Evaluation of the Unsteady Propeller Performance behind Ships in Waves

Abstract Existing computational methods for the design of ship propellers and for the determination of ship-propeller interaction effects only consider the situation in calm water. The propeller is considered to be deeply submerged in all situations and unsteady influences on the operating conditions of the propeller are usually not considered. Oppose to this, a ship operating in heavy seas generates highly unsteady working conditions for the propeller. For typical merchant ships, providing large draughts, the common procedure is sufficient, as the propeller is always deeply submerged. But for smaller vessels, especially for those operating in dynamic-positioning mode (i.e. offshore-supply vessels), the influence of the ship motions and the waves on the propeller should be considered. Due to the large propeller loading in such situations and the small draught of these vessels, the propeller might start to ventilate, which causes large load fluctuations. In the present work a computational procedure is derived, which allows to compute the unsteady working conditions of a propeller in a ship wake under consideration of the wave influence. The method is based on a close and unsteady coupling of a viscous-flow method and a method based on potential-flow theory. Therein, the RANS method is used to compute the flow around the ship hull and therewith the inflow conditions for the propeller, while the inviscid-flow method is used to determine the propeller forces. These forces are afterwards used to model the propeller inside the RANS method by transferring them into a set of equivalent body forces. Henceforth, the influence of the propeller on the ow is considered, but the propeller does not need to be modelled geometrically, which reduces the computational effort significantly. For the computation of the propeller-ship interaction during operation in waves, this procedure is supplemented by a second coupling approach for an efficient wave description. Within this procedure, the RANS method is coupled to an inviscid determined wave description in the far field, which allows the use of very compact domains for the simulation of ship motions in waves and reduces the necessary amount of cells and therefore computational time.