I’m pleased to see my student Dr. Jianhui Cheng defend her Ph.D. successfully at University of Florida.
Title: THE APPLICATION OF INSTABILITY WAVE THEORY ON ACOUSTIC RADIATION WITHIN OFF-DESIGN SUPERSONIC JETS AND ON TWO-POINT STATISTICS WITHIN HIGH-SPEED FLOW OVER SHARP AND BLUNT CONES WITH PLASMA ACTUATION
Abstract:
The intense noise generated by large-scale turbulent structures is the main component of acoustic radiation from jets, especially in the downstream direction, which is harmful to human health. Furthermore, the high-frequency waves created by boundary layer turbulence can cause intense vibrations on vehicle surface structures during rocket launch. It can be simplified as a cone flow. As instability waves are closely connected with the formation of large-scale turbulent structures, instability wave theory is applied to compute the radiated noise in the far-field for free jets and the pressure fluctuations on the cone surface. A phenomenological plasma actuator model is applied to alter the flow-fields and stability properties of the cone flows. We compute the radiated noise from instability waves within two off-design supersonic jet flows. The directivity and azimuthal properties of jet noise from large-scale turbulent structures in the downstream direction are captured. The spectral density of noise in the downstream directions agree well with the experimental measurements, whereas they are lower at the upstream direction. The auto-correlation shows similar behavior both in the downstream and upstream directions. The cross-correlation and coherence shows higher values than the prediction from the Kirchhoff surface method and experimental measurements. The results show that the noise from instability wave models in the upstream and downstream direction are highly correlated. The results could be used for future flow control on noise reduction from large-scale turbulent structures. We examine the effect of plasma actuation on the pressure fluctuations from the instability waves on the cone surface via theory with flow-fields predicted by computational fluid dynamics. We present predictions for a seven-degree half-angle cone at free-stream Mach 2, 3.5, and 5 with varying nose radii. Nose radii range from 0.038 to 38.1 mm and represent both sharp and large leading edge bluntness. For non-actuated flows, we observe that very small radii leading edges do not alter the maximum growth rates. Large radii cones have lower growth rates due to a thicker boundary layer. Spatial coherence of the instability waves decreases with increasing frequency. The growth rates are smaller at higher freestream Mach number. The effect of the simulated plasma actuator adds local heating to the flow-field. We find that plasma actuation stabilizes the flow-field and spatial coherence becomes smaller. The results are beneficial for future flow control to reduce the vibration from large-scale turbulent structures.
Jianhui Cheng, 2022