Trans RINA, Vol 157, Part C1, Intl J Marine Design, Jan - Dec 2015 3.5 HULL REFINEMENT
The work presented in this section describes the results of computations of resistance, trim and sinkage for the mothership concept. The aim of this study was to provide the results of a CFD computation for full scale resi stance prediction and a comparison between an empirical prediction made with the NavCAD Hydrocomp software and at a later stage a sensitivity analysis on a specific variable: the hull roughness.
s
All computations at different speeds reported here were grid
performed using orthogonal similar computational
characteristics as described in the next section with the exception of the prism layer. The prism layer is a model used in conjunction with a core volume me generate
prismatic cells boundaries [15]. The differences next to in the prism
a mesh esh to wall layer
construction were required to keep the average wall y+ value constant at all speeds. The grid generation, pre- processing, computations and the post-processing were performed using CD-Adapco’s STAR-CCM+ soft The solution method is of the finite-volume type. The finite volume method is a discretization method which is well suited for the numerical simulation of conservation laws of various types and uses control volumes of olume
ftware. independent polyhedral shape. In the finite vo
method, the governing partial differential equations are changed into a conservation form, and then solved over discrete control volumes in integral form. In this w conservation of
the flux is guaranteed throughout the
control volume. Wendt and Anderson [16] delineatted the details of discretization and solution methods. The time accuracy in a Virtual Towing Tank type simulation is not of importance because the aim of it is to evaluate the final results and not all transient states leading to it. Therefore the first-order Euler impli cit scheme is used for time
integration. The free-surface tracking
The convective terms are modelled with Resolution Interface Capturing method (HRIC) [18].
3.5(a) Grid Generation
Discretization of the volume elements has been achieved local
using trimmed hexahedral cells with several
wedge-shaped volume. (See figure 10). In the rema areas the mesh size was not required to be very fine and therefore the dimension of the cells was set as large as practicable in order to reduce the computing time. The solution domain extended from 1.0 x Lbp in front of the hull and 3..5 x Lbp downstream 0.5 x Lbp from BL in vertical direction upwards and 1.5 x Lbp downwards; in
refinements. In addition a prism layer was extending from the surface of the hu ll. The refinements were located close to the hull and especially in those areas where a finer mesh was required such as the free surface, around the hull and the wake zone including the diverging waves created by the hull enclosed in a Kelvin aining
added and
locating (the fluid to fluid interface) is performed using the Eulerian method called Volume of Fluid (VOF) [17]. the High
way the Figure 1 0: -Domain discretization, top view 3.5(b) Main Particulars off Hull
The mothership is a 131m Offshore Supply Vessel equipped for the deployment of 4 WFSV and 4 Cabin RIBs. The main particulars:
LOA Lwl
Displacement Displacement
BeamOverall Moulded draft Cb
Depth
131 m 129 m 15978 t 15575 m 32
m
5. .70 m 0. .66 - 12.00 m
Only half of the bare hull in full scale dimensions was considered in the numerica
constrained. The ship vellocities tested ranged from Froude Number
the other 0.154
to freedom were
symmetry across the CL. The model was left free to trim and heave;
al model due to geometric degrees of
0.31. The time step was
adjusted according to the fllow speed in order to satisfy the Courant-Friedrich-Lewy condition with an average convective Courant number of about 1. The tested loading condition was with no heel and even keel.
the lateral direction the K elvin Wedge condition was satisfied to avoid possible unwanted wave refraction on the side boundary, resulting in a total domain width of approximately 2 x Lbp. In the
y+ wall treatment was used. The wall treatment STAR-CCM+ is the set
of
e simulation a Two layer- all in
near-wall modelling
uffer region of the boundary layer). The prism layer is made of 8 layers with a hyperbolic tangent stretching factor. The result of the grriid generation was a domain made of approximately 1.25
of the prism layer was set to reach a value of y+ equal to 50. The choice of this value h
5 million cells. The first layer has been made because it has
shown good result in correlation to the Cf in the ITTC ’57 friction line [19].
assumptions for each turbulence model. The all- y+ wall treatment is a hybrid treatment that attempts to emulate the high-y+ wall treatment for coarse meshes and the low-y+ wall treatment for fine meshes [15]. It is also formulated with the desirable characteristic of producing reasonable answers for meshes of intermediate resolution (that is, when the wall-ce bu
ell centroid falls within the
3
© 2015: The Royal Institution of Naval Architects
C-89
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