Experimental and numerical research into propeller thrust breakdown due to ventilation

When propellers are operating near the free surface, phenomenon called ventilation might occur. Due to insufficient immersion and high thrust loading, the propeller draws air, resulting in a reduced thrust. Reduced thrust may have consequences such as loss of propulsive power, control and steerability, and may therefore be leading to safety issues. Long time exposure to ventilation’s unsteady torque loading can also lead to propulsive unit malfunctioning. As propeller diameters tend to grow bigger, free surface clearance decreases and room is left for air to be drawn. To increase understanding of the phenomenon, current experimental and numerical research was executed. The used propeller was a Wageningen C4.55-propeller, an in design condition lightly-loaded propeller with low blade area, fitting to the trend of increasing diameters. The research was bound by perfect conditions to capture ventilation in the purest form; no influences of wake, waves and ship motion were taken into account. Experimental research showed that free surface ventilation appeared to be the most stable and predictable ventilation regime. Inception through free surface breaking mainly depends on the pressure gradient between the propeller tip and free surface, the tip immersion rate and the ability to draw the free surface. Increased ventilation thrust breakdown showed to be influenced by the local velocity on the blade, which mainly depends on the propeller rotation rate. Vortex ventilation was the most unstable regime in the experiments. Inception seems to be independent of the propeller loading, but influenced by local flow phenomena in the area above the propeller and propeller characteristics . It is believed that vortex inception, shape and wash-out resembles the appearance of the cavitating propeller-hull vortex. Vortex ventilation showed a bistability effect. Experimental results were obtained using a statistical research planning/Design of Experiments, such that polynomial models could be constructed. The model fitted the data well, demonstrated by the fitting coefficient r2 exceeding 0.9. Structural shortcomings were found in capturing the highly unsteady vortex ventilation, variations in mixed ventilation and increased thrust breakdown in free surface ventilating. Numerically, ventilation was simulated using the incompressible VoF-solver ReFRESCO. Vortex ventilation inception was not found, even when a scale resolving simulation was conducted. This is ascribed to insufficient application of the SRS-model in the near blade area, due to insufficient convergence of the omega-equation. Also application of Boussinesqs assumption in the k-equation might be stringent. Free surface ventilation inception was accurately found, both in simulations with a for ventilation adapted actuator disk model and with the propeller. Thrust breakdown was underestimated by CFD. Only breakdown due to surface piercing was found. Underestimation is ascribed to the absence of air entrainment. Application of TNT-EARSM-model (which is not using Boussinesqs assumption) and application of free-slip boundary conditions did not improve the shortcoming. As in literature, other free surface discretization schemes showed the same lack of air engtrainment, the origin might be in the VoF-assumption, being the increased interpolated density used in the momentum equation which prevents air to be convected. Experimental data may only be used with permission of MARIN.Mechanical Engineerin