CFD analysis of broaching for a model surface combatant with explicit simulation of moving rudders and rotating propellers

The mechanisms of broaching in following regular waves are studied by analysis of free model computations. Simulations of the fully appended ONR Tumblehome model DTMB 5613 are performed with the ship hydrodynamics code CFDShip-Iowa v4.5 and validated against experiments of an auto-piloted, self propelled model ship. Appendages include bilge keels, skeg, shafts and struts, moving rudders to control heading, and rotating propellers for self-propulsion. Proportional and proportional–integral autopilots are used. Once validated against experimental data for the proportional controller, the flow field and forces and moments on the hull and individual appendages are analyzed in detail to identify the mechanisms leading to the broaching event. It was found that several reasons contribute to the inability of the ship to overcome the hydrostatic yaw moment caused by the approaching wave. However, it is shown that the use of a slightly better autopilot prevents broaching under identical operating conditions, presenting an opportunity to extend the safe operating envelope of the ship.

► Simulations of broaching of a ship are performed and validated against experiments.
► Keels, skeg, shafts, struts, moving rudders and rotating propellers are included.
► Dynamic overset grids handle ship motions and rotation of rudders and propellers.
► The full broaching process is simulated with no input parameters or correlations.
► For one of the cases, an improved autopilot is proposed that prevents broaching.