Control of Alternating Flow Phenomena in Transonic Shock Wave Boundary Layer Interactions Over Payload Region of a Generic Launch Vehicle Model

Abstract

During the transonic phase of a launch vehicle’s ascent, aerodynamic loads peak due to high freestream dynamic pressure and angle of attack. In addition to steady loads, significant pressure fluctuations arise from Shock Wave Boundary Layer Interaction (SWBLI), where an unsteady λ-shock system interacts with the boundary layer. This phenomenon can cause buffeting over the payload region, leading to structural failure and control issues. To mitigate shock oscillations, NASA recommends limiting the nose cone semi-angle to 15°, classifying such designs as “Buffet-Proof.” However, practical constraints such as payload mass, rocket diameter, and launch pad compatibility often require larger nose cone angles, making vehicles more prone to buffeting. While SWBLI is well understood in two-dimensional flows, research on three-dimensional launch vehicle configurations remains limited. To address this gap, wind tunnel experiments were conducted on nose cones with 20° and 25° semi-angles in the transonic Mach number regime. The study revealed critical flow characteristics, including abrupt pitching moment jumps at small angles of attack (±4°), high-pressure fluctuations, oscillations in the λ-shock system, and destabilizing counter-rotating vortices. Additionally, alternating flow phenomena - where the flow transitions intermittently between supersonic and subsonic states—were observed at Mach 0.90 and 0.94. These effects can significantly impact vehicle stability, requiring effective control strategies. Two approaches for controlling SWBLI were investigated: a passive front-mounted Aerodisc and an active counter-flow jet. The optimized Aerodisc design was tested across different configurations at critical Mach numbers (0.90 and 0.94) and angles of attack (±4°). It achieved a maximum noise reduction of 22 dB in Overall Sound Pressure Level (OASPL) by stabilizing shock oscillations and reducing pressure fluctuations. The second approach used an active counter-flow jet, where compressed air was injected upstream to modify the shock-boundary layer interactions. Tests with varying jet parameters, including exit diameters of 3 mm and 4 mm and a pressure ratio of 3.2, resulted in a OASPL suppression of nearly 20 dB. The counter-flow jet effectively stabilized oscillating shock waves, eliminated counter-rotating vortices, and suppressed upstream-traveling Kutta waves. This research enhances the understanding of SWBLI in three-dimensional launch vehicle configurations and demonstrates effective control techniques. By energizing the boundary layer, both passive and active methods stabilize transonic flow over large nose cones.