When the force coefficients derived from experimental measurements were compared to the values predicted by CFD, the hexahedral mesh gave the most accurate predictions for the tug with no
fin. The average
discrepancy between the predicted side force component and the measured value was 6 percent and the maximum discrepancy was 13 per cent. The largest discrepancy between measured and predicted values occurred at 60 degrees of yaw. For the tetrahedral mesh the predicted forces are consistently under predicted by an average of 18 percent when compared to the measured values, with the maximum discrepancy being 24 per cent.
For the longitudinal force component, which was much smaller than the side force component at the operating yaw angles, the tetrahedral mesh had an average discrepancy of 1 percent and the hexahedral mesh had an average discrepancy of 4 percent.
Comparisons of the forces were made on the basis of the difference between the measured and predicted value of the force component non-dimensionalized by the total measured force ((Fx
2+Fy 4.2 HULL & FIN
Force components and non-dimensional coefficients derived from the results of the CFD simulations for the combined hull and fin are given for the tetrahedral and hexahedral meshes in Table 5. The results of the simulations are compared with the experiments in Figure 7.
2)0.5).
The experimental force data for the hull and fin condition was not available, since this was not a condition required for the original project. All of the experiments with a fin included the protective cage. The effect of the cage was estimated from the complete data set by subtracting the force components for the cage (estimated from the hull only condition and the hull and cage condition) from the hull, fin and cage condition.
The same observations about the accuracy of the
predicted forces apply to the tug with a fin as for the tug without the fin, but the differences between the results with different meshes are smaller. The hexahedral mesh resulted in predicted forces that were typically within 5 percent of the measured values, and never more than 10 percent different, whereas for the tetrahedral mesh, the typical agreement was within 7 percent and the maximum discrepancy was within 12 per cent. The force coefficients predicted from the hexahedral mesh were all within 5 percent of the experiment data for yaw angles between 30 and 40 degrees and within 10 percent at 45 degrees. The forces predicted by the tetrahedral mesh over this range were typically within 10 percent of the measured forces over the same range of yaw angle, but were consistently under
predicted relative to the
measured values. The force coefficients predicted by the hexahedral mesh were a good mean fit to the measured values up to 35 degrees of yaw, but above that the forces predicted by CFD are over predicted relative to the measured values.
Table 5, Comparison of CFD predictions of hydrodynamic forces, tug with fin A
998.2 0.4849
Tetrahedral Mesh
Yaw angle, Speed, deg m/s 10 20 30 35 40 45
0.728 0.728 0.728 0.728 0.728 0.728
Hexahedral Mesh
Yaw angle, Speed, deg m/s 10 20 30 35 40 45
0.728 0.728 0.728 0.728 0.728 0.728
Surge, Total sway, N
N
7.712 6.173 3.721 2.065 0.523 -0.556
21.346 45.906 72.174 84.407 94.16
100.707
0.166 0.358 0.562 0.658 0.733 0.784
0.060 0.048 0.029 0.016 0.004 -0.004
89
102 115 119 128 145
Cq Cl # iterations
Surge, Total sway, N
N
5.878 3.752 1.22
0.418 -0.127 1.146
20.856 42.822 65.079 75.998 84.03 86.53
0.162 0.334 0.507 0.592 0.655 0.674
0.046 0.029 0.010 0.003 -0.001 0.009
224 259 284 293 310 428
Cq Cl # iterations
kg/m3 m2
©2008: Royal Institution of Naval Architects
B-47
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68