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In most of free vortex wake models (FVWMs), the induced velocity is computed by Biot-Savart law. But the details of velocity calculation are still incomplete in their self-integrated loss of adjacent segment's influence. Curved filament correction has already been studied to recover the FVWM in helicopter problems. In this work, an extended FVWM with the correction is developed intended to improve aerodynamic predictions of wind turbines. Numerical simulations are performed on ring vortices and practical modeling of flow state of both fixed and floating wind turbines. It has been shown that the newly-designed technique may generate higher fidelity.
Among multiple modeling methods in aerodynamics of wind turbines, vortex lattice method (VLM) with straight line segmentation have been commonly used. The trailing filaments generated by the blades are assumed to convect freely with material lines of concentrated vorticity in potential flow. Such force free motion is governed by the vortex transportation equation. The governing equation is a partial differential equation which can be solved by various numerical approximation with high-order accuracy in both time and space domain.
It has been studied that for the straight-line segmentation, the approximation of induced velocity is relatively accurate with respect to corresponding theoretical result with the exclusion of self-induced velocity. It means that the collocation points lie in nowhere in vicinity to the discrete vortex segments (Gupta and Leishman, 2005). When it comes to the case that collocation points are extremely close to the discrete segments, the self-induced velocities tend to be infinite. The solutions for this singularity can be eliminated by “cutoff’ process (Bhagwat and Leishman, 2001) and vortex core models (Leishman,2006). These solutions are initially introduced by core regularization to eliminate singularity of the collocation points or simply fulfill the physical mechanism. However, techniques with these processes are incomplete because they fail to add up the total induced velocity.
Li, Gongrong (State Key Laboratory of Ocean Engineering) | Chen, Zhen (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE)) | Luo, Yu (Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration(CISSE))
Based on thermal elasto-plastic finite element method (FEM), the welding simulation of a large-scale ship bottom grillage was carried out in this paper. The structure comprises five transverse floors and three longitudinal girders. In order to enhance modeling and calculation efficiency, shell elements with section integration are adopted to model the entire structure and solid elements to model the local details of weld line region. Linear constraint equations are established between degrees of freedom of shell and solid elements. The techniques of segmented moving heat source model and static substructure are adopted in order to reduce the computation time of models. Some key welding parameters such as heat input, welding speed and welding sequence are considered in the analysis. The welding deformations under two different constraints, including global bending, transverse angular distortion and torsion distortion, are analyzed and compared in this paper.
Welding is a principal jointing method in the construction of ship and offshore structures. It offers several advantages over mechanical jointing methods such as structure integrity, flexibility of design, weight reduction and cost saving etc. (Deng et al., 2007). However, it is inevitable that distortions and stresses are induced during welding due to the non-uniform expansion and shrinkage of material near weld lines (Jang et al., 2002). Welding deformation has negative effects on the accuracy of assembly, external appearance, and strengths of welded structures. In many cases, additional costs and schedule delays are incurred from straightening welding deformation. On the other hand, the design of engineering components and structures relies on the achievement of small tolerance (Deng et al., 2008a). Therefore, understanding and controlling the formation of welding induced deformation are of importance at the design stage of ship and offshore structures.
During the past several decades, thermal elasto-plastic finite element method (FEM) has been proven to be an effective tool for the prediction of welding residual stresses and distortions. Deng & Murakawa (2008a) used the thermo-elastic–plastic FEM to predict welding distortion and residual stress in a thin plate butt-welded joint and the accuracy were verified by experimental results. J. Sun et al. (2014) adopted a developed elasto-plastic FEM to simulate the welding temperature field, residual stress distributions and deformations induced by LBW and CO2 gas arc welding in low carbon steel thin-plate joint. The numerical results agreed well with the experimental results. G. Fu et al. (2014) investigated the welding residual stress and distortion in T-joint welds under various mechanical boundary conditions. FEM analysis and experimental results showed that transverse residual stress, out-of-plane displacement, angular distortion and transverse shrinkage depended on the mechanical boundary conditions significantly. A.A. Bhatti et al. (2015) studied the influence of material properties on welding residual stresses and angular distortion in T-fillet joints based on FEM. The numerical predictions of angular distortion and transverse residual stresses were validated with experimental measurements. However, thermal elasto-plastic FEM is time-intensive and is limited to simulation of small-scale structures such as fillet and butt joints, as mentioned above.
Ma, Chao (State Key Laboratory of Ocean Engineering) | Zhang, Chenliang (School of Naval Architecture) | Huang, Fuxin (Ocean & Civil Engineering Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration Shanghai Jiao Tong University) | Yang, Chi (State Key Laboratory of Ocean Engineering) | Jiang, Yin (School of Naval Architecture) | Chen, Xi (Ocean & Civil Engineering Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration Shanghai Jiao Tong University) | Noblesse, Francis (George Mason University)
Two practical approaches — based on an analysis of experimental data given in the literature or simple flow computations — for estimating the sinkage and trim of a ship that advances at constant speed in calm water are considered. The ‘experimental approach’, based on measurements for 22 ship models, requires no flow computations and yields particularly simple relations. The ‘numerical approach’ involves flow computations based on the Neumann-Michell theory (a practical linear potential-flow method) for the ship hull in equilibrium position at rest; i.e., sinkage and trim effects on the position of the ship hull are ignored in these flow computations. Both approaches are found to yield reasonable predictions of sinkage and trim for a wide range of ships at Froude numbers F ≤ 0.45.
The pressure distribution around a ship hull, with mean wetted surface ΣH, that advances at a constant speed V in calm water evidently differs from the hydrostatic pressure distribution around the wetted hull surface ΣH0 of the ship at rest, i.e. at zero speed V = 0. As a result, the ship experiences a hydrodynamic lift and pitch moment, and a related vertical displacement and rotation that are commonly called sinkage and trim, as well known.
The viscous and wave drags determined from flow computations around the actual mean wetted ship hull surface surface ΣH or the related hull surface surface ΣH0 can differ significantly. Practical methods, notably methods that do not require iterative flow computations for several hull positions, to predict the sinkage and trim experienced by a ship hull are then of practical importance.
In this paper, a new analysis method for optimization of ship’s comprehensive hydrodynamic performance in complicated operating conditions is proposed. The function of ship’s comprehensive hydrodynamic performance is expressed as a weighted sum of the normalized functions of resistance and propulsion, maneuverability and seakeeping performances. The best comprehensive performance is obtained while the objective function gets maximum with all the constraints satisfied. By adjusting the weight value, any combinations of the functions for resistance and propulsion, maneuverability and seakeeping performances can be achieved. By applying a single-factor sensitivity analysis, the optimized results can be used to find the sensitive variables to help determining the ship geometry and kinematics parameters. This method can help to evaluate the design scheme and improve the ship design efficiency.