Controlled Surface Roughness Application in Tip Vortex Mitigation and Inception Delay

The current study summarizes findings of the RoughProp project aiming to develop and utilize controlled surface roughness to mitigate tip vortex cavitation (TVC). The analysis is conducted through numerical and experimental investigations of tip vortex flows over an elliptical foil and a high-skewed propeller by focusing on how the roughness should be applied to have an optimum balance between TVC mitigation and hydrodynamic performance degradation. To model the tip vortex, LES is employed on a proper grid resolution having at least 32 grid points across the tip vortex diameter. The CFD analysis is supported by new sets of experiments conducted in the cavitation tunnel at Kongsberg Hydrodynamic Research Centre. The outcome of CFD results and experimental observations clarify the capability of roughness application in TVC mitigation with a reasonable performance degradation. For the propeller, it is found that the combination of having roughness on the blade tip and a limited area on the leading edge is the optimum roughness pattern that can provide a reasonable balance between tip vortex cavitation mitigation and performance degradation over a wide range of operating conditions. This pattern leads to an average TVC mitigation of 21% with an average performance degradation of 1.5%.