Impact of POD modes energy redistribution on flow reconstruction for unsteady flows of impulsively started airfoils and wings

Obtaining accurate CFD solutions of unsteady flows during the design process ofan aircraft can be a highly-demanding task in terms of computational and time resources. A common practice is the recourse to Reduced Basis Methods (RBM), which manage to reduce the number of degrees of freedom to be solved yet allow preserving high accuracy, as opposed for example to low-fidelity methods like vortex-lattice or panel methods. RBM based on Proper Orthogonal Decomposition have been extensively studied and adopted but limitations are observed when trying to solve unsteady problems, where the temporal sequence of snapshots and the evolving nonlinear dynamics of the flow field need to be addressed carefully while building the reduced model. The present work investigates the problem of the accuracy in reconstructing nonlinear unsteady fluid flows by means of RBM methods for a specific class of impulsively started lifting bodies. The classical snapshot POD approachand a recent variant named Spectral POD will be comparatively studied to assess their capacity to reconstruct unsteady flow fields typical of aerospace applications. The periodic motion past a cylinder will be considered first as a benchmark testwhile the impulsive start of a 2D three element airfoil and a 3D wing in high-lift configurations will be considered as use cases.