Cavitation and cavitation erosion modeling on marine propellers

Cavitation on ship propellers is a complicated fluid mechanics phenomenon that constraints the propeller design. It directly affects the hydrodynamic properties of the propeller, noise, vibration and erosion characteristics. Cavitation erosion is perhaps the most detrimental result of the cavitation on propellers which results in an increase in noise and vibration characteristics, loss of propeller performance as well as high maintenance costs. Cavitation and cavitation erosion are quite complex problems and can only be solved under specific circumstances by making certain simple approaches. There is no cavitation model that can define the entire physics of cavitation together with the flow conditions on the propeller. However, with some assumptions and simplifications, the characteristics of the cavitating flow on the propeller can be partially obtained by numerical methods. In order to solve a cavitating flow correctly, the parameters such as turbulence, two-phase flow, mass transfer from liquid phase to vapor phase, three dimensionality, viscosity, dynamics of cavitation bubbles and interaction between bubbles must be solved at the same time. Most of these properties in the cavitating flow have been ignored for long years. Today, these problems are tried to overcome by means of a rapid improvement in computational technology and application of developed models for turbulence and cavitation. As Computational Fluid Dynamics (CFD) programs have been improved and become more accessible and the computer technology has grown extraordinarily, this results in increased number of postgraduate dissertations and articles, using CFD in recent years. Therefore, an important part of this study has also been carried out using a commercial CFD program. In this Ph.D. study, cavitation on ship propellers and the cavitation erosion is modeled using CFD and the study was aimed to obtain a deep knowledge about the physical characteristics of cavitation and erosion. Therefore, the main purpose of the study is to make a further contribution to the understanding and development of the numerical model of cavitation and cavitation erosion. In this context, cavitation and cavitation erosion were modeled numerically and an experimental study was carried out to investigate the resistance of different propeller materials to cavitation erosion. Numerical analysis was performed using Star-CCM+, which is a Finite Volume based CFD software. The Reynolds Averaged Navier-Stokes (RANS) model was first tried for cavitation predictions but it was decided to use Detached Eddy Simulation (DES) due to the inadequacy of the RANS method in modeling cavities around the hydrofoil and propeller blades. Then for all the CFD analysis carried out within this thesis, the turbulence was solved by DES SST (Menter) k-Omega, two-phase flow was modeled by VOF (Volume of Fluid) and cavitation was modeled by Schnerr-Sauer cavitation model, which uses a simplified Rayleigh Plesset bubble equation. The first principle for accurately modeling of cavitation erosion depends on correct modeling of cavitation. For this reason, the CFD analysis has started with the validation of the cavitating flow on the PPTC propeller, which has cavitation tunnel experiment results in the literature. With the generated CFD model, cavitation predictions and propeller characteristics, such as thrust coefficient, torque coefficient and open-water poropeller efficiency were obtained for six different flow conditions where the first three flow conditions did not have shaft angles and the other three had 12o shaft angles. The calculations were compared with the experimental results to show that the CFD model accurately modeled the cavitating flow. Following the validation study, the same CFD models were used in the analysis of the PPTC propeller was applied in cavitating flow around Delft hydrofoil and NACA0015 hydrofoil as both two hydrofoils have cavitation tunnel experiment results and CFD analysis results in the literature. In our CFD analysis the same flow conditions was used as the experimental ones to compare the results of the lift force coefficient and the volume of cavitation formed on the Delft hydrofoil with the results from the literature. Similar analysis was also performed for NACA0015 hydrofoil. The results of the cavitation on the hydrofoil indicate a good validation of the numerical model with the experimental results and also with some numerical studies performed from the literature. Thus, validation of the generated CFD model in the modeling of the cavitating flow around the hydrofoils was also made. Next step was cavitation erosion modelling following the validation of cavitation. In this step, the erosion intensity on the NACA0015 hydrofoil was predicted by using IFM (Intensity Function Method) and GLM (Gray Level Method). IFM is based on the pressure fluctuations on the surface with time and GLM is based on the change of the cavitation volume on the surface with time. In order to obtain accurate results, both methods should directly related to the erosion intensity thresholds (minimum and maximum values) to be used. It is worth mentioning within the scope of this thesis, a new method has been proposed to determine the threshold values of erosion intensity. According to this new method, the minimum value of the erosion intensity on the hydrofoil surface is obtained from the minimum value of the mean erosion intensity graph. Similarly, the maximum value of the erosion intensity on the hydrofoil surface was obtained from the maximum value of the histogram graph. The erosion estimations of the NACA0015 hydrofoil obtained with the new proposed erosion intensity thresholds were found to be similar than those predicted by IFM and GLM when compared with the experimental results. IFM