Characteristics of closed-loop control in an axisymmetric jet at Mach 0.6

A turbulent compressible Mach 0.6 jet was used as a test bed for applying closed loop flow control schemes, with the control objective of increased mixing in the shear layer observed by a change in the far-field noise. The Reynolds number, based on a nozzle exit diameter of 0.0508m, is 690,000. An azimuthal array of eight zero-net mass flux piezo-electric jets, placed at the jet exit, was used for control authority. The actuators were driven at 1900Hz, corresponding to an exit velocity ~ 50m/s from each actuator. Three control cases were tested, an open loop mode-0 excitation, a closed loop mode-0 (column mode) excitation, and a closed loop mode-1 (helical mode) excitation. A spatial Fourier azimuthal decomposition of the near-field hydrodynamic pressure at 8D in the stream wise is used for state estimation and feedback. The far-field sound was measured with six microphones oriented along a boom at increasing polar angles from 9= 15°-90° at 75D downstream to evaluate the effectiveness of each controlled case. For 9=15° (shallow angle), an overall increase in the sound at lower Strouhal numbers (0.04 - 0.2) of the jet was noticed for all controlled cases. The greatest reduction was noticed for the closed loop mode-0 case. Conversely, an overall decrease in the sound was noticed at higher jet Strouhal numbers (0.2 - 3.7). At 9 = 60° and 90°, there was an overall increase in far-field sound for all controlled cases.