Pressure Drop Measurements in Turbulent Channel Flow Over Superhydrophobic Surfaces With Riblets

In this paper we consider the combined drag reducing mechanisms of superhydrophobicity with riblets. Pressure drop measurements were acquired for turbulent flow in a channel with superhydrophobic walls, riblet walls, and walls with both drag reducing mechanisms. The superhydrophobic structuring was composed of alternating microribs (15 microns tall and 8 microns wide) and cavities (32 microns wide), aligned parallel to the flow. Superhydrophobic surfaces function to reduce drag by minimizing the effective liquid-solid contact area as water will not penetrate the cavities between microribs due to surface tension. The riblets were nominally 80 microns tall, 18 microns wide, spaced with a period of 160 microns and were also aligned parallel to the flow. Riblets function by damping out spanwise turbulent motions. Since turbulence is a three-dimensional phenomenon, this destruction of turbulent motions acts to reduce the average friction at the surface. Fabrication of the drag reducing surfaces was completed with photolithographic techniques on silicon wafers. The wafers were inserted into a channel consisting of a control section with smooth wafers and a test section with patterned wafers. In all cases, the test section walls were structured on top and bottom while the side walls were left smooth. The channel had a hydraulic diameter of 7.3 mm and an aspect ratio of 10:1. Tests were obtained over a Reynolds number range of 5 × 10³ to 1.5 × 10⁴. The superhydrophobic surfaces with riblets showed a maximum drag reduction of 7.0% which was a higher reduction than either the surfaces patterned with riblets or the superhydrophobic surfaces.