| A series of test, that combined two well known turbulent skin friction drag reducing techniques to determine the feasibility of generating enhanced levels of drag reduction over a flat plate, were performed. A long chain, high molecular weight polymer, polyethylene Oxide WSR-301, and gas microbubbles (of both carbon dioxide or nitrogen), were injected into a fully developed turbulent flat plate boundary layer. Each additive is well known for its robust and consistent skin friction reducing capabilities. These two robust drag-reducing techniques were combined in a streamwise tandem injection configuration employing four floating element skin friction balances behind the injection location to measure integrated skin friction and down stream persistence. Laser Doppler velocimetry was employed to characterize the boundary layer integral parameters. Polymer solutions were characterized by high-pressure liquid chromatography, to ensure consistent concentration and molecular weight distributions at the injection point. The near wall region of the flow was sampled at four down stream locations, to provide polymer diffusion rates at these positions. Combined injection drag reduction levels were compared to the two individual mechanism acting in a linearly independent fashion. Enhanced or synergistic skin friction reductions were observed when the maximum injected volumetric flow rates were kept relatively low for both additives. These enhanced reductions were only observed for the test for simultaneous injection of gas microbubbles fore with injection of a polymer solution spatially aft, the reverse injection configuration was less effective. Enhanced skin friction reductions as high as 40% were observed. These enhanced drag reductions levels correlated with local increases in the wall polymer concentration, which suggests polymer diffusion away from the wall is inhibited. |
