Drag Reduction For External And Internal Boundary Layers Using Riblets And Polymers

Two hydrodynamic experiments were conducted to measure drag reduction using riblets in turbulent boundary layers. The first was an external boundary layer experiment using a flat plate in a water tunnel, and the second was an internal boundary layer experiment using a 6-inch diameter pipe. Both experiments used adhesive-backed, vinyl, riblet film, manufactured by the 3M Co., with height and spacing of the symmetric v-grooves equal to 0.003 in. For the flat plate test, free stream velocities ranged from 6.5 ft/sec to 20 ft/sec, and Re sub theta ranged from 1260-7040. The results are in good agreement with aerodynamic data, and show a maximum drag reduction of 8.1% + or - 2.5% at s(+) = 13.1. Much larger percentages of friction reduction (approx. 3x as much) were measured with riblets in the pipe. When a drag-reducing polymer solution was used in conjunction with the riblets in the pipe flow, the total drag reduction was approximately equal to the sum of the drag reductions of the two techniques used separately, with some dependence on Reynolds number. Before these pipe flow results can be extended to general situations, further experiments are needed to investigate the boundary layer transition from smooth to riblet surface.