Open-Source Parametric Airfoils to Study Geometric Effects on Buffet

Recent research into buffet in the transonic flow regime has been focused on a limited number of proprietary airfoil geometries and has mainly considered parametric variations in Mach number and angle of attack. In contrast, relatively little is known about the sensitivity of buffet frequencies and amplitudes to geometric properties of airfoils. In the present contribution, an airfoil geometry construction method based on a small number of parameters is developed. The resulting airfoils and computational grids are high-order continuous everywhere except at the trailing edge corners. The effects of four key geometric parameters, defined by local extrema of coordinates on the airfoil at the design condition and denoted as 'crest' points, are studied using large-eddy simulation, considering both free-transitional and tripped boundary layers. For both states of the boundary layer, buffet amplitude and frequency are found to be highly sensitive to the axial and vertical position of the suction-side crest point, while the vertical crest position on the pressure side affects the mean lift. The present work confirms that buffet can appear for free-transitional (laminar buffet) and tripped conditions (turbulent buffet) with similar sensitivities. For test cases near onset conditions intermediate-frequency phenomena have been observed, which were linked to unsteadiness of separation bubbles. Frequencies scaling based on mean-flow properties of the separation bubble were shown to be in good agreement with previous findings on different airfoil geometries in Zauner et al. (Flow Turb. & Comb., Vol. 110, 2023, pp. 1023-1057). Airfoil geometries are provided as open source: https://doi.org/10.5281/zenodo.12204411