A computational fluid dynamics (CFD) methodology is presented to predict density stratified flows in the near-field of ships and submarines. The density is solved using a higher-order transport equation coupled with mass and momentum conservation. Turbulence is implemented with a k-ε/k-ω based Delayed Detached Eddy Simulation (DDES) approach, enabling explicit solution of larger energy-containing vortices in the wake. Validation tests are performed for a two-dimensional square cavity and the three-dimensional stratified flow past a sphere, showing good agreement with available data. The near-field flow of the self-propelled Research Vessel Athena advancing in a stably stratified fluid is studied, as well as the operation in stratified flow of the notional submarine Joubert BB2 also in self-propelled condition. The resulting density, velocity, pressure and turbulent quantities at the exit plane of the near-field computation contain a description of the relevant scales of the flow and can be used to compute the far-field stratified flow, including internal waves. The generation of internal waves is shown in the case of the submarine for two different conditions, one with the pycnocline located at the propeller centerline, and the second with the pycnocline located slightly below the submarine, concluding that distance to the pycnocline strongly affects the internal wave generation due to the presence of the vessel. It is also shown that, as in the case of surface waves, the generation of internal waves requires energy that results in an increase in resistance. For the case of the surface ship the near field wakes are mostly affected by the separation at the wet transom and propeller mixing. However, in the case of the underwater vessel, the disturbance of the background density profile by the presence of the submarine affects the near-field wakes. Finally, the dead-water phenomenon, which occurs at very low Froude numbers, is studied for R/V Athena. Though the dead water problem has been studied in the literature using potential flow methods, this thesis presents the first attempt at using computational fluid dynamics (CFD) to analyze the flow. Results show that, while CFD can reproduce trends observed in potential flow studies, viscous effects are significant in the wake and the friction coefficient.

The effects of winds and/or waves on ship motions, forces, moments, maneuverability and controllability are investigated with URANS computations. The air/water flow computations employ a semi-coupled approach in which water is not affected by air, but air is computed assuming the free surface as a moving immersed boundary. The exact potential solution of waves/wind problem is modified introducing a logarithmic blending in air, and imposed as boundary and initial conditions. The turbulent air flows over 2D water waves are studied to investigate the effects of waves on incoming wind flow. Ship airwake computations are performed with different wind speeds and directions for static drift and dynamic PMM in calm water, pitch and heave in regular waves, and 6DOF motions in irregular waves simulating hurricane CAMILLE. Ship airwake analyses show that the vortical structures evolve due to ship motions and affect the ship dynamics significantly. Strong hurricane head and following winds affect up to 28% the resistance and 7% the motions. Beam winds have most significant effects causing considerable roll motion and drift forces, affecting the controllability of the ship. A harmonic wave group single run seakeeping procedure is developed, validated and compared with regular wave and transient wave group procedures. The regular wave procedure requires multiple runs, whereas single run procedures obtain the RAOs for a range of frequencies at a fixed speed, assuming linear ship response. The transient wave group procedure provides continuous RAOs, while the harmonic wave group procedure obtains discrete transfer functions, but without focusing. Verification and validation studies are performed for transient wave group procedure. Validation is achieved at the average interval of 9.54 (%D). Comparisons of the procedures show that harmonic wave group is the most efficient, saving 75.8% on the computational cost compared to regular wave procedure. Error values from all procedures are similar at 4 (%D). Harmonic wave group procedure is validated for a wide range of Froude numbers, with satisfactory results. Deterministic wave groups are used for three sisters rogue waves modeling. A 6DOF ship simulation is demonstrated which shows total loss of controllability with extreme ship motions, accelerations and structural loads.