A numerical method to observe thrust behavior of a ducted propeller attached in an underwater vehicle under the influence of flow field of the vehicle's main body is proposed, the thrust characteristics of ducted propeller and the velocity distribution around the propeller when the vehicle in turning motion is studied numerically, the hydrodynamic relationship between the thrust and the velocity components around the propeller is investigated. In the research, 3D geometric models of the duct, propeller and main body of the vehicle are first constructed according to their geometrical feature. Computational fluid dynamics method based on the finite volume method and sliding mesh technique are applied to solve the Navier-Stokes equations which govern the fluid motion around the duct, propeller and underwater vehicle when they are in a turning motion. These equations are solved numerically with the CFD code FLUENT. With the proposed numerical simulation approach, the characteristics of thrust of the ducted propeller and the velocity distribution around the propeller under different working conditions are analyzed, the thrust issued from the ducted propellers and the hydrodynamic relationship between thrust and the velocity components under the influence of vehicle's flow field are observed. Results of the numerical simulation indicate that influence of fluid field caused by the underwater vehicle on the thrust of the ducted propeller is not negligible; there are strong relationships between thrust issued from the propeller and axial induced velocity on the propeller disk, and between the hydrodynamic torque on the propeller and the induced circumferential velocity. It is believed that analyzing correctly the hydrodynamic relationships between thrust and velocity components around ducted propeller in an underwater vehicle is a key to understand more precisely the thrust mechanism of the propeller.
Ducted propeller is one of the major active control devices for manipulation of an underwater vehicle, the trajectory and attitude of an underwater vehicle is usually controlled by user on water surface through an umbilical cable sending operational signals to the ducted propellers attached the vehicle (Avila and Adamowski, 2011; Jaulin, 2009; Li et al, 2005). To understand correctly the hydrodynamic relationship between thrust issued from the propeller and velocity components around it under the influence of the vehicle's flow field is of great importance in designing effective trajectory and attitude control devices for the underwater vehicle. The most common method nowadays to analyze the hydrodynamic performance of a propeller installed in an underwater vehicle is to establish a fitting relationship between the propeller coefficient, torque coefficient and advance coefficient of the propeller by means of open-water propeller test with linear function method or least square method (Bachmayer et al, 2000; Fossen and Blanke, 2000). In these methods, rate of advance on the propeller disk is usually determined based on the vehicle's motion due to the difficulty of evaluating its value on the propeller disk. Obviously the influence of the wake caused by vehicle motion on the thrust evaluation of the propeller is not considered on this computation method although this influence actually exists (Kim and Chung, 2006). Therefore, how to forecast accurately the thrust behaviors of a ducted propeller under the influence of underwater vehicle flow field becomes a basic element for understanding comprehensively the hydrodynamic and control nature of the vehicle when it is driven by the control force of the propeller.