A ROM based flutter prediction process and its validation with a new reference model

Industrial flutter prediction for large transport aircraft requires numerical capabilities to predict unsteady aerodynamic forces induced by harmonic oscillations for the complete flight envelope including transonic and separated flow conditions, with shock boundary layer interaction and inverse shock motion. Adequate numerical methods have to model both flow physical and geometrical complexities in more detail than the standard DLM approach. The application of CFD methods results in a more or less time consuming task. On the other hand the aircraft development process, which leads to structural modifications with impact on the flutter behaviour, requires robust and accurate analyses for various stiffness and/or mass conditions in short iteration times. Thus the CFD calculation process should be decoupled from the structural analysis process. This can be achieved by application of aerodynamic reduced order models (ROM). A new method following this strategy 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 standard DLM based linear flutter prediction process is conserved. The quality of the new approach depends significantly on the choice and number of single CFD computations, as well as on mapping of these results to the DLM aerodynamic data structure. The validation of this approach is performed in a first step by comparing unsteady pressure distributions and generalised airloads for cases of the parameter space, which have not been included in the data base. In a second step the complete process validation requires a common aeroelastic reference test case of adequate complexity. A brief review of available windtunnel data for both unsteady aerodynamics and flutter outlines the shortcomings of these data, like missing measurements of a clear transonic dip significantly below Mach number 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 Drag-Prediction 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 4. A constant lift coefficient of 0.50 has been chosen for all Mach numbers. DLR’s TAU code is applied for attached flow conditions in linear modus. 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 fixed aircraft geometry), without their detailed knowledge.