Overview of criteria to estimate aerodynamic limits of the flight envelope of a transonic aircraft based on RANS simulations

Abstract

The aerodynamic limits of the flight envelope of an aircraft play an important role for flight characteristics and loads, so they must be estimated already in the early stages of aircraft design. However, without wind tunnel tests or costly scale-resolving simulations, it is difficult to make an accurate prediction. From a computational perspective, this problem can be split into two parts: (1) generating a valid flow solution for edge-of-the-envelope conditions and (2) extracting the exact lift limit from these computations. Not only the first point can be very challenging, but also the second point. At low Mach numbers, the lift limit may be detected relatively easily, but already at moderate Mach numbers, maximum lift and the lift limit do not necessarily coincide any more, as the actual achievable lift may be limited by the onset of transonic buffet before maximum lift is achieved. Since buffet itself can only be predicted accurately with high effort using time-resolving CFD methods, indirect criteria are required for estimating the lift limit based on steady RANS simulations. This is the focus of the present paper. For this purpose, various criteria proposed in the literature are presented, applied to a test case and comparatively evaluated. For the test case, steady RANS simulations of the generic wing-fuselage configuration LEISA at different Mach numbers and angles of attack are carried out. Finally, the advantages and disadvantages of the different criteria are identified and discussed.