Roughness Effects on the Tip Vortex Strength and Cavitation Inception

Paper in proceedings, 2019

The possibility and effectiveness of roughness application to mitigate tip vortex flows are evaluated by numerical simulations of an elliptical foil. The analysis includes investigation of the roughness size and area covered by the roughness, as well as the impact on the cavitation inception. Implicit Large Eddy Simulation (ILES) in OpenFOAM has been employed along with a wall-function incorporating the roughness effects to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. The impact of the roughness size on the tip vortex is noted, and it is observed that for the studied condition, the roughness size of 250 micor m is sufficient. The negative effects of roughness on the forces are also observed where application of roughness leads to lower lift and higher drag forces. To minimize the negative effects of the roughness on the performance, the roughness area optimization is conducted and it is found that the application of roughness on the leading edge and trailing edge of the suction side are acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The study shows while the inception occurs at the cavitation number equal to 6.35 in the smooth condition, application of the roughness on the optimum area will delay the inception to the cavitation number equal to 4.1 while only increasing the drag coefficient 1.7 %. This is found to be in a good agreement with the experimental measurements.