Reduced-Order Modeling and Analysis of Unsteady Rotor Hub Flows

Modal Decomposition is used to characterize high-fidelity flow fields for several different helicopter rotor hub variations in forward flight. Computational Fluid Dynamics (CFD) are carried out using the NASA OVERFLOW 2.2, Reynolds averaged Navier-Stokes solver. The simulated flow conditions and computational grids are based on experiments performed in a water tunnel at flight-relevant Reynolds numbers. The grids and computational methods used are discussed in further detail. Fast Fourier Transforms (FFT) are used to examine the force harmonics on the the surfaces of various hubs, revealing a dependence on geometric forcing. Mean-subtracted, space-only POD of the predicted lift and drag show a slow decay of modal energy, implying that a large number of modes is necessary to model the rotor hub. Space-only POD and Spectral Proper Orthogonal Decomposition (SPOD) are also used in an attempt to isolate and identify coherent flow structures in the rotor hub wake.