A novel cavitation erosion risk model, developed by Schenke et al. ["On the relevance of kinematics for cavitation implosion loads,"Phys. Fluids 31, 052102 (2019)], is applied to compute the cavitation implosion loads. The instantaneous energy balance during the collapse of cavitating structures is considered, where the initial potential energy is first converted into collapse-induced kinetic energy, before it is radiated to the surrounding surface at the final stage of the collapse. In this study, we focus on assessing the cavitation development and the risk of erosion on the blades of propellers operating behind a Ro-Ro container vessel. The presence of the hull contributes to the non-uniformity of the inflow. The consequent variation in velocities and angles of attack leads to the amplification of the cavitation dynamics, especially when the blade passes through the top position. Two designs are investigated that experience cavitation erosion on the pressure side. A statistical filter is used to attenuate low-amplitude implosion loads and identify the extreme events on the blade. The results show a very good correlation with the position of the actual erosion damage on the real propeller blades.

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Cavitation is the formation of vapor pockets in a liquid due to a pressure drop or an increase in speed. Cavitation nuisance can be disastrous for mechanical components, and therefore needs to be controlled from an early design stage. However, predicting cavitation dynamics using CFD (Computational Fluid Dynamics) and therefore the performance of different designs in cavitating flow, can be very challenging and computationally expensive. In this study, we make an attempt to understand the mechanisms of cavitation erosion and predict the erosion risk on the blades of marine propellers operating in behind condition. Cavitation erosion is a result of extreme pressure loads taking place at the final stage of the collapse, and the prediction of those requires very small scales in time and space. For engineering applications, it is extremely costly to resolve such small scales, and therefore additional modeling is required to reduce the computational cost....@en

Propeller cavitation erosion prediction at an early design stage is becoming more and more important since it is one of the key constraints in the search for maximum propeller efficiency. Despite the experience from model tests, cavitation erosion research on actual ship scale is very limited. In this study, an attempt is made to assess the erosion risk on the blades of a full-scale steerable thruster of a tug boat. Pressure side cavitation was detected on board for three different propeller designs. For the first time, a cavitation erosion analysis is performed on ship-scale, using a rigorous potential energy approach, which accounts for the focusing of the potential energy at the collapse center during the cavity collapse. A full sensitivity study has been performed for the blade surface accumulated energy. The erosion model shows the erosion risk for different propeller designs applied on the vessel, and different operating conditions, by looking at the surface specific energy on the blade. The erosion analysis shows locations of high erosion risk that show a good resemblance with the actual damage locations on the real blades.

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Predicting the cavitation impact loads on a propeller surface using numerical tools is becoming essential, as the demand for more efficient designs, stretched to the limit, is increasing. One of the possible design limits is governed by cavitation erosion. The accuracy of estimating such loads, using a URANS approach, has been investigated. We follow the energy balance approach by (Schenke and van Terwisga, 2019), (Schenke et al., 2019), where we take account of the focusing of the potential energy into the collapse center before it is radiated as shock wave energy in the domain. In complex flows, satisfying the total energy balance, when reconstructing the radiated energy, has always been an issue in the past. Therefore, in this study, we investigate different considerations for the vapor reduction rate, in order to minimize the numerical errors, when estimating the local surface impact power. We show that when the vapor volume reduction rate is estimated using the mass transfer source term, then all the energy is conserved and the total energy balance is satisfied. The model is verified on a single cavitating bubble collapse, and it is further validated on a model propeller test case. The obtained surface impact distribution agrees well with the experimental paint test results, illustrating the potential for practical use of our fully conservative method to predict cavitation implosion loads on propeller blades.

