Assessment of cavitation erosion risk based on single-fluid simulation of cavitating flows

Doktorsavhandling, 2020

Cavitation erosion is material loss due to the repetitive collapse of cavities near the surface. This phenomenon is one of the limiting factors in the design of high-performance marine propulsors as it restricts their reliability and increases their operational cost. To avoid such consequences, erosion assessment using experimental methods are traditionally performed in the design process of marine propulsors. These methods are, however, expensive and can be applied only in model-scale at a late stage of the design. Alternative to these are numerical assessment methods which can be applied both in model-scale and full-scale at any stage of the design. Development and application of such numerical methods are the main objectives of this thesis.

Two numerical erosion assessment methods are included in this thesis and both are based on single-fluid simulation of cavitating flows. The first method which is developed in this thesis, can assess the risk of cavitation erosion based on incompressible simulations of cavitating flows. This method which considers an energy transfer between collapsing cavities and eroded surface, offers two advantages over other published methods. First, the method takes into account both shock-waves and micro-jets as the mechanisms for cavitation erosion, while previous published methods have considered only one of these erosion mechanisms. Secondly, the method estimates the risk of cavitation erosion based on the collapse induced kinetic energy in the surrounding liquid instead of the potential energy of collapsing cavities, which avoids the uncertainty regarding the calculation of the collapse driving pressure in the potential energy equation. The second numerical assessment method is based on compressible simulation of cavitating flows which has been developed by Schnerr et al. [1] and Mihatsch et al. [2]. This method captures the collapse-induced shock-waves and uses the strength and the frequency of these shock-waves to identify the erosion-sensitive areas. These numerical assessment methods are implemented in the OpenFOAM framework and the implementation has been validated against analytical solutions and an experimental study.

Using the above mentioned numerical assessment methods, three types of cavitating flows are investigated. These are 3D leading edge cavitation over a wing, a cavitating flow in an axisymmetric nozzle, and cavitating flows in water-jet pumps. For the leading edge cavitation, the numerical methods are combined with experimental techniques in order to investigate the relation between the shedding mechanisms of transient cavities and aggressive collapse events. This investigation shows that the leading-edge cavitation leads to the shedding of small and large-scale cavitating structures, both of which are associated with high risk of cavitation erosion. The small-scale cavitating structures are, however, shown to possess a higher risk of cavitation erosion, as they result in a large number of aggressive collapse events which are close to the surface. The second studied case is the cavitating flow in an axisymmetric nozzle which is simulated with the objective of validating the numerical erosion assessment methods included in this thesis. The risk of cavitation erosion predicted by these methods is compared with the experimental erosion investigation by Franc et al. [3] and this comparison shows both methods are capable of identifying areas with high erosion risk. Furthermore, using the numerical results, the hydrodynamic mechanism responsible for the high risk of cavitation erosion at the inception region of the sheet cavity is investigated in detail. This investigation indicates that the high erosion risk in this region is closely tied to the separation of the flow entering the nozzle. The third type of studied flow is the cavitating flow in water-jet pumps. These cavitating flows are numerically investigated with two specific objectives, 1) to identify the mesh resolution requirement for high quality simulation of water-jet pumps, 2) to perform numerical erosion assessment on water jet pumps. For the first objective, the AxWJ-2 pump from Johns Hopkins University is simulated using different mesh resolutions and the results are compared with available experimental data in the literature. For the second objective, the cavitating flows in a commercial water-jet pump are investigated. The investigation includes applying the developed incompressible erosion assessment method to two flow conditions with different risk of cavitation erosion. The results of these numerical erosion assessments are compared with the experimental paint tests performed at Kongs- berg Hydrodynamic Research Centre (KHRC). This comparison shows that the developed numerical erosion assessment method is not only able to distinguish between the conditions with different levels of cavitation erosion risk but also capable of identifying the regions of high erosion risk in the most erosive flow condition. Furthermore, the hydrodynamic mechanisms leading to different risk of cavitation erosion in the two studied conditions are investigated using numerical simulation results. It is shown that this difference is due to a different distribution of axial velocity in the flow entering the pump in the two studied conditions.