Passive Control of the Counter-Rotating, Vortical Structures on a Backward Facing Ramp

The formation of twin, counter-rotating, vortical structures on the surface of an adverse pressure gradient backward facing ramp geometry is a well-known phenomenon. Many groups have looked into delaying separation for the purpose of improving pressure recovery via active or passive flow control. However, very few have looked into directly controlling these structures, which were believed to be caused by the presence of the sidewall boundary layers. This presentation will discuss a series of surface oil film flow visualization experiments conducted on a 0.9 meter chord, backward facing, two-dimensional ramp geometry of 0.91 meter span, complete with an adjustable internal wind tunnel ceiling for streamwise pressure gradient control. A detailed description of the surface topology of the three-dimensional separating flow will be presented along with results from the various passive control schemes, directed at controlling these structures. The results will be used to elucidate the inherent sensitivity of the counter-rotating, vortical surface flow patterns and show that although the sidewall boundary layers may feed into their development and influence their sign of rotation and overall spatial extent, they do not appear to cause their formation.