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In the maritime industry, cavitation erosion prediction becomes more and more critical, as the requirements for more efficient propellers increase. Model testing is yet the most typical way a propeller designer can, nowadays, get an estimation of the erosion risk on the propeller blades. However, cavitation erosion prediction using computational fluid dynamics (CFD) can possibly provide more information than a model test. In the present work, we review erosion risk models that can be used in conjunction with a multiphase unsteady Reynolds-averaged Navier-Stokes (URANS) solver. Three different approaches have been evaluated, and we conclude that the energy balance approach, where it is assumed that the potential energy contained in a vapor structure is proportional to the volume of the structure, and the pressure difference between the surrounding pressure and the pressure within the structure, provides the best framework for erosion risk assessment. Based on this framework, the model used in this study is tested on the Delft Twist 11 hydrofoil, using a URANS method, and is validated against experimental observations. The predicted impact distribution agrees well with the damage pattern obtained from paint test. The model shows great potential for future use. Nevertheless, it should further be validated against full scale data, followed by an extended investigation on the effect of the driving pressure that leads to the collapse.

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This study presents a novel physical model to convert the potential energy contained in vaporous cavitation into local surface impact power and an acoustic pressure signature caused by the violent collapse of these cavities in a liquid. The model builds on an analytical representation of the solid angle projection approach by Leclercq et al. ["Numerical cavitation intensity on a hydrofoil for 3D homogeneous unsteady viscous flows," Int. J. Fluid Mach. Syst. 10, 254-263 (2017)]. It is applied as a runtime post-processing tool in numerical simulations of cavitating flows. In the present study, the model is inspected in light of the time accurate energy balance during the cavity collapse. Analytical considerations show that the potential cavity energy is first converted into kinetic energy in the surrounding liquid [D. Obreschkow et al., "Cavitation bubble dynamics inside liquid drops in microgravity," Phys. Rev. Lett. 97, 094502 (2006)] and focused in space before the conversion into shock wave energy takes place. To this end, the physical model is complemented by an energy conservative transport function that can focus the potential cavity energy into the collapse center before it is converted into acoustic power. The formulation of the energy focusing equation is based on a Eulerian representation of the flow. The improved model is shown to provide physical results for the acoustic wall pressure obtained from the numerical simulation of a close-wall vapor bubble cloud collapse.

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Cavitation occurrence on the propeller blades may have severe effects, such as cavitation noise, vibrations in the aft ship, and material damage on propellers or rudder. While noise and vibrations affect comfort and stability, cavitation erosion might lead to a significant reduction of the propeller efficiency and structural integrity. Cavitation erosion prediction is therefore extremely important. However, it still remains a challenging issue, especially on marine propeller applications where computational time limit in a practical propeller design environment is imperative. This paper focuses on cavitation aggressiveness on the propeller blades of the KCD-193 model propeller. Cavitation intensity is assessed by the potential power density. Following the idea that the potential power is the basis of the cavitation aggressiveness, we found that by computing the potential power of the first layer of cells, in contact with the blade surface, it gives a useful first estimate of the regions that are exposed to a high erosion risk. The use of a mean field pressure, which is the time average of the local pressure, as the driving pressure behind the cavity collapse, was necessary to get a good match with experimental observations.@en

The last decades there is a strong interest in predicting cavitation dynamics as it is a prerequisite in order to predict cavitation erosion. Industrial applications require accurate results in an acceptable time span and as a result there is a focus on large scale dynamics. In this paper the RANS equations are used to investigate the shedding frequency of sheet cavities in two-dimensional simulations. First a verification study is made for the NAC A 0015 in 6 degrees angle of incidence. A grid sensitivity study is conducted in wetted flow and in steady (non-shedding) cavitating condition (σ=1.6). Then an investigation is conducted in order to capture the shedding frequency. The results show that only when a correction for turbulent viscosity at the cavity-water interface is used it was possible to capture the shedding frequency as found in other numerical studies. Furthermore, a validation study is conducted on a NACA66-312 α=0.8 for two different angles of attack. The obtained results are compared and validated with the experimental data from Leroux et al. They indicate that the 2D shedding frequency predicted by the numerical simulations is in good agreement with the frequency obtained in the experiment.

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