The roll instability observed in buoyantly rising small to medium sized submarines is analyzed in this work using a computational fluid dynamics (CFD) RANS solver coupled to the six degree of freedom (DOF) solid body equations of motion for the submarine. The theoretical framework and the numerical implementation, in particular the fluid-rigid-body interaction methodology, are outlined in detail in conjunction with models for control, propulsion, and ballast blowing. The submarine-specific models are conventional while the RANS flow field predictions are compared with a coefficient-based model and validated against steady state wind tunnel data spanning the onset flow conditions of interest. Detailed analyses of full-scale rising stability scenarios identify the magnitude and onset behavior of contributing moments to the instability. The rolling moment generated by the sail was found to be the primary cause of the instability. Simulation results confirm the finding from a previous study that the emergence roll angle of the submarine is very sensitive to the initial heel (roll) angle. When the initial heel angle is a fraction of a degree, the emergence roll angle is too small to be of concern but an initial heel angle of 2° results in a significant emergence roll angle of 10°. Unsteady viscous effects due to vorticity trailing from the sail were observed but they were found to have an insignificant impact on roll instability for the scenarios investigated.
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