradients, are investigated experimentally. Experiments were limited to upstream velocity and turbulence intensity of 10.5 m/s, 3.8%, respectively. A single hot-wire probe was used for measurements of instantaneous flow velocities within the region of interest. 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. Boundary layer surveys of the mean longitudinal velocity and rms of the velocity fluctuations were obtained at several streamwise locations. Experimental results were used to determine variations of streamwise skin friction coefficient and boundary layer shape factor versus the local Reynolds number. Evaluation of the final results showed that favorable pressure gradient in the laminar region causes the onset of transition to shift downstream. Also adverse pressure gradient leads to accelerating the bypass transition process and shortening the length of transitional region.

Keywords:

Transitional boundary layer , Bypass transition , Streamwise pressure gradient , Hot wire anemometry