Numerical evaluation of Flettner-rotor setups for cargo ships

The urgent need for alternative ways of vessel propulsion over the past few years created a growing interest in rediscovering wind as a source of propulsive force. While many concepts for harnessing the power of the wind have been developed so far, their actual implementation on real ships still remains at a scarse level since it brings with it many challenges. These challenges include the design of the wind assisted propulsion systems to fit the confines and meet the demands of each individual vessel they operate on. The re-discovery of the Flettner Rotor – an idea first brought up by German engineer Anton Flettner – opened up a new realm of possibilities for using that concept on modern cargo and passenger vessels. This research project aimed to deepen the already existing base of knowledge by extending it and providing a methodology within the context of numerical simulations for developing and assessing rotor sails and their placement on the decks of cargo vessels. This includes a detailed analysis of the influence of some of the main design parameters of these rotors. A dual rotor setup was examined to highlight potential interactions between the individual rotors of a multi rotor setup. This dual rotor setup was compared to a target rotor. The latter was designed based on the findings from an analysis of rotor design parameters. Both the target rotor and the dual rotor setup are subjected to three different wind conditions, varying in their direction. The spin ratio was observed to be the primary operational parameter to influence the generation of lift and drag, with higher spin ratios resulting in higher lift and drag coefficients throughout all examined geometric configurations. The analysis also found that both the aspect ratio and disc diameter ratio have an influence on the lift and drag coefficients, with lift coefficients being proportional to rotor aspect ratio, while the drag coefficient was observed to be inversely proportional to aspect ratio. The disc diameter ratio was found to have a major influence on drag, with higher disc ratios yielding lower drag coefficients, especially at higher spin ratios. The dual rotor setup showed a noticeable interaction between the rotors in a beam wind condition due to a change in the angle of inflow on the downstream rotor. Pressure distributions on both sides of the rotor were also analyzed to further highlight the reasons behind the observed effects for both parts of the analysis, showing dominating effects on the suction side.