Characterization of Transonic Buffet on a Swept Wing by Means of Cryogenic PIV

Abstract

Transonic buffet is one of the major factors in the aerodynamic design of modern transport aircraft. The phenomenon is encountered on the suction side of airfoils in transonic flow. It is associated with low-frequency shock oscillations coupled with a periodic modulation of the large-scale shock-induced flow separation. Buffet can induce detrimental structural vibrations (buffeting) and thus strictly limits the flight envelope. Various aspects of the phenomenon remain not fully understood, in particular where realistic 3D aircraft wings and high, flight-realistic Reynolds numbers are concerned. In the present study, we characterize transonic buffet on a swept wing at a Mach number of Ma=0.84 and a high Reynolds number of Re=12.9 Mio (based on the mean aerodynamic chord), thus fostering a comprehensive understanding of 3D buffet near the limits of the flight envelope. A full-span model of the XRF1 airplane geometry designed by Airbus was investigated in cryogenic conditions (T0=180 K) in the European Transonic Wind Tunnel (ETW), Cologne, Germany. An indepth discussion of the velocity field measured by means of cryogenic particle-image velocimetry (cryoPIV) is presented. Two consecutive and spatially linked fields of view on the wing and in its wake allow to capture the flow physics in a large region simultaneously. The nature of the unsteady flow topology induced by an oscillating shock wave at established buffet conditions is explored in terms of phase-averaged velocity maps. A thorough analysis of the mean and turbulent velocity field is complemented by a statistical evaluation of turbulent quantities. High-speed PIV sequences in the wing wake are explored to capture the spectral signature induced by transonic wing buffet and to assess the fluctuations imposed on the wing wake. Inherent fluctuations continue to exist as they are convected downstream, and maintain a spectral composition typical of 3D buffet. Consequently, buffet severely affects also the downstream flow conditions.