Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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Prediction of Slam Loads on Wedge Section Using Computational Fluid Dynamics (CFD) Techniques
Prediction of Slam Loads on Wedge Section Using Computational Fluid Dynamics (CFD) Techniques
Pages 708-720
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From page 708... ...
The main conclusions drawn are that close correlations with the existing experimental data are obtained, and the effects of impact velocity variations, domain size and three dimensionality are significant on the numerical results for slam loads. ~ Introduction Ships operating in open seas undergo severe motions with increasing speeds and experience slam loads on the hull due to continuous buffeting from the waves, leading to loss of ship control and speed, discomfort to crew and passengers, increased wet decks and structural damage.
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From page 709... ...
The present study considers numerical prediction of slam loads consisting of total impact force and pressures during water entry on a V-shape similar to the case of a wedge dropped onto calm water surface. The present numerical prediction method using CFD techniques accounts for the key factors affecting the accuracy of these slam loads using CFD techniques to achieve better correlations with experiments.
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From page 710... ...
With the further assumption that viscous effects on the free surface boundary are negligible, in which case normal stresses are neglected, the pressures on either side of the free surface are equal, which are simply atmospheric. Walls (Sw~The vertical boundary at far end of the computational domain is considered to be wall where no-slip boundary condition similar to that of body boundary is applied i.e., the normal velocity across the boundary is defined to be zero.
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From page 711... ...
For the present case of simulating slamming phenomena, implicit scheme for transient terms is considered due to its unconditional stability and ability to obtain physically realistic and bounded results and does not put restrictions on the size of the time step through courant condition. Free Surface The shape and position of the free surface keeps changing with time during slamming of the ships and if the cells, which lie on the free surface boundary, are known, implementation of the free surface boundary conditions is straightforward.
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From page 712... ...
Impact velocity profiles, Vertical impact force on the whole section, and Pressures at certain locations on the surface.
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From page 713... ...
Grid and Cell sizes Since, the shape of the impacting section does not change along its length, grid size along x-axis is kept at one cell or CV, whereas grid sizes along y and z axes are chosen in such a way that the total number of cells in the whole computational domain is kept at optimum level to reduce the computational time but produce grid independent results. Distribution of grid points is carried out using stretching functions so that the cells closer to the body boundary are as small as possible apart from ensuring that the neighboring cells at each segment boundary do not vary much in size to achieve numerical stability during simulation.
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From page 714... ...
54 112 222 456 IMin. |time step l size dt | |secl e4 1 .25e4 4.70e-S |2.80e-5 ICPU Time I [has: | mind 10 o4 0:15 1:25 Table 5: Computation time for different grid sizes Numerical results consisting different slamming parameters like impact velocity profile, pressures at distinct locations and the impact force on the whole wedge section are presented in Figure 4.
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From page 715... ...
Actual dimensions of the computational domain sizes will be dependent on the physical size of the section. Simulations are carried out considering independent variation of the domain size ratios (DB, DD)
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From page 716... ...
152 152 l Min.time step size dt |sec 8.9e-S 8.9e-S CPU time h ml 10:17 14:25 Table 9: Details of computation time for different 3D grid sizes of Wedge section Slamming impact forces obtained for different grid and domain sizes along x-axis are shown in Figure 9 and Figure 10 respectively. Figure 9: Time series of slammingforces for different grid sizes along x-axis 9
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From page 717... ...
Figure 10: Time series of slammingforces for different domain sizes along x-axis Results from Figure 9 indicate that the effect of grid size along x-axis is not significant, understandably due to the fact that the section shape does not change along the section length. Results from Figure l O indicate that a domain size ratio of Do= 5 along section's length is enough to obtain domain independent results.
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From page 718... ...
Vertical Impact forces obtained from 2D and 3D cases match well during initial stages of impact but differ during later stages of impact with the 3D case predicting smaller magnitudes indicating occurrence of cross flow.
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From page 719... ...
There are two main conclusions: Firstly, close correlations of the present numerical results with the experiments have been obtained considering the histories of both impact velocity and slam loads for the water entry of wedge section during drop tests. Secondly, Effect of impact velocity variations and computational domain size, cross flow and threedimensionality on the numerical accuracy of the results is significant.
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From page 720... ...
and Peric, M (19999: Application of a Two Fluid Finite Volume Method to Ship Slamming.
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