Characterizing Unsteadiness in Supersonic Retropropulsion Flows
Ho, Brenton Willoughby
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Supersonic retropropulsion (SRP) is an increasingly important entry, descent, and landing technology with applications to both planetary exploration and commercial spaceflight. Unsteadiness, and the conditions under which it appears, in SRP flows is poorly understood. Improved predictive models of SRP flow phenomena will lead to more efficient and effective use of the technology. This thesis studies the effect of forebody size on SRP flowfield phenomena using high-speed schlieren imagery. A series of retronozzles and forebodies were tested over a range of stagnation pressure ratios in order to explore the conditions under which SRP flow unsteadiness occurs and the modes of unsteadiness found under these conditions. An analysis method was developed which effectively stabilizes unwanted retronozzle motion, identifies and tracks the shock fronts, and extracts frequency spectra for unsteady motion. Mean and RMS images for each test case were also examined to highlight flow structure and regions of unsteadiness. For each of these cases, one of two types of unsteadiness were observed: bow shock oscillation or bow shock rippling/penetration. The appearance of one or the other was found to be dependent on forebody size and jet pressure. Significantly, frequencies of bow shock motion and Strouhal numbers of the order 10^(-1) were extracted for each unsteady case and found to point to the possibility of a single underlying mechanism which may drive SRP unsteadiness.