| Driven micro-cavities embedded in the wall beneath turbulent supersonic boundary layers are analyzed using two-dimensional computational fluid dynamics. This concept is a passive flow control technique in which very small cavities formed by arrays of thin vertical walls are oriented transverse to the flow direction and underlie the boundary layer. The purpose is to reduce or eliminate skin friction drag. Various micro-cavity configurations were analyzed at locations (0.1 m and 1 m) downstream of the leading edges of flat plates, for free-stream Mach numbers of 1.2, 2.0, and 3.0. Results focus on net drag reduction achieved, cavity flow-field effects, perforation effects in vertical cavity walls, cavity scale effects, mesh refinement issues, and the stability of the solutions.;Skin friction drag was eliminated over micro-cavity regions for all configurations tested. Drag in these regions was due to pressure effects on vertical walls and exhibited a linear increase with downstream distance. Drag reductions as high as 18-20% (compared to a reference flat plate section) were obtained for 52-cavity geometries at Mach 2.0 and Mach 3.0 downstream of the 10 cm and 1 m flat plates, respectively. Perforation of the cavity walls showed no effect on net drag reduction for these cases. Stability issues were observed when using a fine grid mesh for the Mach 2.0 case, with significant oscillations seen in the drag. A parametric investigation in which cavity scale, number, and wall configuration were varied was also performed for two free-stream Mach numbers of 1.2 and 3.0. Drag reductions between 18--40% were seen for these cases. It is shown that drag reduction was reduced with increasing cavity length and that the steadiness of the solution increases with the number of vertical cavity walls present. |
