Numerical Analysis of Bow Tunnel Thruster Performance

A previous work at Royal IHC, following the observation from sea trials of trailing suction hopper dredgers (TSHD) that the turning performance of bow tunnel thrusters significantly reduced at slow forward speeds, studied the flow behaviour and performance of bow tunnel thrusters at slow forward speeds using Computational Fluid Dynamics (CFD). Simulations were performed, on model scale, first with a simplified wedge model of a containership and then with a simplified wedge model of a TSHD with the thruster modelled by an actuator disk. The numerical results deviated significantly from experimental results from literature. The present work aims to investigate this deviation. Simulations are also performed with a full thruster unit to compare with the actuator disk approach. Also, possible improvement in thruster effectivity is explored by altering the tunnel geometry of the TSHD wedge. First, a grid convergence study is performed on the containership wedge. Next, simulations are carried out for a range of forward speeds - for selected thrusts for the actuator disk approach, and corresponding rpm’s for those thrusts for a full thruster unit. In case of the actuator disk approach, the numerical results show a steady increase of transverse force (Fy) with increasing forward speed, contrary to observations in sea trials. The circulation around the wedge, resulting from interaction of the jet from the tunnel and the cross-flow due to the forward speed, results in the wedge to behave as a slender body in a flow. This results in a transverse force, analogous to Kutta-Joukowski lift, that results in increased Fy. A full thruster unit in place of the actuator disk result in a different flow behaviour and variation of Fy with forward speed. In this case, an initial decline in Fy is observed before it increases monotonically. Comparison with experimental result of the containership wedge and numerical results by both actuator disk and thruster unit show considerable deviation. The transverse force due to the circulation around the hull presents a plausible explanation. The conclusions from the grid convergence study for the containership model are utilized in meshing the TSHD wedge. Simulations performed for a range of forward speeds, first with actuator disk approximation, indicated lowered thruster performance as was observed in sea trials. The pressure contours over the hull imply that the resultant of the forces around the inlet – due to low pressure caused by high velocities of the flow into the tunnel – and those around the outlet – due to low pressure caused by high velocities of outflow – is such that there is a net thrust deduction. Simulations with thruster unit also show lowered thruster effectivity. The tunnel of the TSHD wedge is then given a forward bend of 45o at the tunnel inlet and outlet in order to explore the possibility of improving thruster performance. It is expected that the changed geometry will result in better inflow at tunnel inlet, and the action of the cross-flow on the jet will result in a ‘straightened’ jet with lesser interaction with the hull compared to a jet from a straight tunnel. The CFD results, with actuator disk approximation, indicate that thruster effectivity improves in case of higher forward speeds and reduces for lower speeds. It is advisable to perform the study with smaller tunnel bending angles such that the jet-flow interaction is favourable for the entire range of speeds.Mechanical, Maritime and Materials EngineeringMaritime and Transport Technolog