Steering Torque Determination of Pulling Open Thrusters

The use of azimuth thrusters as main propulsion device and for manoeuvring of ships is increasing over the past decades. However, the understanding of hydrodynamic loads acting on such devices in oblique inflow conditions is still an interesting area of research as it is generally believed that such loads can be partially responsible for the failure of parts of power transmission system to the thruster. A deeper understanding of such loads can provide critical mechanical design considerations for the parts involved in transmitting power such as bevel gears, bearing and seals as well the parts of thruster housing. Thus, the aim of this research is to provide an insight into the behavior of hydrodynamic loads of a pulling open type steerable thruster particularly the steering moment, including extreme oblique inflow conditions at various ship speeds and power absorption. A numerical investigation of steering thrusters is performed for various conditions. For this a turbulence model is used and for simulating manoeuvring tests transient conditions are used where as open water tests are simulated using quasi steady conditions. These simulations are based on RANS method for solving the flow with standard k-epsilon model with a high y+ wall treatment around an azimuth thruster system. A commercial code of Star CCM+ is used for performing the simulations. Post processing results give an insight into the critical flow regions and contribution of loads and moments of the system. Validation is done with available model scale data, tested at MARIN institute at Netherlands. The results showed an acceptable agreement to do further analysis of the steering moment behaviour at various steering angles and advance ship speeds. Behaviour of various hydrodynamic loads is discussed in the results section. In general, the various hydrodynamic forces were found to be increasing with increase in steering angles and symmetrical for mirrored conditions. As numerical simulations provide the feasibility of studying loads on individual components, the contribution of individual parts of the thruster for different forces and moments was also analyzed. The side force moment created by the propeller side is balanced mainly by anti moments created by the skeg and the remainder is taken up by the steering motor torque to keep the thruster at a particular steering position under such external hydrodynamic loads. Detailed asymmetrical flow fields are discussed which arise due to oblique angle and spinning of fluid due to propeller rotation. Finally, the pulling thruster under study was compared with a ducted pushing thruster for hydrodynamic loads and the results are discussed in detail. The steering moments in both cases rise very steeply with steering angles and are significant at higher steering angles. Also, in case of a pulling thruster there is a fluctuating steering moment present even in straight inflow conditions unlike the pushing unit, which can be of interest when comparing these two type of configurations.

Subject

Hydrodynamics of Steering Thrusters
Turbulence Modeling
Numerical Modeling of Thrusters in Oblique inflow
Transient behavior
Pushing v/s Pulling Thrusters

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master thesis

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