Trans RINA, Vol 157, Part C1, Intl J Marine Design, Jan –Dec 2015 3.5(c) Comparison Process and Results


The comparison process has been carried out analysing the Pe [kW] data retrieved from the Computational Fluid Dynamic Analysis and the Pe [kW] calculated with the NavCAD software


which solved the empirical


formulation of Holtrop – 1984, with the following conditions:


 Cf type: ITTC  Correlation allowance: 0.00038  Hull roughness:150-6m  1+k:1.3340  Speed dependent correction  Appendage method: Holtrop-‘88


The formulation was also resolved for a rather heavy sea state:


 Significant wave height: 5.0 m  Modal wave period: 9.7 sec


The sensitivity analysis was carried out on 2 different hull surface roughness values, 0.0015mm and 0.001mm respectively. The roughness in StarCCM+ is a module which can be activated and tuned in order to reach the required roughness height


and specification. The


activation of the module modifies the wall treatment to incorporate roughness [20].


Table 6 shows the Pe and Total Drag values retrieved from the CFD simulations at different speed for the hull free to trim and sink with the following characteristics:


 LCG: 53.44 m from AP  VCG: 7.70 m from BL  Hulls surface roughness height: 0.0015 mm


The NavCad Pe Total represents the values of Pe calculated using the empirical formulation with SWH of 5 metres and a modal wave period of 9.7 seconds. The lower roughness Pe represents the Pe for the simulation of the hull with a surface roughness of 100 µm.


Table 7 shows the sensitivity analysis comparing the Cf, Cr and Ct of the hull resistance computed with two different surface roughness height. The range of velocities has been reduced to the most relevant, namely between 12 and 18 knots.


3.5(d) Discussion


The numerical analysis shows a good correlation between the empirical formulaion for a Fn between 0.154 and 0.246. For the range of speed above 0.246 the differences are rather significant. The reason for this discrepancy is due to the overall dynamic trim effect on the total resistance. The hull hence reaches a significant trim by the bow and the bulb dives consistently, creating


C-90 [deg]


0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35


813 [m]


‐0.60 ‐0.50 ‐0.40 ‐0.30 ‐0.20 ‐0.10 0.00


813 18 [knots] Sinkage [m] 18 Figure 12: Trim values


an important bow wave. The added resistance due to the bow wave can’t be predicted by the


numerical


formulation and this is therefore the reson for the large discrepancy at higher Fn. See Fig. 14 to Fig 19.


10000 12000 14000 16000 18000


2000 4000 6000 8000


0


8 10121416182022 Full Scale Ship Velocity in Knots


Figure 11: Effective Power values [kW]


CFD Pe [kW] NavCad Pe Bare NavCad Pe Total LowerRoughnessPe


Trim* [deg]


[knots]


Figure 13 – Sinkage values measured in COG


© 2015: The Royal Institution of Naval Architects


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  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210