Control of Flow Separation in a Rocket Nozzle Using Microjets

Recent advances in commercial space transportation emphasize rocket engine reusability, reduced number of stages, and its operation over a wide range of operating conditions. The rocket nozzles typically underperform at sea level where they operate at highly overexpanded pressures. This leads to flow separation inside the nozzle that lowers nozzle efficiency. In addition, asymmetric flow separation inside the nozzle will generate side loadings and large amplitude pressure fluctuations resulting in nozzle fatigue and a catastrophic mission failure. The main objective of this study is to characterize the performance of a scaled convergent-divergent supersonic rocket nozzle by identifying flow separation locations along the diverging section at various nozzle operating conditions. This information was then used to guide the implementation of active flow control technique using transverse secondary gas injection microjets near the separation line. Measurements include pressure distributions inside the nozzle with and without microjet control over a range of nozzle pressure ratios (NPRs) and velocity field measurements in the jet plume near the nozzle exit. Results show that the flow separation location is a function of the NPR for the baseline configuration. An increase in NPR results in a delay in the flow separation marked by a downstream movement of separation location. Microjet control appears to be very effective in delaying the flow separation, increase the jet diameter at the exit plane, and thereby improve the stability of the jet plume.