EFFECT OF BLUFF-BODY WAKES ON SKIN FRICTION AND SURFACE HEAT-TRANSFER IN A TURBULENT BOUNDARY-LAYER

Experiments were conducted in a turbulent boundary layer which interacts with the wake of a bluff body mounted upstream with its axis spanwise. Measurements reported here include surface shear stress and heat transfer rate. Wakes from both stationary and vertically-oscllating cylinders, with circular and square cross-sections and of different sizes, were considered. Measurements were also made with a 16 mm circular cylinder translated to different locations from the plane of the flat plate, and with a cylinder rotated ('rolled') about the centerline of the plate to different angles in the cross-stream plane. The results show that the skin friction in the turbulent boundary layer decreases significantly in the presence of the wake from the stationary cylinder. The effect of the wake from an oscillating cylinder appears to be closely the same as deduced from a set of measurements with a cylinder fixed at different vertical positions. The streamwise distribution of skin friction shows the expected trend for the case of the translated cylinder in that the surface shear stress changes only when the cylinder is above the plane of the plate. The spanwise distribution of skin friction in the case of the rolled cylinder shows similarly significant changes on one side of the centre line as expected. The net effect of the wake on the Stanton number, on or off the axis of the plate, is always small: reduction due to decrease in mean velocity in the wake roughly balances the increase due to the wake turbulence.