Comparison of shock-buffet dynamics on a supercritical airfoil with and without a pitching degree of freedom

Abstract

With the goal of understanding the dynamics of the transonic flow around an OAT15A airfoil model, velocity field measurements were performed by means of high repetition rate particle image velocimetry. The experiments were performed at free-stream Mach numbers from 0.70 to 0.77 and at a Reynolds number of Rec≈3×106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_\ extrm{c}\\approx 3\ imes 10^6$$\\end{document}. The variation of the Mach number allowed for an investigation in the pre-buffet, buffet and close to buffet-offset regime. A fixed version and a spring mounted version of the model were used to investigate the effect of the pitching degree of freedom on the shock buffet. The dominant structural frequency of the airfoil’s pitch motion was adjusted to be in the range of the natural buffet frequency of the flow with inhibited pitching motion of the model. Flow field measurements with an acquisition rate of 4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$4\\,$$\\end{document} kHz allowed for the detection and analysis of the shape and the motion of the compression shock. With released pitching degree of freedom, shock buffet started at a lower Mach number and showed a larger amplitude for the shock oscillation. Furthermore, the shock motion appeared more harmonic compared to the model without pitching degree of freedom. For a Mach number of M∞=0.72\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_\\infty =0.72$$\\end{document} and 0.74, the change of the angle of attack and the shock location correlated strongly with each other. From the measurements, the phase lag between both quantities during the coupled motion could be determined. From the correlation of the shock position at different heights, it can be concluded that the shock motion is controlled by events at the shock foot. The movement of the upper shock part is only a reaction to the movement of the lower part.