Drag reduction for a uniform velocity flow past a sphere with highly waterrepellent
surface and a gas liquid interface has been investigated experimentally and
analytically for Reynolds numbers ranging from 10 to 104. The surface of a sphere had
high water repellency (contact angle of ~150°) and the gas liquid interface exists at the
surface with many fine grooves. Experimental results showed that the separation point
of the boundary layer around a sphere moved downstream compared with that of a
smooth surface sphere. It was also shown, by measuring the sphere drag, that drag
reduction for a sphere with highly water-repellent surface occurs at Reynolds numbers
less than 104 and that the maximal drag reduction ratio is 28.5 % at Re = 7.2. By
considering that such a phenomenon occurs due to an apparent fluid slip at the gas
liquid interface of a hydrophobic surface, the flow patterns around a sphere were
analyzed by applying the gas liquid two-phase model at the surface proximity. Results
of numerical simulations were obtained for Reynolds number ranging from 100 to 450.
The boundary condition for fluid slip was given by assuming an effective slip boundary
condition of the surface. A comparison of the simulation results with the experimental
results shows a close agreement concerning the flow patterns of the wake and drag
coefficient.
Keywords: Drag reduction, external flow, sphere, flow drag, highly waterrepellent
wall, wake, gas-liquid interface, numerical simulation, fluid slip, drag
coefficient, falling ball test, flow pattern, separation, pressure profile.