Observations of Instability in Spatially Periodic Channels with Different Aspect Ratios

Laboratory experiments are conducted for channel flows with one wall textured by spanwise grooves placed periodically in the streamwise direction. Prior numerical simulations indicate that the pure flow becomes unstable at a critical Reynolds number of order 100; the resulting secondary flow is predicted to exhibit stable waves reminiscent of Tollmien-Schlichting modes. Here, we examine instability onset and the resulting secondary flow as a function of the channel aspect ratio. Characterizing textured channels in low aspect ratio regimes could suggest new ways to improve mixing and transport at low Reynolds numbers with small additional costs in system pumping power.