Effect of Active Control on Under-Expanded Jets

In an effort to reduce the aircraft jet noise, control of jets has become one of the highly explored areas. In this work, we examine an underexpanded jet subjected to control with Localized Arc Filament Plasma Actuators (LAFPA) to complement prior results on perfectly expanded flow. High fidelity, Large Eddy Simulations (LES) are employed with a simple model for the actuators, eight of which are placed along the periphery of a Mach 1.2 converging nozzle exit. The axisymmetric mode (m=0) is excited at two different Strouhal numbers of 0.3 (corresponding to the most amplified jet-column mode) and 0.9, based on the exit diameter of the nozzle. Baseline (no control) simulations at two different Reynolds numbers (100,000 and 1.2 million) are also performed. Results indicate a good correlation between the numerical and the experimental results. Undulations are observed in the mean flow, which correspond to the increase and decrease of the flow velocity as the jet traverses the complex shock cell structures generated as a result of the under-expansion of the jet. Baseline simulations at the two chosen Reynolds numbers reveal no significant difference between the two cases indicating that the effect of Reynolds number is negligible. Phase-averaged results, for St=0.3, indicate the presence of large vortical structures generated as a result of amplification of the natural structures due to actuation. Two different kinds of structures are generated corresponding to the switching on and switching off of the plasma actuators. These structures are absent when the flow is actuated at St=0.9. Quantitative near field acoustic analysis is conducted using two-point correlation technique. The qualitative effect of forcing on far-field noise propagation is also investigated.