A multi-point focused laser differential interferometer for characterizing freestream disturbances in hypersonic wind tunnels

A three-point Focused Laser Differential Interferometer (FLDI) instrument was implemented to investigate the freestream disturbance environment in a Mach-6 shock tunnel. The FLDI beams were split such that one pair was aligned with the flow direction and the other along the Mach angle, allowing for simultaneous measurements of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Transfer functions for interpreting the FLDI signal were applied to the free-jet nozzle flow: the instrument’s depth of focus and sensitivity to disturbance wavenumber were estimated, as well as the influence of the nozzle shear layers in corrupting the FLDI signal. Experiments were performed at varying unit Reynolds numbers (3.2-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-$$\end{document}14.4×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document}106m-1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^6\,\mathrm m^{-1}$$\end{document}) and total enthalpies (0.57-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-$$\end{document}0.89 MJ/kg). Streamwise convection velocities measured with the FLDI were found to be lower than theoretical freestream velocities, consistent with earlier studies detailing the noise convection velocity, though some frequency dependence was observed. Both broadband and core-flow density-based turbulence intensities exhibited a decrease with increasing Reynolds number. The relative contributions of the acoustic and entropic disturbances to the overall noise environment was also examined through a study of the correlated signal pairs. The acoustic contribution was found to be dominant in all cases, with no clear trend with Reynolds number. Finally, the disturbance intermittency at the various tunnel conditions was investigated; the intermittency was greatest at the lowest Reynolds number condition, suggesting that the nozzle boundary layer may be transitional at this condition.