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Experimental Characterization of Acoustic Wave Propagation through a Supersonic Ducted Flow

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Technical Report

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University of Maryland College Park United States

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In scramjet combustors, if pressure waves could propagate upstream through subsonicboundary layer flow, it would set up an acoustic feedback mechanism that could lead to selfsustainedcombustion instability. To investigate the possibility of upstream wave propagation,non-reacting supersonic flow experiments were conducted in a specially-designed supersonicflow duct, which simulated the internal flow path of a dual-mode scramjet combustor.Furthermore, to experimentally simulate combustion instability, large-amplitude pressureoscillations were created by passively exciting the exhaust jet flow using screech mechanism,which resulted in large-amplitude pressure oscillations with dominant frequencies rangingbetween 2.7kHZ and 4.2kHz. Then, the acoustic signal was tracked along the supersonic flowduct using four high-frequency-response Kistler pressure transducers that were flush-mounted atthe combustor and isolator walls. Schlieren visualization was conducted to characterize theinternal supersonic flow field, and an analytical approach was used to estimate the turbulentboundary layer growth and displacement thickness. Ten sets of experiments were conducted atvarious stagnation pressure values ranging from 35psi to 125psi, and four sets of experimentswhere strong resonances were observed were repeated over ten separate runs for reproducibility.Fast Fourier Transform was used to quantify the changes in pressure oscillation amplitude ineach case. The results conclusively show that the downstream disturbances were propagatingupstream, and they were being attenuated at different rates depending on flow conditions andduct geometry. Possible reasons for this new phenomenon were examined and discussed.

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