CFD based optimization of the wave-making characteristics of ship hulls

Ship optimization based on computer simulations has become a decisive factor in the development of new, economically efficient and environmentally friendly ship hull forms. An important task at an early design stage is the optimization of the wave-making characteristics of the ship hull since for fast ships a considerable resistance component stems from the steady ship wave system. Moreover, the ship waves cause adverse effects in the far field as the wash hits the shore line or other vessels. A novel hull form optimization approach has been developed and implemented. It builds on a CFD (Computational Fluid Dynamics) based evaluation of the nonlinear ship wave pattern, on realization of the cause and effect relation of hull variations and their impact on wave formation, which is accessed by a perturbation approach, and on wave cut analysis (WCA). WCA yields an excellent assessment of the wave-making characteristics of a hull form in terms of its free wave spectrum and the wave associated pattern resistance. These features are highly integrated and controlled by a fully automated optimization scheme. The scheme is specific in the way it tackles the optimization problem: • The hull adaptation is driven directly by hydrodynamics, avoiding prerequisites to the shape representation and to the hull modification method. • Wave cut analysis yields the objective function of optimization in terms of the free wave spectrum and the wave pattern resistance. This considerably improves the system identification allowing a focused optimization. • The hull optimization is carried out directly for the effective wetted hull portion of the advancing ship including the effects of dynamic trim and sinkage and wave formation along the hull. • The optimization process is established in terms of an iterative marching scheme of successive sub-optimization loops. In each loop a region of the solution space is mapped to a simplified convex quadratic image which merely possesses a single minimum determined by the active constraints. This enables a straightforward solution procedure and a simultaneous treatment of a large number of locally acting optimization variables, introducing much freedom to the hull variation. All aspects of the proposed optimization approach are presented. Applications of the optimization scheme to practical ship hull forms show the following: • The wave-making characteristics can be considerably improved with a tangible reduction of the wave (pattern) resistance. • Hull variations are driven by the optimization to the expected optimum hull shapes with the hull geometry fully self-adjusting according to the optimization requirements. • Optimal interferences of local wave trains are enforced both in amplitude and phase which results in a beneficial cancelation of the wave trains. • The specific fingerprints of locally confined hull variations (e.g., at the bulbous bow) can be effectively traced in the wave spectrum and in the wave pattern resistance to allow a focused minimization of particular adverse wave components. • The system identification can be significantly improved by utilizing additional information in terms of the wave spectrum and the distribution of the wave pattern resistance over the waves range. 4 J. Heimann · CFD Based Optimization of the Wave-Making Characteristics of Ship Hulls