BVI Noise Prediction of the Split Tip Vortex Rotor Using a Time-Marching Free-Wake and Acoustic Analogy

A split tip vortex rotor is one of the BVI noise reduction methods and is also known as sub-wing tip or vane tip. Previous studies show good noise reduction effect and the optimized values of split tip vortex parameters. In order to design a configuration of split tip vortex rotor with the optimized parameter values, new prediction tool is required. The major requirements of a new tool are that the strengths of each split tip vortices can be determined with consistence of the governing equations considering the tip shape and the split tip vortices trailed and self rolled up can mutually rotate according to the governing equation. In this study, the time marching free-wake method, which has been validated in various cases is employed for the high resolution airloads calculation. The method has to be modified considering the multi domain vortex lattice and tip and/or swept leading edge trailing vortices. For noise prediction, the Farrasat formulation 1A based on the FW-H equation is applied, considering the blade flapping motion. First, newly improved method is validated in OLS rotor case. And then, the BVI noise of the split tip vortex rotor and the baseline generic BERP rotor is simulated. The noise contours clearly show the typical BVI directivities. However, a noise reduction effects of the split tip vortex rotor is very little in the specific tip shape. According to the investigation of the tip vortices strengths in hover, it seems the inner tip vortex strength is quite small so that the noise reduction effect is very small.