CFD-Based Optimization of Auxiliary Channels for a Waterjet Intake Duct : Multi-Objective Optimization of Auxiliary Channels for Improved Pump Inflow Quality and Reduced Cavitation Risk during Low-Speed Operation

Sammanfattning

Waterjet propulsion systems are widely used in high-speed marine applications due to their high propulsive efficiency, cavitation resistance and low levels of noise and vibration. However, limited research has focused on intake duct performance during low-speed operation, where unfavorable flow phenomena such as flow separation, recirculation and vortex formation deteriorate the inflow to the pump and increase the risk of cavitation around the intake lip. This study investigates the implementation of auxiliary channels in a waterjet intake duct as a method for improving low-speed performance. Computational Fluid Dynamics (CFD) was coupled with CAESES to perform a multi-objective optimization of the auxiliary channel geometry and placement under bollard pull conditions, corresponding to zero vessel speed. The optimization aimed to maximize outflow uniformity and perpendicularity while minimizing cavitation risk areas. The optimized geometry was evaluated across an operating range between 0 and 55 knots and compared with the original intake configuration. In addition, two full-system transient intake-pump simulations including cavitation modelling were performed for the original and optimized duct geometries at zero vessel speed to evaluate system-level effects on mass flow rate, pump efficiency and vapor formation. The results showed that the optimized auxiliary channels reduced the cavitation risk area by 82.6% and increased outlet flow uniformity by approximately 9%, while perpendicularity was only marginally affected. Improvements in pump inflow quality were primarily observed for zero vessel speed, whereas the influence of the auxiliary channels was negligible at higher vessel speeds. Regarding cavitation risk, a favorable reduction was observed for vessel speeds below 20 knots, while higher speeds resulted in unfavorable pressure redistribution and an increased cavitation risk area. For vessel speeds above 30 knots, outflow through the channels was observed, due to the pressure difference across the channels being reversed at higher operating conditions. The results argue for further investigation of auxiliary channels with full system transient simulations across a wide operating range. The transient intake-pump simulations showed increased mass flow rate and pump efficiency for the optimized geometry, together with reduced vapor formation close to the duct surface. The results demonstrate that auxiliary channels have the potential to improve pump inflow quality and reduce cavitation risk during low-speed operation by redistributing the pressure field within the intake duct. As a result, measurable improvements in waterjet system performance were observed. The study therefore indicates that auxiliary channels represent a promising passive flow-control method for low-speed waterjet operation, with further improvement potential through further optimization of the channel geometry and configuration.