Flow simple low-molecular fluids such as water or glycerin solutions, which are Newtonian fluids, fluid slip at the solid boundary is ordinary negligible, and it is well known that the calculated result obtained under the no-slip boundary condition agrees well with the experimental results. On the other hand, recently the possibility that some hydrophobic walls produced by the development of coating materials has been realized. The objective of this study is to clarify the effect of fluid slip on the drag reduction of a duct or a sphere with a highly water-repellent wall. Consequently, the drag reduction occurs in laminar flow range at the highly water-repellent wall with many fine grooves, and the formation of the gas-liquid interface in the grooves plays a very important role for the occurrence of the apparent fluid slip.
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