Flow modelling around propeller for a deep drafted vessel in very shallow water

Abstract

Computational fluid dynamics (CFD) codes, are recently used as efficient tools to understand flow characteristics such as wake development around propeller. This thesis presents numerical modelling of flow characteristics in the stern region for a deep drafted LNG carrier with the effect of propeller and rudder in shallow water. The modelling was conducted based on the B5-75 type propeller, with a diameter (D) of 7.7m, which was designed at MARIN in the Netherlands. The ANSYS Fluent version 12 software was used to solve the Reynold Averaged Navier Stokes (RANS) equations, and ICEM CFD as a mesh generator. The propeller was meshed using tetra unstructured mesh in a flow field based on 3D incompressible Navier-stokes solver. Two turbulent models were applied in the ANSYS Fluent; which are the standard k-epsilon (k-e) model for the steady simulation and transient shear stress transport (SST) k-omega (k-?) for the unsteady simulation. For the computational domain, the propeller blades were mounted on two finite long constant radius cylinders. The two types of cylinder domains, were developed; stator domain and rotor domain. For the stator domain, the inlet flow was 2D from blade, the outlet flow at 6D and the outer boundary was 3.6D. The upstream for the rotor domain was maintained at 0.2D but the downstream was extended between 0.4D and 0.7D, and the outer boundary at 1.4D. The turbulent model was simulated in the rotor domain by using the stator-rotor approaches such as the multiple reference frame (MRF) and the sliding mesh (SM) method. Comparisons with the published experiments were presented, and the dependence of the numerical solutions on the computational parameters was studied extensively. The thrust and torque of the propeller were generally predicted with a small error when it was compared with the published experiments. The difference in performance of propeller in the open water test is about 10 percent, likely due to mesh strategy as well as mesh resolution and quality. The performance of the propeller was also studied. It was found that the rudder placed in front of propeller increased the efficiency of the propeller and produced greater thrust increments when the rudder was deflected to -70 and -200. The presence of the rudder which acts by cancelling the trailing vortices from the tip of propeller slipstream leads to increase of thrust and torque of propeller. There was, as expected, a difference in the velocity concentration between propeller only and propeller-hull interaction. The effects of propeller and rudder on the velocity profiles in the region for the LNG carrier in shallow water are clearly identified. Especially in very shallow water, (h/T = 1.1), the extreme velocity profile is concentrated in vicinity of top part of the stern and seabed regions.