Flutter Computations for a Generic Reference Aircraft Adopting CFD and Reduced Order Methods

Abstract

A new ROM for a CFD based flutter analysis at transonic and separated flow conditions is presented. It relies on a limited number of unsteady CFD computations forming the ROM data base, combined with an arbitrary number of Doublet Lattice computations. Thus compatibility with the standard DLM based linear flutter prediction process is conserved.The validation of this approach requires a common aeroelastic reference test case of adequate complexity. A brief review of available windtunnel data for both unsteady transonic aerodynamics and flutter outlines the shortcomings of these data, for example the lack of clear transonic dips at Mach numbers significantly below one, and of inverse shock motions.A new common test configuration with a transonic dip flutter boundary in the Mach number range between 0.80 and 0.95 is proposed. The aircraft geometry from the DragPrediction Workshop 4 fulfils the above mentioned unsteady aerodynamic requirements. It is extended to a flutter model of a generic aircraft. The capability of this model is demonstrated by applying the above flutter process. An unsteady aerodynamic ROM is generated in the 3 dimensional parameter space of Mach number, reduced frequency and elastic mode shape. For selected points of this parameter space a sufficient number of unsteady RANS simulations is performed to display unsteady pressure distributions at Mach numbers between 0.6 and 0.90, and reduced frequencies up to 2. A constant lift coefficient of 0.50 has been chosen for all Mach numbers. DLRs TAU code is applied using both harmonic forced motion and pulse responses for attached as well as for detached flow conditions. The ROM is completed by performing this procedure for several so called synthetic modes, which are chosen properly to display all realistic structural modes of the aircraft geometry, without their detailed knowledge.