MODELING THE TIME-DEPENDENT FLOW OVER RIBLETS IN THE VISCOUS WALL REGION

The flow over drag reducing riblets is examined computationally using a time-dependent model of the viscous wall region. The flow at the upp er bound of the computational domain (y+ congruent-to 40) is described using a streamwise eddy model consisting of two scales. A control-vol ume finite-element element method utilizing triangular meshes is used to exactly fit the riblet cross-sectional geometry. Observations of th e transient flow conditions suggest that the riblets limit the lateral spread of fluid inrushes towards the wall and retain low momentum flu id in the riblet valleys effectively isolating much of the wall from s uch inrushes. The generation of intermittent secondary vortices within the riblet valleys also occurs; however, these appear to be quite wea k and fairly short-lived.