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

The flow over riblets is examined computationally using a time depende nt model of the viscous wall region. This ''21/2 D model'', developed by Hatziavramidis and Hanratty (1979) and modified by Nikolaides (1984 ) and Chapman and Kuhn (1981, 1986) assumes homogeneity in the streamw ise direction so that the flow is solved only in the cross-sectional p lane. The flow at the upper boundary of the computational domain (y+ c ongruent-to 40) is described using a streamwise eddy model consisting of two scales, one of the streak spacing (lambda+ congruent-to 100), w hich dominates vertical momentum transport, and a larger scale that ac counts for the influence of large outer flow eddies. The protrusion he ight concept (Bechert and Bartenwerfer, 1989) is used to define a y+ = 0 location for surfaces with riblets. A control volume finite element method utilizing triangular meshes is used to exactly fit the riblet cross-sectional geometry. Results obtained using fairly large riblets compare well with the limited experimental evidence available. Observa tions of the transient flow suggest that the riblets interact with the near-wall streamwise vortices, weakening them by the generation of in termittent secondary vortices within the riblet valleys. The riblets a lso appear to limit the lateral spread of inrushes towards the wall an d retain low momentum fluid in the riblet valleys effectively isolatin g much of the wall from such inrushes.