Non-viscous calculation of propeller forces under consideration of free surface effects

Abstract This thesis addresses the issue of determining dynamic loads acting on propellers based on potential theory simulations. In particular, the focus is set on loads due to free water surface influences resulting from a shallow propeller immersion and due to ventilation when the propeller is only partly wetted. This situation occurs e. g. when vessels are supplying offshore platforms, which are usually relatively small and therefore ship motions can be large. Moreover, these vessels often operate in harsh weather conditions and also may operate in dynamic positioning mode. The resulting loads on a ventilating propeller can lead to mechanical failures of the propulsion system because the thrust and torque evolution shows large amplitudes. The development of numerical methods follows a two-way approach. The first approach begins with purely potential simulations starting with a BEM for submerged bodies, in which the boundary conditions at the free water surface are introduced with different numerical implementations for the steady and the unsteady case. In the steady case, the kinematic and dynamic boundary conditions are combined and solved at the same time, whereas they are solved separately in the unsteady case by applying a numerical time-stepping scheme. The second approach is a hybrid method which is based on the coupling of a BEM and a RANSE solver. In this approach, the propeller forces are computed by the BEM under consideration of the viscous velocity distribution present in the RANSE domain. The propeller forces are computed by the BEM and act as a field of body forces distributed on the grid cells in the RANSE computation domain. The simulation method is capable of representing the three-dimensional propeller geometry and the actual blade positions. The thesis gives insight into the flow phenomena occurring at a propeller in operating conditions and provides a literature survey on numerical methods that have been developed and experimental campaigns that have been conducted so far. This is followed by a discussion on the numerical methods and free water surface models applied. A comprehensive validation of the baseline methods with a concentration on propellers, unsteady effects and free water surface influences is given. The last section offers two application examples for propellers operating shallowly immersed in calm water and in waves. Comparisons are made towards experimental data obtained by MARINTEK.