Hydrodynamic performance of a submarine is significantly affected when it sails near free surface. It has been studied that the hydrodynamic characteristics of a SUBOFF sailing near surface with different conditions of free surface (calm water and regular waves), using an inhouse CFD code HUST-Ship in this paper. Herein, an unsteady RANS equation has been solved with SST k-ω turbulence model. Firstly, grid/time-step convergence studies have been conducted in order to verify the numerical method and the uncertainty analysis of the numerical simulation results of resistance has also been evaluated. The results of simulation showed good matches with the experiment. Afterwards, it has been simulated that submarine sails below calm water with different depths and altering the calm water to regular waves with different wave lengths in constant depth and velocity after the research of calm water. The hydrodynamic performance of submarine has been acquired and the influence of conditions of free surface has been compared. The results demonstrate that hydrodynamic performance has greet discrepancies between calm water and regular waves. Particularly, force and moment perform nonlinear tendency with the wave length changing.
Restricted by the requirement of replenishment and communication, it is necessary for a submarine floating beyond the free surface periodically. When it sails close to the free surface, due to the interaction between flexible surface and dynamic pressure distribution along the hull, the surface waves will be generated and the hydrodynamic performance of submarine will be highly affected. As a result, the submarine maneuvers will get worse, making more requests about control systems. To explore the mechanism of free surface affecting an underwater vehicle and provide reference for submarine self-propulsion and maneuvering, it is essential studying the hydrodynamic performance of a submarine sailing near free surface.
Wilson et al (2010) and Dawson (2014) conducted several experiments to investigate the influence of free surface on the drag, lift and moment of submerged vehicles, which revealed that the free surface affected the wave-making resistance and dynamic pressure distribution along hull, causing the lift and moment changing. With the development of computational fluid dynamics (CFD), many studies utilizing this method has been presented. Amiri et al (2018) numerically simulated a SUBOFF sailing beneath free surface and evaluated the mechanism of free surface affecting the hydrodynamic performance of submarine. The study indicated that the interaction between the bow and aft shoulder waves performed a larger influence than the interaction between the bow and stern waves. On the other aspect, some studies related to the self-propulsion of submarine near free surface have been conducted. Carrica et al (2019) and Wang et al (2020) performed experiment and simulation to study the self-propulsion of submarine near free surface. Carrica focused on the effect of different control strategies on the propeller wake and parameters of propeller motions. Wang investigated the detailed propeller wake with different shaft depths and advance coefficients. Both of them evaluated the interaction of free surface to the hull and propeller, which showed that the free surface changed the force and moment as well as inflow to the propeller, affecting the advance, thrust and torque coefficients and complicating control of the vehicle.
