On The Importance Of The Objective Function And Scale Effects In Hull Design Optimization

M. Orych, L. Broberg

FLOWTECH

Abstract

The demands on ship efficiency continue to increase due to the EEDI regulations and also ship operator expectations. This calls for more advanced design procedures. The evaluation of ship performance is still based mainly on the towing tank measurements complemented with extrapolation methods and correlation factors. Based on the knowledge in hydrodynamics, experience from towing tank tests and some intuition the designers can improve the hull lines. This is a slow and costly process that requires validation data from experiments and sea trials. The introduction of Computational Fluid Dynamic methods makes it possible to evaluate more hull variants between tests and new generations of ships could be developed faster and with larger improvements. However, most of the CFD evaluations are performed in model scale exactly like the towing tank tests. In this paper we show how a change to this approach may reveal more gains and result in more optimal design solutions. An optimization is set up for the Japan Bulk Carrier based on a partially parametric modelling technique. The computations are performed with three different approaches. The first one is using the total resistance as an objective function that is being minimized. The second optimization approach is looking at the delivered power. Both of these are performed at model scale. The third optimization is using delivered power as an objective function and executed at full scale. The results of all the three rounds of optimizations illustrate the importance of finding the balance between the hull resistance and the propulsive efficiency as well as the scale effects. Using the delivered power as an objective function takes the hull-propeller interactions into account implicitly and in effect results in a superior hull shape as compared to the one optimized solely for minimum resistance. The procedure at full scale gives an insight into additional gains through simulating flow conditions closer to a real ship and is also of significant importance for design of energy saving devices. In addition the effect of hull surface roughness on the wake is illustrated.