Control of acoustics and store separation in a cavity in supersonic flow

Abstract

The supersonic flight community is currently faced with two cavity-under-cross-flow related problems, one being the high noise levels inside the cavity and the other being the return of a store into the cavity after being released from inside. This thesis provides a systematic framework to understand the dominant physics in both problems and to provide solutions for ameliorating the problems. For the first problem, an innovative cavity acoustics model is developed that rigorously explains the role of leading edge microjets in cavity noise suppression and predicts the magnitude of noise reduction for a given control input (that is the steady pressure at which the microjets are fired). The model is validated through comparison of its noise reduction predictions with experiments done using the Florida State University cavity and wind tunnel for different microjet pressures and under Mach 2.0 and Reynolds number 3 million flow, with the microjets being of diameter 400 microns. Based on the cavity acoustics model, optimization of the control input is performed for microjet-based noise suppression of a general cavity under external cross-flow. The resulting control strategy for the FSU cavity is that of an open loop steady microjet firing with the pressure being uniform along the leading edge. This corresponds to a noise reduction of 9 dB OASPL and 20 dB SPL at the dominant tone. The cavity also exhibits saturation in noise reduction for microjet pressures higher than 30 psig. The second problem that the thesis is concerned with, is that of unsuccessful store drops from an external bay of an aircraft in flight. A group of researchers under the DARPA-funded HIFEX Program is currently developing an effective control mechanism to ensure safe release

(cont.) of a slender axi-symmetric store from a rectangular cavity under supersonic external cross-flow. The actuator being tested under this program is based on a tandem array of microjet flow injectors distributed in the spanwise direction near the leading edge of the cavity, and the control input is the steady pressure levels at which the microjets are fired. In order to optimize the control input to ensure safe store departure, a low order model that reliably predicts the trend in the store drop trajectory in the presence of microjets becomes necessary. In this thesis, a suitable low-order model is developed with separate components to predict the pitch and plunge motion of the store when it is inside the cavity, when it is passing through the shear layer at the mouth of the cavity and when it is completely outside the cavity. The model is based on slender axi-symmetric body aerodynamics, thin shear layer at the cavity mouth, high Reynolds number external cross-flow, plane shock waves associated with the microjet actuators, no-flow condition inside the cavity and inconsideration of the cavity acoustic field. The model is validated by comparing with the results of store drop experiments performed under the HIFEX Program at Mach 2.0 and 2.46 using a generic sub-scale weapons bay for different control inputs. The store drop was observed experimentally and predicted by the model to fail when microjets were switched off and successful with microjets on. However, with an increase in microjet pressure, the store drop became unsuccessful ...