A general solver for the prediction of flutter and buffet onset

ifasd2024 Tracking Number 112

A versatile solver capable of predicting both flutter and buffet onset (also under the influence of a flexible structure) values is introduced. The methodology involves computing the (generalized) aerodynamic forces using a linear-frequency-domain (LFD) solver based on linearized unsteady Reynolds-averaged Navier-Stokes equations (URANS) with an appropriate turbulence closure model. A state-space model of the aerodynamic forces is generated through interpolation of the frequency-domain samples, which is the basis of the p-L method [1]. Eigenvalues corresponding to fluid modes may be directly determined, eliminating the need for a priori pole selection as required by traditional rational function approximation techniques. Consequently, the (pre)buffet frequency is accurately represented by the imaginary part of the fluid mode’s eigenvalue. For automated stability analysis encompassing flutter and (elastic) buffet phenomena, a two-step procedure is employed. An algorithm extracts the dominant fluid mode first. Aerodynamic and structural models are coupled then, and a scalar parameter q_m multiplying the (generalized) mass matrix is varied from infinity to unity, while the dynamic pressure is set to a minimal value. A following sweep involves increasing the dynamic pressure, with both structural and dominant fluid modes included. Classical flutter is encountered if aeroelastic eigenvalues corresponding to structural modes cross the imaginary axis, while (elastic) buffet is predicted if the dominant fluid mode’s eigenvalue does. Figure 1 illustrates an application for the OAT15A airfoil at Mach number 0.73 and angle of attack 4° [2]. In the first sweep (left), q_m decreases as indicated by the arrow. Buffet onset is caused by an increasing density value, represented by the point at which the dominant fluid mode’s eigenvalue crosses the imaginary axis (right). This solver also accurately predicts classical flutter onset under different conditions, as detailed in the full paper.