A comparison of linear and moving mesh CFD-CSD aeroelastic modelling of the BACT wing

Consideration of the aeroelastic and aeroservoelastic behaviour of an aircraft is important throughout design, as catastrophic failure may occur in extreme cases, and more generally aeroelasticity can impact upon safety critical factors such as fatigue, control and vibration, and hence lead to limits upon the safe operating envelope. At low speeds, linear representations of the structural and aerodynamic interactions may be employed with reasonable success. In the transonic region, however, the non-linear nature of the flowfield means that a linearised model no longer accurately represents the physical behaviour, and more complex forms of analysis become necessary, such as fully coupled non-linear computational schemes. Such methods are validated against experimental test cases, and in turn the linear methods compared with both. This usually results in broad agreement between all methods up to the transonic range, whereupon the linear methods diverge from both experiment and the more advanced non-linear schemes. The BACT wing, however, is somewhat unusual in this regard in that the most accurate prediction of the flutter speed in the transonic dip has been produced by a (highly localised) linear representation, whilst coupled codes have struggled to predict even lower speed flutter points, and completely missed the dip itself. The reasons for this are investigated in this paper through a comparison of a model based on linear aerodynamics and an unsteady, fully coupled CFD solver. It is found that using the reported data, the linear method significantly under predicts the flutter speed. Using data derived from unsteady CFD simulation, a close correspondance is found with the full simulation, despite an over prediciton of flutter speed of about 10% at Mach 0.77. Although compared to previous CFD simulations, USCRANSMB performed well, the boundary produced was found in places to vary significantly from the experimental result. A theory for this discrepency advanced elsewhere relating to the centre of gravity location could not be reproduced in this case. Although inconclusive, some evidence was produced suggesting that the differing flutter boundary is caused by the effects of the wind tunnel itself (not modelled in the CFD), and/or other errors relating to shock position and strength.