Ahmad Fakhar - Department of Mechanical Eng., Sharif University of Technology Tehran, Iran
Rad - Department of Mechanical Eng., Sharif University of Technology Tehran, Iran
Afkhami - Department of Mechanical Eng., Sharif University of Technology Tehran, Iran

The drag coefficients on discs rotating in a cylindrical tank has studied in relation to the nature of a composite coating attached to the underside of the disc, and direct comparisons were made with reference discs of identical geometry which were completely rigid. The composite coating was formed by: (1) bondig of velvet fabric to the metal disc with velour face outward, (2) casting a premixed silicone composition over the velvet fabric, (3) covering the silicone layer with a thin, stiff polymer film. As a result of the tests in certain cases drag coefficient was reduced by 20% or more as compared to reference rigid discs. Two conclusions are possible: (1) True reduction of energy exchange in the turbulent boundary layer was achived, perhaps by damping of the turbulent bursting process; (2) A significant increase in the laminar region at the center of disc was obtainded (transition of laminar to turbulent delayed). The stiffness of the top film influenced the performance of the coating. The change of drag coefficient of discs to be strongly influenced by the dynamic mechanical properties of the silicone elastomer network.

Compliant Coating, Drag Reduction, Transition