Estimation of Free Surface/Solid Walls effects on Loads

This technical report concerns with a preliminary analysis on the confined water effects on a submarine advancing in steady zero a non-zero drift motion by accurate numerical simulations. In this analysis numerical computations have been carried out by means of Xnavis solver, which is a general purpose Unsteady RANS solver developed at CNRINSEAN CFD code; in order to keep more insight into the effects of a boundary effects on the flow field, and consequently on the global loads (forces and moments), simulations are performed for a submarine advancing in the straight ahead and with a pure drift motion, both close to a free surface and a solid wall. In particular the free surface has been modelled as a moveable (considering gravitational effects), whereas a solid wall is a rigid, non-deformable surface. The main purpose of this analysis is to verify the suitability of an alternative connection of the submarine to the measuring device during the experiments in order to reduce as much as possible interference effects, which can alter the out of plane loads which constitutes one of typical aspect affecting the submarine manoeuvring qualities. For the sake of completeness, the theoretical model adopted at CNR-INSEAN is described and related assumptions and limits are discussed; the solution algorithm is outlined and the main features of the developed solver are described. The latter involves grid generation, influence of numerical parameters, convergence properties and uncertainty. Un-propelled simulations have been performed and used to help the physical investigations. Numerical results will be presented and analyzed in order to quantify the effect of considering a rigid (viscous) free surface with respect a deformable (non-viscous) one. The features of the flow around the submarine are described in terms of velocity and pressure fields, and the influence of body geometry and configuration is investigated with and without solid/deformable boundaries. The numerical uncertainty and the order of convergence are also reported. Differences among the two free surface models are discussed and the global loads results are investigated in terms of body configuration and presence of the boundaries. Globally the simulations carried out permit to emphasize differences resulting from considering the free surface deformable or rigid; in particular the most noticeable differences regard the out of plane loads (namely pitch and vertical force due to drift).