A ship advancing in a stratified fluid: the dead water effect revisited

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.