Numerical and Experimental Assessment of Parameters Influencing the Development of Secondary Vortex Structures in Rotor Wakes

Abstract

A common phenomenon in modern high-fidelity CFD simulations is the breakdown of the primary vortex system in hover due to secondary vortex braids. In the current work, the development of secondary vortex structures in the wake of a two-bladed rotor was investigated by combining stereoscopic particle image velocimetry measurements in different measurement planes and high-fidelity simulations. An analysis was performed to investigate how various numerical inputs including sub-iteration convergence, numerical dissipation, blade pitch offset, grid-resolution, and rotor thrust affect the primary and secondary vortex characteristics. A parametric study on near- and off-body solver sub-iteration convergence demonstrated that the primary and secondary vortex characteristics converge as the sub-iteration convergence of both solvers increase. The breakdown of the primary vortex was shown to be directly linked to the number of secondary vortices. Dissimilarities in the blade pitch angle, which could not be avoided in the experiment, were modeled by intentionally using an offset in the blade pitch angle of the two blades. It was shown that as blade pitch angle offset increases, vortex pairing becomes more distinct. When vortex pairing occurred in both the experiment and simulation, the decay of secondary vortices in the experiment and simulation agreed best. Grid resolution was decreased from 5% chord to 3% chord, and the finer mesh simulation with higher sub-iteration conver gence agreed best with the measured primary and secondary vortex characteristics. Differences in the measured and predicted number of secondary vortices for the lower thrust case was attributed to differences in tip vortex breakdown in the experiment and simulation.