Numerical studies on effects of leading-edge manufacturing defects on marine propeller cavitation performance

During the propeller manufacturing process, grinding of propeller surfaces can introduce imperfections and deviations from the desired geometry. These defects could lead to degradation of propeller performance in terms of efficiency, cavitation, vibration and noise. However, there is a lack of scientific literature available that specifically addresses the subject of manufacturing tolerances of propellers. In this dissertation, numerical simulations were conducted for foils with constant DTMB modified NACA-66 a = 0.8 sections using the steady Reynolds-Averaged Navier-Stokes(RANS) solvers in Star-CCM+ to investigate the effects of manufacturing tolerances. 2-D simulations were first performed for the modified NACA-66 (a = 0.8, t/c = 0.0416and f/c = 0.014) foils without and with the leading-edge(LE) defects in infinite flow. Convergence studies were carried out to examine the effects of domain size, grid distribution, grid resolution, and turbulence model on the solution. Using the best-practice settings for2-D simulation, verification studies were carried out for the cavitation buckets of NACA-66(a = 0.8, t/c = 0.2and f/c = 0.02) foils without defect. CFD simulations with best-practice settings were then extended to themodifiedNACA-66(a = 0.8, t/c = 0.0416and f/c = 0.014) foils with three different sizes of LE defects, representing three levels of manufacturing tolerances within International Standards Organization(ISO) 484 Class S. The results showed that the LE defects have significant effects on the cavitation performance of 2-D foils in terms of reduced cavitation inception speed in the typical design range of angle of attack. To investigate the differences in 2-D and 3-D simulations and further quantify the effect of LE defect in future validation studies, 3-D simulations were carried out for the modified NACA-66(a = 0.8, t/c = 0.0416and f/c = 0.014) foils in 1.0m and 0.525m spans with and without LE defects in cavitation tunnel. Effects of RANS modelling parameters, such as domain size, grid aspect ratio, first-grid spacing, y⁺, and turbulence model, on the solutions were carefully examined. Using the corresponding recommended settings, the cavitation buckets, the reduction of cavitation inception speed and the efficiency due to LE defect were predicted. Additionally, preliminary validation studies were performed on two sections of 0.525 m span with no and 0.5mm defects. Furthermore, this dissertation extended 3-DRANS studies on the foils in cavitation tunnel to full-scale propellers, based on the geometry of David Taylor Model Basin(DTMB) 5168 propeller, with and without LE defects. Effects of simulation parameters, including domain size, grid size, stretch ratio, first-grid spacing, y⁺, and turbulence model on the solutions were carefully examined and the best modelling practices for the full-scale propeller was developed. Since there is no full-scale data available, convergence studies were performed for the model-scale propeller followed by validation studies in order to develop the best-modelling practices for model-scale propellers. The wakefield, open-water and cavitation performance were presented and compared with the experimental data. The best-practice settings for the model-scale and full-scale propellers were compared. Using the best-practice modelling settings for full-scale propellers, simulations were carried out to full-scale propellers without and with 0.10mm, 0.25mm, and 0.50mm LE defects. The results showed that the LE defects within Class S tolerances narrow the cavitation buckets. As a consequence, such LE defects can result in more than 40% reduction in cavitation inception speed, which is similar to the conclusions drawn from earlier 2-D studies.