Modal decomposition analysis of hovering rotor wake breakdown

The breakdown of a rotor wake in hover is a common phenomenon in eddy-resolving computational fluid dynamics simulations. While such breakdown can be alleviated by employing specific grid topologies and tailored solution procedures, the exact process by which this breakdown occurs and the extent to which it should occur are unknown. In this work, the S-76 rotor in hover is simulated over a range of collective pitches and wake breakdown is investigated by applying proper orthogonal decomposition (POD) to the full solution. POD determines the optimal bases on which a data set can be projected and subsequently reconstructed, providing guidance on coherent structures and fundamental behaviors in the solution. The NASA OVERFLOW 2.3 solver is used with hybrid RANS/LES modeling enabled. Grid generation and computational methods are described. The POD modes of the wake structure are investigated and compared, revealing that there are coherent structures in the wake, but these exist primarily in the young wake-age regions. These features were dominated by the 4/rev forcing of the blades. The wake was analyzed in an isolated state by removing the 4/rev forcing; however, there was no appreciable coherence found in the POD modes with this removal.