Measurement of Drag Reduction on Parallel Trench Superhydrophobic Surfaces in a High-Speed Water Tunnel
Superhydrophobic (SHPo) surfaces are capable of trapping air under water, and this air plastron may lubricate the shearing of the water flowing by and reduce its skin-friction drag. While the slip effect and the resulting drag reduction on SHPo surfaces have been well understood in laminar flows, the results have been controversial and inconsistent in regard to turbulent flows. This thesis describes the progressive modification and adaptation of a low-profile testing module that measures the relative drag of two surfaces for flow tests in a high-speed water tunnel. After a series of experiments at Naval Undersea Warfare Center (NUWC) in Newport, Rhode Island to improve the testing module until it functions reliably in their water tunnel, we obtain a functional prototype and complete successful measurements. Testing a set of SHPo surfaces in boundary layer flows along the parallel trenches at Reynolds numbers up to 9.7x106, we obtain drag reduction up to ~34% compared with a smooth surface. The results support previous numerical studies predicting that the rate of drag reduction increases with Reynolds number, visualize the wetting phenomena of SHPo surfaces in high-speed water tunnel tests, and lay the groundwork to study the effect of SHPo surface trench parameters.