Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.

                                                                                                                                            —Albert Einstein

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High-Frequency (HF) Resonance Enhanced Microjet (REM) Actuators For Jet Noise Control

Two High Frequency (HF) REM actuators with nominal frequencies of 20 kHz and 60 kHz have been developed and characterized with an objective of actively controlling high speed free jets for noise reduction. Large scale coherent structures are known to be the most dominant source of noise in high speed jets. These structures usually dominate the first two potential core lengths of the jet and are primarily responsible for noise radiation in the aft angle. The goal of high frequency excitation is to force fine scale turbulence in the jet shear layer and potentially promote a ‘direct’ transfer of energy from large to fine scale structures. This is expected to modify the evolution of large scale structures and result in a favorable impact in terms of noise.

HF-REM Actuators- Implentation on a Mach 0.9 Free Jet Flow

A preliminary implementation of these actuators was performed on a Mach 0.9 free jet flow field. Eight nominally 20 kHz actuators were azimuthally distributed at the nozzle exit, with the resulting microjets penetrating the shear layer at an angle of 60° from the vertical. Cross stream velocity measurements showed formation of strong streamwise vortex pair as a result of high frequency injection. These vortex pairs are believed to be responsible for local entrainment and thickening of the initial shear layer. A reduction in turbulent quantities was observed. Far field acoustic measurements were performed at r = 100D measured from the jet exit. A uniform reduction in low frequency noise across all polar angles was observed.

a) b)

Figure 4: Implementation of HF-REM actuators on a Mach 0.9 jet showing (a) Individual source jet actuator assembly showing the actuator orientation at the nozzle exit. (b) Actuator manifold with the stagnation ring, actuators, and the nozzle.

Figure 8: Near field jet acoustic measurements acquired at r = 15D, showing microphone results (a) 90° (b) 150°

Figure 9: Directivity showing reductions in OASPL levels across all polar angles. OASPL is calculated between St = 0 and St = 1:6.