We investigated the drag reduction phenomena on highly water repellent surfaces, which have micron to sub-millimeter order of projections. In present study, the drag reduction is estimated by means of friction coefficients for laminar flow in 2mm×8mm rectangular duct, where both the top and the bottom walls in the tested section consist of the water-repellent micro-structured surface. The sizes and pitch of the projections on the surfaces were treated as the main parameters of the present work. Among three different surfaces tested, the one that could hold the air layer caused reduction of friction coefficient of 4-5%.
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