The Structure and Statistics of Turbulent Flow Over Riblets
PRISM calculation on an Intel Paragon
A direct numerical simulation of turbulent flow at Rer = 140 was performed for a channel with its lower wall mounted with riblets of h+ = 19, h/s = 0.5 using a hybrid spectral element Fourier numerical scheme. The statistical results from the study indicate that there is a thickening of the viscous sublayer as well as a reduction in turbulence intensities and Reynolds stress at the riblet wall as seen by other researchers. Analysis of the turbulent flow structure is carried out in the light of current drag reduction mechanism hypotheses. We also present new quadrant analysis data for velocity ance vorticity to support the idea that riblets do not affect the flow field beyond a range of y+ <= 20 and the presence of riblets results in a lack of coherence in the structures near the wall - an analysis which leads to new speculations concerning the mechanism of riblet drag reduction. The main contribution of this work is the identification of drag reducing and drag inducing flow structures in wall-bounded flows.