New Insights into Roughness Applications in Tip Vortex Cavitation Inception Mitigation

Paper in proceeding, 2020

Tip vortex cavitation (TVC) is usually the first type of cavitation that appears on a propeller. Therefore, it is considered as the main cavitation characteristics to avoid in the design procedure of low-noise propellers, where their operating profiles demand very low radiated noise emissions. The current study includes both numerical and experimental analyses of blade surface roughness application in order to mitigate TVC inception. The investigation consists of applying roughness application on a classical benchmark, an elliptical foil, and on a propeller selected from a Kongs-berg research series of highly skewed propellers having a low effective tip load. The numerical simulations are performed on an appropriate grid resolution for tip vortex propagation, at least 32 cells per vortex diameter by using Implicit Large Eddy Simulation (ILES) for unsteady simulations, and RANS using the SST k − ω model with a curvature correction for steady simulations. Two approaches are considered to include roughness in the numerical simulations: using a rough wall function and resolving the flow around the roughness elements. To minimize the negative effects of the roughness on the propeller performance, the roughness area is optimized by simultaneous consideration of the tip vortex mitigation and performance degradation. Experimental measurements of the elliptical foil are conducted to support the CFD study at different operating conditions and with different roughness patterns while LDV and high-speed video recordings are used to collect the data. The tested conditions include both cavitating and inception of TV flows on the smooth and roughened foil to provide further insights on the usage of roughness. For the elliptical foil, it is found that the application of roughness can reduce the cavitation number for cavitation inception, σ i , by 35 % while keeping the performance degradation less than 1% compared to the smooth foil condition. The average reduction of the TVC inception number achieved by using roughness on the propeller is around 21% with a performance degradation of around 1.5% compared to the smooth propeller condition.