The effect of longitudinal microstriations and their profiles on the drag of flat plates

Abstract

The use of surface modifications as a means of reducing viscous drag on a body has potential aerodynamic and hydrodynamic applications. V-grooves of specific dimensions are machined in a longitudinal direction onto the surface of a smooth plate and the resulting effect on the drag force of the plate is observed. Experiments show that V-grooves (riblets) could reduce turbulent skin friction drag by up to 7 per cent, depending on the size of the groove. The drag-reducing performance of riblets with aspect ratios, h/s, of 0.22 and 1 are examined. A boundary layer analysis of the turbulent flow characteristics over the smooth surface and the riblet surfaces indicated an increase in the laminar sublayer thickness and local Reynolds number while reducing the boundary layer thickness for the ribbed surfaces. A maximum drag reduction of 6.83 per cent was recorded for the surface covered with the symmetric riblet, at a Reynolds number of 117 101. It is felt that riblets hamper the momentum and turbulent energy exchange from regions of high velocity to lower-velocity regions. Riblets impede the cross-flow of stream-wise vortices that prevail in the viscous sublayer of a turbulent boundary layer. By suppressing these streamwise vortices, turbulent mixing and hence turbulent shear stress are reduced. Results obtained agree with results suggested from research elsewhere.