Ship specific optimization of pre-duct energy saving devices using reduced order models

Abstract

Hydrodynamic energy saving devices (ESD) have been widely explored as an effective alternative to improve energy efficiency of vessels by reducing losses across propellers, especially in the presence of strong interactions between hull and propellers. In this work, we propose a computational optimization framework for the design of pre-duct ESD for a specific hull form. For optimization of duct geometries, the framework is based on computational fluid dynamic (CFD) approach for prediction of hull resistance as well as propeller performance under changes in the design of pre-swirl duct geometrical parameters. We have implemented two improvements in the numerical procedure to speed up simulations; enabling the affordable computational design of the duct. First, an overset grid approach is used to realize a change in duct geometries on a background solution of flows over the hull and propeller only. This accelerates the convergence of RANS simulations for a specific design. For every single optimization loop, the propulsive energy efficiency was calculated using proper-orthogonal decomposition (POD) based reduced-order model for the hull, propeller, and duct. Using the POD interpolation technique it is possible to predict the resistance of hull and duct as well as propeller thrust at a particular design parameter using offline flow solutions pre-computed with a sample of design parameters. The accelerated CFD approach has been applied for design of pre-swirl duct on the Japan Bulk Carrier hull using the surrogate-based optimization approach. The performance of the propeller at the optimal design of the duct is simulated with CFD and compared with an earlier proposed duct design. The optimization framework demonstrates the potential of the approach for future applications for the design of ESDs for specific hulls, especially in retrofitting ships.