Viscous Drag Reduction on Axisymmetric Bodies using Helical Riblets

A series of subsonic wind tunnel tests was conducted on an ogive-cylinder-flare configuration at zero angle of attack to study the effects of small helical protuberances on viscous drag reduction behavior. The experiments have been carried out on a smooth model and two wire-wrapped models with different spacing between the helical riblet rings, known as pitch length. In the present experiments, the pitch length to model diameter ratio for the wire-wrapped models were 0.5 and 1.0 and the velocity profiles at 19 streetwise locations as well as the pressure distribution on the model at the same positions have been measured and compared for the smooth model and the two wire-wrapped ones. The results show about 10% reduction in the local skin friction drag for the wire-wrapped model in comparison with the smooth one. The investigations also showed that depending on the boundary layer thickness, optimum values could be found for the riblet diameter and its pitch length for which, the maximum reduction would be achieved in the local friction drag.