Micro-vortex generators and recirculating flow control of normal shock stability and position sensitivity

Experiments have been conducted herein at Mach 1.4 to determine the effect on normal shock stability of a micro-ramp array, ramped vane arrays, porous plates over a cavity, and an open cavity as compared to no control (solid wall) in the region of a 5o diffuser. For each of the control devices, the mean position and standard deviation in position of the normal shock were determined for a set stagnation pressure over the range of tunnel stagnation pressures from 134.4 to 148.2 kPa using schlieren photography. For each of the set stagnation pressures, the boundary layer static pressure fluctuations downstream of the diffuser entrance were also obtained for each control device. An array of three micro-ramps and an array of three ramped vanes, whose heights were scaled to 40% of the incoming boundary layer thickness, were placed ahead of the normal shock, as well as an array of two ramped vanes with a height of 0.6 the incoming boundary layer thickness. Three porous plate configurations with a porosity of 5% were also examined with varying lengths and positions. The open cavity used was 13.5 and 5 incoming boundary layer thicknesses long and deep, respectively. It was demonstrated that the control devices did affect shock stability in the region of the diffuser, both increasing and decreasing shock stability depending on shock position. Schlieren photography showed the micro-vortex generators reduced fluctuations of normal shock position when the shock moved into the diffuser, while slightly increasing fluctuations in shock position when the mean shock position was upstream of the diffuser entrance. The porous plate that terminated 3.5 incoming boundary layer thicknesses upstream of the diffuser entrance reduced the shock position fluctuations near the diffuser entrance. High frequency wall pressure measurements showed that the open cavity should not be used to improve stability. Also, the standard deviation of the wall static pressure fluctuations inside the diffuser was improved with the micro-vortex generators when the mean shock position was near and downstream of the diffuser shoulder.