results obtained using the erosion intensity thresholds determined by the above technique were more similar than those of CFD studies using different erosion intensity threshold in the literature with the experimental results. After the validation of erosion on the hydrofoil using the two methods, hydrofoil and propeller erosion were predicted by using a new method for the first time for hydrofoils or propellers in the literature. This new method is called EPM (Erosive Power Method). The EPM equation used in this method has been mentioned in several studies in the literature, but has never been applied to the propeller erosion predictions. The EPM equation consists of the sum of the IFM and GLM equations and involves the properties of both two methods. Therefore, it depends on both the pressure change and the cavitation volume on the surface. The images of the cavitation erosion expected to occur on the NACA0015 hydrofoil surface obtained by the EPM were found to be more similar than the obtained IFM and GLM results, when compared with the experimental results obtained from the literature. In Chapter 5, cavitation and cavitation erosion estimations on KCD-193 model propeller using the above cavitation and cavitation erosion models were carried out for six different flow conditions. Cavitation development on the propeller surface and performance predictions of the propeller were compared with the experimental results. In the study, velocity and turbulence intensity data obtained from the LDV measurements were imported to the Star-CCM+ program as the real flow characteristics in the inlet flow conditions; instead of by making assumptions. Afterwards, the erosion images of KCD-193 propeller obtained for two different flow conditions were compared with the images obtained with EPM, IFM and GLM described above. The results of the EPM that used for the first time in the literature were compared with the results obtained from the cavitation erosion experiments. The prediction of the erosion region of the propeller blades was found similar to the experimental results. The experimental part of the study described in Chapter 6 began with air jet erosion tests of the materials dyed with different dyeing techniques. The tests indicated not any apparent damage on the surfaces of the samples. It was revealed that the use of air jet instead of water jet in the tests did not generate enough pressure difference enough to damage the surface. Therefore, no visible erosion on the surface of the sample occured. Following the air jet tests, it was decided that the study should be repeated by installing a water jet test rig instead of air jet. Thus, initially a simple water jet test rig was set up yielding a dental flosser device which is normally used in dentistry. As the system did not generate desired results, the decision have been made to move to the use of a a high pressure water jet rig to study cavitation erosion properly as given in ASTM (American Society for Testing and Materials) standards. Thus the system was set-up in the Ata Nutku Ship Model Test Laboratory of the ITU, Faculty of Naval Architecture and Ocean Engineering, based on procedures given by ASTM. In the validation study of the water jet test rig, cavitation erosion tests were performed for Al-6063 material with different Stand-off (Soff) distances for 30 minutes test durations, with consideration of data obtained from the literature. Similar to the literature, the erosion rate, which is the ratio of mass loss by the time, decreases when Soff distance increases. Furthermore, in the surface roughness analysis performed by optical profilometer, the results showed that the deepest pit height (hmax) formed on the sample surface changes inversely with Soff distance. In the main part of the cavitation erosion tests were conducted according to ASTM G-134 standards, to evaluate the cavitation erosion resistance of three propeller materials, such as Cu1 (manganese-bronze), Cu3 (nickel-aluminium-bronze) and Cu4 (manganese-aluminium-bronze) alloys. The erosion tests were performed for three different cavitation numbers by the time durations of 30, 60 and 90 minutes. The erosion rate was used as an indicator for damage behaviour on the material. Moreover, the damaged sample surfaces were examined and the surface roughness characteristics, such as Ra, Rq, Rt and maximum depth were measured by a 3D optical profilometer. Although Cu4 has the superior mechanical properties, such as yield stress, tensile strength, strain rate, Brinell hardness, Rockwell B hardness and fatigue strength; the study showed excellent erosion resisting performance of Cu3 compared to that of Cu4, contrary to expectations. In addition to that, it was found that the erosion becomes more pronounced with increasing testing duration as well as decreasing cavitation numbers independently of the material. On the other hand, in some conditions erosion rate increases as the cavitation number increases. The ultimate aim of the experimental study carried out is try to explore similarity of the cavitation erosion formation between tests in water jet test rig and erosion tests at cavitation tunnels for propellers. This will enable the replication of the propeller material and cavitation characteristics as an erosive indicator in a much simpler setup with more number of samples tested. As a result, in the Ph.D. study cavitation was modeled accurately using CFD and a new method for estimation of cavitation erosion was introduced to the literature. Besides, a high pressure water jet test rig was set up to investigate the resistance of different materials to cavitation erosion. Cavitation erosion tests were carried out for three different cavitation numbers, three varying time duration and the resistance to cavitation erosion of the materials was investigated within the parameters determined.