Numerical and Experimental Investigation of Hydrodynamic Mechanisms in Erosive Sheet Cavitation

Licentiate thesis, 2018

Cavitation erosion is one of the limiting factors in the design of hydraulic machinery as it is associated with the reduction in the operating life-time of a hydraulic machine and a significant increase in maintenance cost. In order to be able to design a hydraulic machine with a low risk of cavitation erosion, understanding the hydrodynamic mechanisms controlling the cavitation erosion is of great importance. A precondition of these hydrodynamic mechanisms is the creation of a transient cavity that collapses violently as it travels into the high-pressure regions; this creation is often called the shedding of cavity structures. Therefore, understanding the shedding process of the cavity structures plays an important role in providing the knowledge related to hydrodynamic mechanisms of cavitation erosion.
In this thesis, the dynamics of cavitating flows over a NACA0009 foil are investigated using numerical and experimental methods. The shedding behavior of cavity structures is analyzed based on the results from the numerical simulations and high-speed videos. For erosion assessment, the location of erosive collapses in the cavitating flow is determined using a paint test method. These locations and the detectable collapse events in the high-speed videos are used to find the relation between the erosion patterns and cavitation dynamics. In order to numerically assess the areas with high risk of cavitation erosion, the cavitating flow is simulated using a compressible solver, capable of capturing the shock-waves upon the collapse of cavities. The areas with high risk of cavitation erosion, identified with the compressible solver, is compared with the results from paint test. The results from the compressible solver are used to investigate the hydrodynamic mechanisms of erosive collapses.