The possibility of fluid slip has received considerable attention in recent years. Laminar drag reduction is achieved by using a hydrophobic wall with fluid slip. Fluid slip is closely related to the gas-liquid interface formed at a solid surface with many fine grooves. The friction generated by the solid boundary is modified considerably because the gas-liquid interface provides a zero-shear stress boundary condition. The purpose of this study is to experimentally clarify the flow characteristics and drag reduction of a hydrophobic wall sphere by visualizing flow and by measuring the drag. In addition, the flow patterns were numerically analyzed by applying a wet boundary condition for fluid slip. The flow visualization results showed that the Vortex Loop was not exist at Re < 400 in the hydrophobic wall sphere and the separation point moved downstream compared with that of a conventionally smooth sphere. Drag reduction occurred in the flow and the maximum drag reduction ratio was 14.6% at Re=93.2. In this simulation, the flow patterns for the numerical simulation results agreed with those of the flow visualization results.

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