A surrogate-assisted propeller optimisation in propulsive and regenerative operations

Abstract

Engineering has become more and more optimisation and simulation based. This causes an increase in the use of optimisation methods for complex problems. This thesis focuses on the implementation of a surrogate-assisted optimisation method for propeller optimisation, to design a better propeller in a shorter timespan. Therewith reducing both: computational expenses, and fuel consumption when a propeller has been built. This thesis tries to reach two objectives: The first goal is to implement a surrogate-assisted method for propeller optimisation. The other objective is to design a propeller for both propulsive as well as for regenerative operations. To succeed, this thesis has been limited to the testing and coupling of SAMO-COBRA (A Fast Surrogate Assisted Constrained Multi-objective Optimization Algorithm) with PropArt. The testing has been based on benchmark tests where SAMO-COBRA was tested against CMOPSO and NSGA-II (the two algorithms that MARIN currently uses for propeller optimisation). Here, SAMO-COBRA outperformed NSGA-II and CMOPSO on over half of the test cases and is therefore considered for the rest of the research.

The thesis evaluates propellers using a Boundary Element Momentum theory, which is a mathematical method that is the basis of MARINs in-house propeller tool PROCAL. To be able to model propellers that operate at high J values correctly certain improvements regarding the wake expansion and alignment have been made. For the wake expansion three methods are proposed. After comparing the open water diagram of a F4-63-0.6 propeller, that is evaluated by PROCAL, and the one that was made after physical testing. It can be concluded that the disk theory is the most versatile implementation for propellers that operate at a range of advance ratio. Results show that from the two proposed methods, the method where the wake pitch is prescribed by the advance ratio results in the best wake alignment.

The test case optimises 3 configurations, two separate designs, one for propulsion and one for regeneration. A normal CPP that is loaded under a negative angle of attack for regeneration and a propeller that can be fully reversed for regenerative operations (the propeller is loaded from the trailing edge in regeneration). The propeller is optimised to reach a maximum regenerative power whilst minimising the propulsive power. The optimum propeller has to satisfy multiple constraints based on cavitational-, geometrical- and separation of flow constraints. The optimisation using SAMO-COBRA did not yield feasible results for any of the three cases. After removing the cavitational constraints a new analysis of the results has been done, it is found that a propeller that is rotated 180 degrees can provide the highest regenerative power. This thesis proposes multiple hypotheses that cause the lack of feasible results of the test case. The proposed causes are: errors in the PropArt model, problems in the compiled PropArt version or a too low convergence level of COBYLA.