The Potsdam propeller test case in oblique flow - prediction of cavitation patterns and pressure pulses

We report computations performed as part of the Second International Workshop on Cavitating Propeller Performance, using the Potsdam Propeller Test Case. The requested computations differ from the previous workshop in that the propeller was tested with a shaft inclination of 12 degrees. Hence all computations were performed unsteady. Results are reported for both open-water (non-cavitating) and cavitating conditions, including prediction of pressure pulses inside the cavitation tunnel, using the computational fluid dynamics code ReFRESCO. Three unstructured grids were generated: one designed for use with wall functions (Coarse); and two with a wall resolved grid (Medium and Fine). The Fine grid also had increased refinement in the region of the sliding interface. Small differences in the prediction of the propeller performance coefficients were found between the three grid types (less than one percent). The grids were seen to resolve the wake flow well, including transport of the tip vortex and capture of the shaft vortex which impinges onto the strut. Due to the shaft inclination, a strong circumferentially varying propeller loading distribution and cavity pattern was seen. Differences in the predicted cavity patterns were small between the Coarse and Medium grids. The reduction in thrust and torque due to cavitation was underpredicted compared to the experimental observations. Propeller thrust increases slightly for the pressure pulses case, due to the reduced distance between the blade tip and the tunnel ceiling. The pressure pulse amplitudes are seen to increase due to cavitation, including additional higher frequency content.