Study on Frictional Resistance of Drag Reducing Solid Wall

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In this study, we studied on the flow characteristics of a tap water or glycerin solutions for the flow in a duct, the flow past a sphere, and the flow in an enclosed rotating disk with a drag reducing surface.For a flow in a duct with hydrophobic coating surface, it was clarified that the laminar drag reduction occurred in the Reynolds number range of 150 <Re < 770. The maximum drag reduction ratio is 12.6% at Re = 649.For the flow past a sphere, it was clarified that the vortex street was not found at the Re < 320 in the hydrophobic wall and the separation point moved downstream compared with that of a conventional smooth wall. The drag reduction occurred in the flow and the maximum drag reduction ratio was 14.6% at the Re=93.2. In this simulation, the method for tracking gas-liquid interfaces has been used the VOF technique, and the method for modeling surface tension effects on fluid motion has been used the CSF model. The flow patterns for the numerical simulation results agreed with that of the flow visualization results.For the flows around an enclosed rotating disk with the fine spiral grooves, it was seen that the drag reduction occurs in the turbulent flow range at the non-dimensional axial clearance (s/a) = 0.011, 0.22 and 0.33, the maximum drag reduction ratio is approximately 15%. The fine spiral groove has the effect which suppresses the development of the fluctuation of transition region, and which delays the generation of the local turbulence in the transition range.