Investigation into wakes generated by surface piercing periscopes

For operational purposes, submarines are required to pierce the free surface with masts, such as periscopes and snorkels. These masts generate a large plume structure which can significantly increase the chance of detection. This thesis develops a numerical technique to accurately model the plume structure generated by a mast piercing the free surface, examining four critical aspects: bow wave height, plume height, plume length, and drag coefficient. The Computational Fluid Dynamics (CFD) model was validated through an experimental programme in a towing tank and against published data. The numerical validation showed that CFD-RANS modelling failed to predict the plume height and plume length while the CFD-LES accurately modelled the plume structure under Froude numbers of 3.0, whilst predicting the overall shape with a small under prediction at Froude numbers greater than 3.0. The results show aspects of the flow regime within the plume structure and experimental configurations required to accurately replicate real world scenarios. The most critical flow characteristic found for the plume structure was the combination of two flow regimes within the plume structure. The forward section of the plume is generated by the bow wave, whilst the aft section is caused by the submerged section of the mast. When replicating the mast using a surface piercing cylinder with no end body, it was found that an immersion to depth ratio greater than eight is required to avoid end effects. Additionally, the use of an end body can effectively reduce the impact of the end effect on the plume structure. Utilising the validated numerical model, several mast configurations were tested in order to reduce the plume signature. Cylindrical double mast configurations and streamlined NACA0012 mast configurations were tested showing potential reductions of the overall plume size. This data can be used to improve the submarine mast configuration, thus reducing the risk of detection due to the generation of a large plume structure.