Numerical optimization of hull/propeller/rudder configurations

Doktorsavhandling, 2008

The new framework of the global economy has stimulated and expanded the shipbuilding and shipping industry, especially in Asia in the twenty-first century. This induces urgent and high requirements on designing and building both conventional and new types of ships with high performance, such as high speed, manoeuvrability, seaworthiness and so on. Simultaneously, environmental concern and a constant increase in the price of fuels have put more pressure and demands on designers to minimize the energy consumption, maximize the protection of the marine environment and maximize the efficiency and economy of maritime operations. As the “heart” of a ship, the propulsion system has to be improved continuously to satisfy these requirements. Therefore, how to optimize the interaction between a ship’s hull, propeller and appendages, while minimizing the side effects of cavitation on the surface of propeller and rudder, plays an important role in improving ship performance and increasing the economy of ship operations. The work presented in this thesis is a research project at the Rolls-Royce University Technology Center (UTC) at Chalmers University of Technology. The objective of this project is to numerically simulate, analyze and automatically optimize the interaction between a ship hull, a propeller and a rudder. All the numerical calculations are carried out by the software SHIPFLOW, a RANS method-based code. The effect of the propeller is simulated by a lifting line method or a lifting surface method via body force. Two turbulence models, i.e. k-ω SST and EASM, are applied. Both gradient-based and genetic algorithm optimization methods are used in optimizations. In order to systematically investigate hull/propeller/rudder interaction and to verify and validate the numerical method, a series of computations are made for (1) a bare hull, (2) a propeller in open water, (3) a 3-D rudder in a free stream, (4) a hull/propeller combination, (5) a propeller-rudder combination in open water and (6) a hull/propeller/rudder combination. Problems with the lifting surface method are, however, identified when a rudder is added behind the propeller. The problem is resolved by adopting a lifting line method coupled with the RANS method. Among the different cases, three particularly interesting ones are optimized: (1) a single cavitating propeller in a given wake, (2) the hull/propeller configuration, and (3) the hull/propeller/rudder configuration. The project has yielded a validated numerical model for bare hull resistance, propulsive factors, open water propeller characteristics with or without rudder and hull/propeller/rudder configuration flows; a framework for automatic optimization and a design tool for cavitating and non-cavitating propellers. The main conclusion of this thesis is that coupling a RANS solver and a potential flow-based propeller model via body forces can yield a useful and practical tool for designing and optimizing hull/propeller/rudder configurations for both academic and industrial purposes.