The 3D separation behaviour of a micro-ramp controlled oblique shock-wave reflection

Abstract

Oblique Shock-Wave Reflections or Shock Wave-Boundary Layer interactions (SWBLI) are complex phenomena which commonly occur in high speed flight. SWBLIs can have catastrophic consequences on the aircraft due to shock induced separation and unsteadiness of the interaction. These adverse effects can be mitigated by manipulating the boundary layer, either before it enters the interaction, or in the interaction region itself. Micro-ramp vortex generators or micro-ramps have been identified as promising boundary layer control methods in supersonic engine intakes. They are small ramp devices with heights less than the boundary layer thickness (typically 50% of boundary layer thickness). It is known that, micro-ramps generate a counter-rotating vortex pair, due to which a fuller and more separation resistant boundary layer is obtained upstream of the SWBLI. They successfully prevent separation at their centrelines and their effectiveness reduces away from their centrelines. Since a micro-ramp generates a highly 3D flow, the main aim of this study is to quantify the 3D velocity fields in the micro-ramp controlled interaction. Further, this study is motivated by the fundamental question whether the total volume of separated flow in the interaction is reduced by employing micro-ramp control. The results from tomographic-PIV experiments confirm that micro-ramp control has a beneficial effect on SWBLI in reducing separation. For the experimental conditions and micro-ramp geometry used in this thesis, the total volume of separated flow is reduced to 31% compared to the interaction without control. Further, the maximum separation probability is also reduced by using micro-ramp control, with the largest reductions occurring along the micro-ramp centreline. Additionally, the findings indicate that a moderately negative correlation coefficient exists between separation bubble sizes at spanwise stations on either side of the micro-ramp centreline. This indicates that the separation bubble size at a particular spanwise location is coupled to its size at another spanwise location in the micro-ramp controlled interaction. Along with the 3D velocity fields in the interaction, these results extend the knowledge on micro-ramp control significantly. It is conjectured that the spanwise correlation between the separation bubble sizes may play an important role while using micro-ramp arrays for applications in supersonic engine intakes. Therefore, an immediate possible investigation would involve studies of 3D effects of micro-ramp array control and the spanwise correlation of the separation bubble sizes.