Microbubble Drag Reduction in Rough Walled Turbulent Boundary Layers

Abstract

Experiments were conducted in the 12-inch diameter tunnel at ARL/PSU using the tunnel wall boundary layer facility to determine the influence of surface roughness on microbubble drag reduction. To accomplish this, carbon dioxide was injected through a slot at rates of 0.001 m3/s to 0.011 m3/s, and the resulting skin friction drag measured on a 317.5 mm long by 152.4 mm span balance. In addition to the hydrodynamically smooth balance plate, additional plates were covered with roughly 75, 150 and 300 micron grit. Over the speed range tested of 7.6, 10.7 and 13.7 m/s, the roughness ranged from smooth to fully rough. Not only was microbubble drag reduction achieved over the rough surfaces, but the percentage drag reduction at a given gas flow rate was larger for larger roughness. A new scaling parameter that collapses all of the data is also introduced.