In this paper a three-dimensional mathematical model to simulate the hydrodynamic behavior of a tethered underwater robot system is proposed. The fluid motion around the robot is governed by mass and momentum conservation equations (Navier-Stokes equations). These highly nonlinear differential equations are solved numerically with the CFD code Fluent. From the numerical solution we obtain the hydrodynamic forces on the robot which are linked to the different relative fluid velocities and different incident angles to the robot. With such a method the hydrodynamic loading on the robot can be predicted as a whole. In the model the governing equation of umbilical cable linked to the robot is based on the Ablow and Schechter (1983) method. The boundary conditions for the robot system are presented. The sixdegrees- of-freedom equations of motion for submarine simulations are adopted to estimate the hydrodynamic performance of the underwater robot, and the dynamic effect of the active control thrusters to the robot is also incorporated into the model. The hydrodynamic behavior of underwater robot under different control manipulation introduced by control thrusters is observed numerically based on the established mathematical model in this paper.
The trajectory and attitude of a tethered underwater robot is usually controlled by user on board sending control signal through an umbilical cable to manipulate the active control mechanism installed on the robot, most of control mechanism applied in an underwater robot system nowadays is azimuthing thruster unit. When the trajectory and attitude control problem of a tethered underwater robot is studied it is necessary to couple the main body of underwater robot, umbilical cable and active control mechanisms together to form an integrated mathematical model so that the robot is in a comprehensive dynamic equilibrium condition and influence of umbilical cable and the thruster on the robot behavior can be considered properly.
