Numerical investigation of the effects of free-surface flow past submerged bluff and streamlined bodies

Abstract

The last decade has been marked by a continuous growth in computational power, which has allowed for elaborate modelling techniques like LES and DES applied at engineering scales which generally imply a Reynolds number in excess of 1×106. One field that has seen a rapid growth in use of numerical methods in design and performance analysis is the naval sector, especially in the design of submarines where the free-surface boundary plays an important role. This thesis is devoted to the study of free-surface flow past submerged bodies, with the objective of numerically studying free-surface flow past a submarine fairwater at periscope depth near actual operating conditions. This work is motivated by DSTL, who have reported that near periscope depth submarines exhibit an increased drag (private communication). In this work both LES and DES modelling approaches are also utilized, while the submerged body is accounted through the use of an immersed boundary method and the free-surface is through the use of a part moving mesh. The thesis is split in two main parts. The first part of the thesis focuses on low and moderate Reynolds number flow about a submerged cube (bluff body) for various submergence depths. Two configurations are examined one being that of a single cube in a uniform flow, while the other is that of a matrix of equally spaced cubes. Results show that for both cases a reduction in submergence depth causes the forces, the fluctuation in the forces and shedding frequencies to alter, while the level of interaction between vortices and free-surface to increase. The second part of this thesis focuses on the main study of free-surface flow past a submerged fairwater at a high and near operating Reynolds number. It is found that for both Reynolds numbers the flow behaviour shows little change, while the effects of reducing submergence depth results in the forces, the force variation and shedding frequency to increase. Surface waves are found to disperse by an angle of up to 40° for all submergence depth and are of the Kelvin wave kind. No direct interaction between vortical structures shed from the fairwater and free-surface are found, whilst the increases in forces acting on the fairwater are directly attributed due to wave motion.