University Of Tasmania
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Effect of outrigger position on motions and added resistance of a trimaran in various sea conditions

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posted on 2023-05-28, 12:13 authored by Nowruzi, L
Trimaran hull forms provide the ship with larger deck areas and allow for arranging more weights on the upper deck. Also, high slenderness ratio of the centre hull makes it more efficient in reducing the required power. Based on the excessive dependence of trimaran vessels on the position of outriggers, and in order to improve the passenger comfort, it is imperative that the outriggers be carefully spaced near the main hull in order to increase dynamic stability and at the same time, not to increase added resistance of the vessel. Operational circumstances such as wave conditions and operating speed have great impacts on the seakeeping of trimaran vessels. Therefore, many researchers have investigated the resistance of trimarans and some have focused on developing numerical methods for assessing their dynamic performance. Linear Strip theory-based methods have not proven reliable among shipbuilders due to lack of accessibility and comprehensiveness. For instance, linear strip theory-based methods are not reliable when it comes to solving problems considering free surface viscous flows, breaking waves and high speeds. Disadvantages of the current predictive methods as well as limited available experimental data on trimaran motions were the driving forces behind carrying out a computational and experimental study of trimaran behaviour in different sea conditions. As a result, a Finite Volume based CFD software, STAR CCM+, is used to solve Reynolds Averaged Navier Stokes (RANS) equations for trimaran motions in various operational speeds and wave conditions. Variations of gridding systems and time steps are investigated, and reliability analysis was performed in solving the RANS equations to improve the accuracy of the results. Moreover, different turbulence models were investigated, and the SST Menter K ˜ìv¢ turbulence model proved a more accurate model than Realizable K-˜í¬µ model. The outcomes were validated against experimental results, which were obtained from a 1.6 m trimaran model tested in various conditions. The tests were carried out in Australian Maritime College Model Test Basin. The impact of transverse, longitudinal and vertical position of outriggers as well as wave interference between the three bodies on the motion response amplitude of the trimaran was investigated. Comparing the results against secondary data, the CFD model was primarily proved as an effective and reliable method to predict the motions of a trimaran in regular head waves. Then, it was further improved to create the capability of analysing the motions and added resistance of a trimaran in oblique waves. The results suggest that unlike strip theory, the effect of breaking waves, hull shape above waterline and green seas are among those considered in CFD application. Wave deformation as a result of wave-current-wind interaction in CFD was identified as the main source of discrepancy. Considering the complex wave system between the main hull and outriggers, it was found that the position of outriggers and the hull shape above waterline have significant impact on dynamic performance of the model. With an increase in Stagger ratio (ST, longitudinal spacing of main and demi hulls transoms/overall length (x/L)), Separation ratio (CL, separation of outrigger centre lines/overall length) and Buoyancy fraction of outriggers (B, % with respect to centre hull), the roll motion is reduced, heave motion and added resistance are increased, and pitch motions are slightly influenced. The transverse position of outriggers caused 37% variation in roll peak amplitude, and it was found that the longitudinal position of outriggers can reduce the roll motions peak amplitude by 80%. The motion behaviour of trimaran hull appeared unique in terms of RAO, where two peaks were identified in roll and heave motions. This is mainly due to trimaran geometry and the effect of reflected waves between the hulls. It was noticed that heave motions and added resistance vary considerably with different outriggers arrangements. The added resistance peak is maximized in configurations leading to minimum roll motion peak amplitudes. Overall, CFD overestimates the roll responses and underestimates the pitch responses and resistance forces. CFD methods appear to deliver reliable results in predicting both motions and added resistance of trimarans whilst showing similar trends recorded by experimental test in MTB. The maximum discrepancy is 10%-14% in frequency range, where maximum responses occur. The findings further highlight the importance of investigating the impact of operational conditions and outriggers' configurations on the trimaran's dynamic performance as well as the effect of wave interference when undergoing different oblique wave encounter scenarios. The proposed CFD model can be utilised to better understand the wave interference between three bodies of the trimaran. The results of this thesis form the basis for further analysis to investigate other parameters such as extended wave conditions and speeds, size of the outriggers and the weight distribution. This could help establish a more systematic basis for effective design and operation of trimaran vessels in the future.


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