In the year 1998 the SVA Potsdam managed for the first time world-wide to simulate the flow around a ship with rotating propeller numerically. With this milestone new approaches for the investigation of ship and propeller as an integrated system were at hand.
Until that time the improvements were made for ship and propeller separately, but an optimisation of isolated components rarely leads to an optimal overall system. Due to the unsteady nature of the interaction between ship and propeller, the simultaneous consideration of both is a major precondition to capture the physics and impose improvements to the entire system. Again and again investigations proof that a ship with minimal resistance not necessarily requires minimal power.
On basis of this success nowadays the numerical simulations are conducted with consideration of e.g. an inclined rudder or other appendages such as fins, ducts or shaft brackets.
The following animation shows the comparison of CFD calculations with the corresponding LDV measurements conducted in the towing tank of the SVA Potsdam of the flow behind a ship with rotating propeller and demonstrates the accuracy of state of the art numerical simulations.
The following pictures show that the propeller can be simulated, in dependency of the required task, as body-force model (left) or as rotating propeller (right). With the first method the simulation of the suction effects of the propeller is possible, while the second method enables the analyses of the unsteady interaction between ship and propeller with a higher accuracy.
Costa bulb
By means of the rudder positioned in the propeller slipstream the energy of the propeller induced vortices can be partially converted into thrust. The part of energy that can be recovered has to be maximised, which can be accomplished with the application of e. g. Costa bulbs. In systematic numerical investigations the propulsion coefficients of the ship and the rudder forces have been determined, in dependency of the geometry and the arrangement of the rudder with Costa bulb.
The figures show two variants, for which the diameter of the Costa bulb relative to the diameter of the hub and the distance between bulb and hub were altered. The run of the streamlines reveals a significant difference in the flow around the Costa bulb. With increasing distances between bulb and hub the hub vortex is not suppressed.
Influence of appendages on the propulsion characteristics
With the aid of additional appendages such as vortex generators the wake field and thereby the propulsion characteristics of the ship can be favourably manipulated.
The picture shows the comparison of the aft ship flow without (left) and with (right) a vortex generator fin. The vortex generator reduces the velocity gradients in the flow field and thus acts beneficially on the propeller.
