Experimental investigation of the transonic buffet cycle on a supercritical airfoil

Abstract

Transonic buffet behaviour of the supercritical airfoil OAT15A was investigated experimentally at flow conditions $$Ma=0.7$$ M a = 0.7 and $$\alpha =3.5^\circ $$ α = 3 . 5 ∘ , using schlieren and particle image velocimetry (PIV). The general behaviour of the buffet cycle was characterised with short-exposure schlieren visualisation and phase-averaged PIV measurements. A spectral analysis showed that the shock oscillation occurs with a dominant contribution at 160 Hz (St = 0.07, in good agreement with the literature) and between 25 and 55 % of the chord of the airfoil. Proper Orthogonal Decomposition (POD) was applied to the PIV data to extract the main modes connected with buffet. It is found that the first three most energetic modes capture around 65 % of the total fluctuating kinetic energy. The first and the third modes have a main frequency peak at 160 Hz and are well representing the separated area and the shock oscillation. The second mode was, instead, associated with an asymmetrical behaviour of the separated area and of the shear layer and displays a main peak at 320 Hz, being double the main buffet cycle frequency. Finally, it was shown that by using the 11 most energetic POD modes, an accurate reduced-order model (ROM) is obtained, which when subtracted from the instantaneous velocity fields allows the visualisation of the small-scale structures present in the flow, such as the upstream travelling waves (UTWs) and the vortex shedding in the separated area near the trailing edge. The analysis allowed to estimate the velocity of the UTWs, obtaining values in good agreement with the literature. In contrast, the analysis of the vortex dynamics in the trailing edge area revealed that vortices shed at the shock foot, which convect downstream in an area detached from the airfoil surface, cannot be considered responsible for the creation of UTWs in view of the mismatch in frequency of the two phenomena. Graphic abstract

Experimental study of the effect of wing sweep on transonic buffet

“…The values of the average shock position and STD reported for the airfoil, are very similar to the values reported in Ref. [24] for same flow conditions, as obtained with schlieren and PIV techniques, confirming the accuracy of BOS for 2D applications. When analysing the shock position, it is evident that for the unswept wing case, the shock behaviour is less periodic, with higher frequency contributions being observable in the shockwave behavior.…”

“…For the airfoil the most developed buffet condition is obtained for Ma=0.7 and α=3.5°, as shown in a previous experimental study [24] in the same wind tunnel. Correcting the Mach number for blockage effects, a value of Ma=0.73 is achieved, which is in good agreement with the value found by Ref.…”

“…Both the configurations display a main peak at 160 Hz (similar to Ref. [24]). However, the general behaviour of the PSD greatly differs for the two models, with the airfoil configuration having only minor contributions at frequencies other than 160 Hz, in contrast to the unswept wing.…”

“…In addition to the three wings, an airfoil having a chord of 10 cm and a span of 28 cm (clamped at both sides of the wind tunnel) has been tested as well. This model has been extensively studied in the same wind tunnel for buffet applications in previous experiments [24]. For the finite-span wings, the distance of the wing tip to the side windows (3 cm) has been chosen to ensure that the wing was not immersed in the turbulent boundary layer developing on the side wall of the test section.…”

“…In this paper the main investigation is conducted by using stereo-particle image velocimetry, for reconstructing the three velocity components in several planes of measurement oriented along the free stream direction, at different spanwise locations. The use of PIV is not uncommon in the study of transonic buffet on airfoils (see for example [5,24,25]), however, to the best of the authors' knowledge no previous study has considered PIV to study transonic buffet on swept wings. The application of PIV could help to visualize and analyze the main flow structures characterizing transonic buffet on swept wings, which is not always allowed with wall pressure measurements.…”