Influences of micro ramp vortex generators on the performance of flush waterjet propulsor

An array of micro-ramp vortex generators (MRVGs) is deployed on the bottom of the ship's hull just before the inlet, aiming to mitigate flow separation on the ramp wall at low inlet velocity ratios (IVR). Four parameters, including height, wedge angle, side length, and spanwise spacing, were analyzed using the numerical methods. Through parametric screening, the optimal height ranges from 20 to 40% of the hull boundary layer thickness upstream of the inlet, while wedge angle and length have negligible effects. MRVGs have been shown to improve outflow uniformity and total pressure recovery, particularly under IVR < 0.7 conditions. At the representative condition (IVR = 0.5), the inlet with optimized MRVGs achieved reductions of 41.5% in nonuniformity, 74.0% in swirl, and 61.6% in total pressure distortion, while increasing total pressure recovery by 8.4% compared with the original design. MRVGs improved the thrust of the waterjet propulsion system by 2.86% and enhanced the propulsion efficiency by 1.60%. Furthermore, the amplitude of the axial frequency (f_n) pressure pulsation on the inlet pressure side of the impeller was reduced by 40.86%, and the amplitude of excitation force on the impeller at f_n was significantly reduced by 60%. This work demonstrates the valuable application of the MRVGs device in improving the overall performance and the potential application in vibration and noise reduction of flush waterjet propulsion at high cruise speeds via simple passive flow control techniques.