For the tunnel thruster, the strut of the gearbox may impose excessive vibration and noise. The current study separately adopts RANS and DES method to study the influence of the cross-section shape of the gearbox strut on the unsteady hydrodynamics of the tunnel thruster. It mainly focuses on the unsteady hydrodynamic forces and the vorticity distribution of the tunnel thruster with struts of different cross-section shapes. It shows that the elliptical strut and the streamlined strut can reduce the flow separation around the strut, thereby reduce the amplitude of blade excitation force and the unsteady force of the gearbox strut.
In recent years, tunnel thrusters have been widely used in autonomous underwater vehicle (AUV), which permits operations in very confined areas, with virtually independent horizontal and vertical motion at velocities starting at 0 m/s, thereby increasing the maneuverability and robustness of the AUV (Cruz, 2008). However, the gearbox used to drive the propeller rotation is usually close to the propeller due to the limitation of the tunnel space, and the gearbox has a serious blocking effect on the incoming flow when the gearbox is in front of the propeller, resulting in a large velocity gradient of the incoming flow. Nonuniform incoming flow will cause the components in the tunnel to be subjected to unsteady forces that vary periodically, intensifying the vibration of each component as well as noise, thus bringing about structural damage and noise overload. Therefore, it is of great importance to study the effect of the gearbox geometry on the unsteady hydrodynamic characteristics of the tunnel thruster.
At present, the influence of geometric parameters of the tunnel and propeller on the hydrodynamic characteristics of the tunnel thruster is of great interest in the industry engineering. In terms of theoretical derivation and model test, Taniguchi (1966) conducted a systematic experimental study of the tunnel thruster and discussed the effects of parameters such as the number of blades, propeller disk ratio, hull tilt angle, tunnel length, bottom dip depth and tunnel guide circle radius on the thrust of the tunnel thruster. Wang et al. (1996) pointed out that the K-shaped bow thruster tunnel has the advantages of increasing thrust and slewing moment, reducing vibration and noise. Baniela (2009) pointed out through his analysis that the tunnel shape, the angle between the tunnel and the hull, the length and diameter of the tunnel, the propeller diameter and the rotational speed all affect the efficiency of the tunnel thruster. Dang and Ligtelijn (2019) pointed out that the vibration and noise levels of the propeller can be reduced by using the S-shaped chordwise radian distribution of the blade profile based on the tunnel thruster model experiment. CFD method has been widely used in the research of tunnel thrusters with the development of computer technology, Zhai and Cao (2015) used RANS (Reynolds-Average Navier-Stokes) method to reach the conclusion that increasing the radius of the inlet guide circle can make the propeller inflow more uniformly, while it will reduce the propeller thrust. Yan (2012) numerically studied the effects of propeller pitch ratio, disk ratio and hub diameter ratio, tunnel length, as well as the tunnel length and tunnel vertical tilt angle on the hydrodynamic performance of the tunnel thruster and the surrounding flow field based on RANS method. Brizzolara et al. (2017) proposed a stepped tunnel scheme and changed the diameter and the shape of the joints on this basis, and found that the stepped tunnel form had better hydrodynamic performance through comparative analysis of CFD calculations. Yu et al. (2018, 2019) discussed the effect of blade tip unloading on the unsteady force of the tunnel thruster based on model tests and numerical simulations, and found that the propeller with blade tip unloading can reduce the fluctuation pressure at the wall of the tunnel near the blade tip, but increase the amplitude of the excitation force of the propeller itself. Kong and Hu (2021) adopted RANS method to investigate the effect of sacrificial anode block arrangement in the tunnel on the unsteady hydrodynamic force of the tunnel thruster.