Towards Feedback Control of High-Speed and High-Reynolds-Number Jets

We present a first attempt and preliminary results on the development and application of feedback control to high-speed and high Reynolds number axisymmetric jets. In particular, we demonstrate control authority on the near pressure field of a Mach 0.9 jet with a Reynolds number based on jet diameter of 7.8 × 10. Open-loop forcing experiments are presented wherein our in-house developed plasma actuators are shown to have two distinct effects on the near pressure field. At low forcing Strouhal numbers (StDF’s) near the jet column mode instability, a sharp peak in the pressure fluctuations is observed. At higher StDF’s (close to the initial-shear-layer instability) a broad minimum is observed in the nearfield pressure fluctuations, and especially in its axisymmetric mode. An online gradientbased extremum-seeking feedback control scheme is implemented; it uses the RMS of the azimuthal-mode-filtered pressure fluctuations to optimize the StDF. The actual cost function for optimization can be selected as the RMS of various individual pressure azimuthal modes, or a combination thereof. The controller can be setup to either seek a maximum or a minimum with negligible reconfiguration. Preliminary experimental results are presented to demonstrate satisfactory performance in both operating regimes. Starting from either side of the minimum or maximum, the controller is shown to drive the StDF to the respective optimum and maintain the value over time.