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Linear Photodiode Arrays For Time-Domain FLEET Velocimetry In Hypersonic Flows

Douglas W Carter, Steven J Beresh

Sandia National Laboratories, Albuquerque, United States


Linear photodiode arrays are applied to Femtosecond Laser Electronic Excitation Tagging (FLEET) to determine the stream-wise velocity of flow at Mach 8 in the Sandia Hypersonic Wind Tunnel (HWT). Incident light emitted by nitrogen gas upon FLEET excitation is collected by intensifying optics that amplify and transmit the light to the photodiode elements as the gas advects downstream. Two photodiode array configurations are considered: one aligned with the flow direction and one that is perpendicular to it. Conventional FLEET using a CMOS image sensor is also performed for quantitative comparison. The configurations are tested for purely free-stream cases as well as the wake of a seven-degree half-angle cone model at various Reynolds number. The influence of the bi-exponential decay of the FLEET line and the transfer function imposed by the intensifying optics are identified as key considerations on the time-domain method accuracy. Overcoming these effects leads to promising application spaces due to the compact nature of photodiodes and low data overhead.

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