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Published July 8, 2022 | In Press
Journal Article Open

Supersonic Freestream Density Fluctuations from Focused Laser Differential Interferometry and Pitot-Probe Measurements


Focused laser differential interferometry (FLDI) is used to make freestream density fluctuation measurements in the von Kármán Gas Dynamics Facility (VKF) Wind Tunnel D at Arnold Engineering Development Complex. A comparison is made between pitot-probe and FLDI measurements after converting both to freestream pressure fluctuation spectra. A modification of Stainback and Wagner's theory, incorporating recent numerical results from Chaudhry et al., is used to interpret the pitot data, while a new inversion algorithm is applied to the FLDI data. These methods of interpretation make the assumption that the tunnel freestream disturbance environment is dominated by the acoustic Kovásznay mode, in line with the theoretical and experimental evidence. Close agreement is found between the two sets of spectra, showing that accurate quantitative data can be obtained with FLDI, and the instrument can be used to extend spectra beyond the pitot bandwidth.

Additional Information

© 2022 by California Institute of Technology. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Received 4 November 2021; revision received 10 April 2022; accepted for publication 12 May 2022; published online 21 June 2022. This work was partially supported by the Office of Naval Research award N00014-16-1-2503 with Eric Marineau as program officer and the Air Force Research Laboratory award STTR FA8651-17-C-0071 with Daniel Reasor as program officer. The authors would like to thank B. Valiferdowsi for the design of the frames used to mount the focused laser differential interferometry onto Tunnel D. Thanks also to J. E. Shepherd for his involvement and encouragement toward our efforts to understand the details of how focused laser differential interferometry works. J. M. L. would like to thank R. S. Chaudhry for a useful conversation about his pitot-probe work, and L. Duan for generously providing the data and permission for Fig. 4.

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August 22, 2023
October 23, 2023