Influence of Reynolds Number on Transonic Buffet Conditions on a Supercritical Airfoil

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Transonic flow past airfoils and wings is often associated with shock-wave/boundary-layer interactions and flow separation, which may induce a self-sustained oscillatory shock motion (buffet). The resulting structural response (buffeting) and variation in separation length restricts the flight envelope of passenger aircraft. To promote a better understanding of the exact conditions that initiate buffet onset, the transonic flow over the supercritical OAT15A airfoil is investigated experimentally for three Reynolds number regimes, and over a wide range of Mach number and angle of attack (AoA). First, the buffet-onset conditions are precisely identified, as well as the relevant Mach/AoA regime in which the strongly oscillating shock-wave/boundary-layer interaction characteristic for the buffet state is maintained. Therefore, a variation in terms of Mach number between 0.68 and 0.80, and angle of attack in a range of 3.3 to 7.0 deg is carried out. High-speed focusing schlieren imaging is employed to capture the shock motion temporally, and to extract buffet-relevant conditions. Several large-scale parameters such as mean shock position, buffet frequency, periodicity of shock motion, and mean shock amplitude are discussed in detail to quantify the relevant operating conditions. The discussion spans different regimes from pre-buffet, buffet onset, fully-developed buffet, to buffet offset conditions. Primary focus is to identify the inherent buffet behavior upon a variation of the chord Reynolds number. To capture onset and fully-established buffet conditions at different Reynolds number regimes three wind tunnel models with chord lengths 100 mm, 150 mm, and 200 mm were investigated. Based on the identified cases most relevant to buffet, detailed insight into the spectral nature of all examined configurations is provided.