Investigation of Thrust Deduction on Azimuth Thrusters in Bollard Pull Condition using CFD

Examensarbete för masterexamen
Lemonakis, Foivos
This report is result of a master’s thesis project performed together with Caterpillar Propulsion AB, which is a revisit of an older master’s thesis with title "Hydrodynamics of Conventional Propeller and Azimuth Thruster in Behind Condition" (Matin, 2011). In the previous master’s thesis, the thrust deduction factors of open shaft propellers and ducted azimuth thrusters, installed on an offshore vessel, are studied. However, during the recent years the ducted azimuth thrusters have become more and more common and especially for vessels such as tug boats, drilling vessels etc. For such vessels, the bollard pull condition is really important, which indicates high power operation with high loading (Funeno, 2009). Thus, the purpose of the current master’s thesis is the investigation of thrust deduction on ducted azimuth thrusters in bollard pull condition using Computational Fluid Dynamics (CFD). In order achieve this type of study, a harbour tug boat with twin ducted azimuth thrusters and a combination of steady state Multiple Reference Frame (MRF) and transient Sliding Mesh Interface (SMI) modelling methodologies have been used. MRF has been proven to be inaccurate in behind condition and at low or high advance coefficients, as it usually results in relatively strange flow fields in the vicinity of the propeller while SMI has shown higher accuracy. However, SMI is a much more computationally demanding methodology (Gullberg & Sengupta, 2011). Thus, SMI has been used for critical regions and MRF for regions of lower interest. Outcome of this study is that during the bollard pull condition the contribution of the nozzle is higher than the propeller in terms of thrust and the contribution of the gear case housing is higher than the hull in terms of resistance. The highest thrust deduction appears on the gear case housing with 8.92% while the highest merit deduction, which shows a lower performance, appears when placing the whole propulsion unit behind the hull with 9.09%. Furthermore, the measured bollard pull force of the open water case is about 8.23% higher than the one of the Wageningen CD Series. However, this high difference is not realistic and the reason has to be identified. Finally, by comparing the MRF and SMI methodologies, it is concluded that the MRF approach gives appropriate results for initialization and quite reasonable results for medium advance coefficients at low computational cost while the SMI approach is needed for the critical regions of low and high advance coefficients.
Propulsion, Hydrodynamics, Thruster, Thrust deduction, Bollard pull, CFD, Multiple Reference Frame (MRF), Sliding Mesh Interface (SMI)
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