Use of CFD techniques to improve the energetic efficiency of a hull form with particular reference to bulbous bows

Abstract

After significant effort in the marine research, hydrodynamics have begun to venture into computational prediction of hydrodynamic behavior of surface ships. The flow pattern around ship hull plays an important role in its resistance. In this work CFD has been used for calculating ship resistance, and investigating its variation when changing the ship hull form due to varying the geometrical parameters of the bulbous bow, which represents a very important task in the optimization of the efficiency of the ship hull form. Commercial software is used for grid generation and flow solution. In the present work the STARCCM+ software is used to simulate ship resistance.

Introduction:

In recent years, the design of a large motor yacht is focused on higher speeds, as it’s known that in generals the yachts are working at the cruising speed. This indicates the need for focusing hull design to have less resistance rather than good hull efficiency at cruising speed. Using an optimization methods to optimize the area of immersed transom, bulbous bow and hull form…etc., it’s shown that the bulbous bow has a big effect on the wave making resistance rather than on the total ship resistance. This drives to study the ship resistance and its components with particular reference to the influence of the bulbous parameters on the wave making resistance.

In order to define the influence of the bulbous parameters on the ship resistance, the Kracht theory will be use. In his method Kracht defined six parameters for the bulb, three linear and three nonlinear, each parameter having a different effect on the wave making resistance either on interference or breaking wave. This knowledge allows us to use the systematic method to optimize the bulbous bow, this method based on the modification of each parameter of the bulb separately, and note the influence of each one on the ship resistance. Unfortunately it’s not the case for the bulb where it cannot modify each parameter independently.

The yacht model has been provided by AZIMUT-BENETTI shipyard, where this model has already tested in Towing tank, with initial bulb. The methodology of this project is to simulate the ship resistance using Computational Fluid Dynamics by using STARCCM+ Software which is able to solve this kind of problems (Free surface). The initial hull will simulate in STARCCM+ and the result obtained will be compared with the one obtained from experimental test, in order to check the validity of the mesh generated which is consider the main important task in ship simulation. The valid error required by the company AZIMUTBENETTI is about of 5% of ship resistance.

Once the ship resistance simulates and the results correspond to the requirement of the company, the next task will be the optimization of the bulbous bow shape. In order to optimize the bulb the KRACHT method (theory) is used, following to this theory the bulb shape will be modify according to the criteria of the kracht experience. All alternatives shapes of the bulb will be tested and the optimum bulb will be compared with the initial bulb shape in order to appreciate the influence of the bulb on the wave making resistance further than total ship resistance.