This experimental study has the objective of providing new insight into the role of upstream traveling waves (UTWs) in the transonic buffet phenomenon, using the background-oriented schlieren (BOS) technique and corroborating the results with particle image velocimetry. The experiments were carried out on the supercritical OAT15A airfoil under transonic conditions, at a Mach number of 0.7, an angle of attack of 3.5°, and a chord-based Reynolds number of x6. The specific scope of the investigation is the characterization of the spanwise organization of the buffet phenomenon; therefore, the measurements consider a streamwise-spanwise-oriented field of view on the suction side of the airfoil. A particular topic of interest is the propagation and orientation of upstream traveling pressure waves (UTWs) that occur in transonic buffet. The experimental setup used allowed to confirm the two-dimensionality of the velocity field and of the shockwave, but revealed that the UTWs propagate at a non-zero orientation. Processing of the BOS images with two different procedures (normal and differential), has furthermore allowed to extract the frequency and propagation velocity of the UTWs, which have been confirmed to behave as acoustic waves, traveling at the speed of sound relative to the flow. A further analysis has given hints that the strength of the UTWs is modulated during the buffet cycle and, therefore, in support of the feedback-mechanism description of transonic buffet.
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