Measurements of freestream density fluctuations in a hypersonic wind tunnel

Birch, Byrenn ORCID: and Buttsworth, David and Zander, Fabian ORCID: (2020) Measurements of freestream density fluctuations in a hypersonic wind tunnel. Experiments in Fluids, 61 (7):158. pp. 1-13. ISSN 0723-4864

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Density disturbances in the freestream of the University of Southern Queensland’s Mach 6 wind tunnel (ρ∞≈34gm−3) have been measured using a focused laser differential interferometer (FLDI). The direct contribution of the turbulent shear layer from the Mach 6 nozzle to the FLDI signal was largely eliminated by mechanically shielding the FLDI beams from these effects. This improvement significantly enhanced the low wavenumber FLDI spectra which allowed a von Kármán spectrum fit and demonstrated a −5/3 roll-off in the inertial subrange and enabled the identification of the integral length scale (28–29 mm). The normalised root-mean-square density fluctuations were found to change over the flow duration (typically between 0.4 and 0.6%) for the 1–250 kHz frequency range which corresponds to the wavenumber range of 6– 1600 m−1 in this Mach 6 flow. Previous disturbance measurements using intrusive methods have identified a narrowband 3–4 kHz disturbance that is first measured in the core flow about 65 ms after the flow begins and remains until the flow terminates. The onset of this narrowband disturbance was previously correlated with transition-to-turbulence in the subsonic test gas supply to the nozzle. This correlation was investigated further herein, and the 3–4 kHz feature was inferred to be entropy mode disturbances by showing the departure of the FLDI measurements from Pitot pressure measurements. Through the comparison of FLDI and Pitot pressure data, Pitot pressure probes were demonstrated to produce a poor estimate of the static pressure fluctuations when non-isentropic disturbances are non-negligible.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Faculty/School / Institute/Centre: Current - Faculty of Health, Engineering and Sciences - School of Mechanical and Electrical Engineering (1 Jul 2013 -)
Faculty/School / Institute/Centre: Current - Institute for Advanced Engineering and Space Sciences (1 Aug 2018 -)
Date Deposited: 27 Jul 2020 05:35
Last Modified: 07 Oct 2020 00:54
Uncontrolled Keywords: Nozzles; Pressure measurement; Shear flow; Density fluctuation; Disturbance measurement; Freestream density; Hypersonic wind tunnels; Inertial subrange; Narrowband disturbances; Transition to turbulence; Turbulent shear layer
Fields of Research (2008): 09 Engineering > 0901 Aerospace Engineering > 090107 Hypersonic Propulsion and Hypersonic Aerodynamics
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