Experiments and CFD for the propeller wake of a generic submarine operating near the surface

Abstract

An experimental and computational fluid dynamics (EFD and CFD) study of the appended notional submarine DARPA Suboff fitted with the 7-bladed E1658 propeller and operating near the free surface is presented. The flow measurements were taken using particle image velocimetry with a multi-camera configuration in the Large Free Surface Cavitation Channel at INM. Single-phase level set numerical simulations with an overset approach were performed for three advance coefficients (J=0.5,0.65,0.82) at three shaft depths (z/D=1,1.5,2). The results reveal that the presence of the hull and the interaction with the free surface strongly affect the inflow and wake of the propeller, producing higher local advance coefficient and blade loads near the surface. Strong free surface fluctuations at small shaft depth also cause instability and breakdown of the propeller tip vortices in the near field. CFD simulations show elliptic instability before vortex breakdown. Comparison between CFD with EFD shows that CFD matches overall trends well, mainly for the phase-averaged flow field, but underpredicts wake fluctuations and thus displays more coherent phase-averaged vorticity. In addition, CFD results also underpredict tip vortex strength and predicts vortex instabilities farther downstream from the propeller plane due to the unresolved upstream turbulence.