Development of a cable dynamics module for the flow solver CFDShip-Iowa

Abstract

Abstract-A cable dynamics module was developed and implemented in the Reynolds Averaged Navier-Stokes Equations (RANS) solver CFDShip-Iowa to model the dynamics of towed cable systems in high sea states and during the launch and recovery of tethered underwater vehicles from behind a ship. In previous cable dynamics simulations, the vehicle forces are computed using hydrodynamic coefficients which are only accurate for steady, uniform flows. For unsteady, energetic flows in sea states and/or in the ship turbulent wakes, this coefficientbased approach is inadequate, and high-fidelity RANS solution is needed to predict the unsteady fluid forces acting on the vehicles. The cable module is based on an existing cable program with modifications for integration in CFDShip-Iowa and additional capabilities including dynamics modeling of the towing vehicle. The cable itself is represented by a series of rigid cylinders or links connected end-to-end by spherical joints. Cable physical parameters and external forces are lumped at these joints, and all vehicles are treated as rigid bodies. This paper focuses on the theoretical development of the cable module and presents sample simulations of an underwater vehicle towing another vehicle in both calm water and in Sea State 3. More extensive simulations and validations will be included in a future paper.

Variable-Length Cable Dynamics of Payout and Reel-in With a Vertically Tethered Underwater Drill Rig

“…The numerical calculations used the Newton iteration method and implicit time integration method. Reference [20] regarded underwater cables as being connected by rigid joints through spherical joints, and the mass of the cable and the external forces are concentrated on the joints. Reference [21] used the absolute node coordinate method to model the tow cable between the semi-submersible and the underwater towed body, considering the added mass and water resistance of the cable, and through the direct sail and U-turn numerical examples verified the rationality of the model.…”