Angel Beltrán Roig Miquel Gómez Garcés
6.1. JUSTIFICACIÓN DE PRECIOS
6.1.2. PRECIOS AUXILIARES
The plugin allows automatic domain creation and mesh settings based on a given file with different inputs. (See Figure
2.3.7-3) Explanations of inputs:
• Clean Project: closes current project.
• Load Parasolid/CATPart File: opens a parasolid or CATIA v5 file.
• Load STL/Dom File: opens a STL or domain file.
• Application: we usually assume that the domain size can be defined according to the type of
application, the Froude number and based on the Lpp of the boat. Hence default number of Lpp are given per application but the users are free to enter their own values using the "user-defined" application. —Resistance: LppBefore = 1 LppBelow = 1.5 LppAbove = 0.5 LppSide = 2
if Froude <= 1, LppBehind = 3 ; if Froude > 1 and Froude < 1.5, LppBehind = 4 ; otherwise, LppBehind = 5. —Seakeeping: LppBefore = 1 LppBelow = 1.5 LppAbove = 1 LppSide = 2
if Froude <= 1, LppBehind = 3; if Froude > 1 and Froude < 1.5, LppBehind = 4, otherwise, LppBehind = 5. —Decay: LppBefore = 3 LppBelow = 1.5 LppAbove = 1 LppSide = 3 LppBehind = 3
FIGURE 2.3.7-3 Domain Constructor and Mesh Setup window
• Configuration: defines if the case should be symmetric or not. If the geometry file is already
split in two, ’Symmetric’ should be selected. In case the geometry file is the entire body and the goal is to simulate half of it, select ’Symmetric’. In case the geometry file is the entire body and the goal is to simulate the entire body, select ’Non-symmetric’.
• Body orientation: defines the orientation of the reference frame related to the location of the
center of gravity of the body.
• Reference: defines the reference length (automatically or not) and speed.
• Initial free surface position: defines the initial free surface position in Z-direction.
• Scale: performs the scaling on the ship with the scaling factor specified. The speed value and
position of the free surface are automatically updated to ensure the Froude’s similarity between the two scales.
• Mesh density: Three different mesh levels are available: Coarse, Medium and Fine. They all
correspond to a different initial mesh size but also different refinements on the patches of the geometries according to the table shown below. The number of cells per length of the ship (LOA) is fixed whatever the ship type is. Reference length (LOA) is used as the reference value for the mesh generation parameters.
—Example:
Number of cells per LOA for coarse = 3, medium = 4 and fine = 5.
For mesh adaptation on each patch, the type and number of refinements as well as the diffusion level will be defined depending on its name based on the table shown below.
If one can define names in the CAD software, the script will be able to recognize them and define mesh settings accordingly. In the script, a ’Coarse’ mesh correspond to the 1st level, ’Medium’ to 2nd level and ’Fine’ to 3rd level. Hence if the selected mesh density is ’Medium’, the 2nd number of refinement level will be selected. The same idea applies for the diffusion.
—Example 1: if the geometry contains a patch named "Deck", a Target cell size criterion will be applied with a maximum number of refinement of 4 and target sizes of (0,0,0). The diffu- sion will be set to global (2 by default).
—Example 2: if the geometry contains a patch named "Rudder", if the user selected a ’Fine’ mesh, the curvature and the Target cell size criteria will be applied with a maximum number
of refinement of 10 and target sizes of (0,0,0). The diffusion will be set to 1.
If the face name is not in this table, the script will assign a target cell size criteria with a value of 6 for the maximum number of refinements, using a global diffusion.
The patches names can be written in different ways: capital letters or not, with a suffix or prefix, etc. For instance, "surf1_domship_Deck_2" will still be considered as "Deck".
If a patch names contains two different names from the table, the highest values for the refinement will be used.Each refinement has been elaborated to ensure a good Richardson extrapolation. Hence, if one would like to perform a mesh dependency study using 3 levels, the final value can be extrapo- lated with a very good precision.
• Extra refinement of wave field: based on Froude’s number (and wave length), the purpose of
these extra refinements is to add extra surfaces to refine accurately the bow wave and the wave pattern behind the ship. Hence, for shorter wave lengths more refinements will be added in the X and Y-direction for instance. This option adds a significant number of cells and should be used with caution. For resistance and motions prediction, this option is not essential. It will however ensure a very good description of the wave field.
• Merge patches with the same name: merges faces which have the same name. It supposes that
the faces are correctly named with Computation Aided-Design software (such as Cadfix). Therefore, names should be defined carefully in the CAD software to avoid that too many faces are merged.
• Merge tangential faces: groups all faces which are contiguous and more or less tangential. A
minimum angle should be chosen. If two contiguous faces have a maximum angle between 0 and , then the faces are grouped. This method is especially helpful when the geometry imported is a Parasolid file with many faces. A low angle will merge more faces whereas an angle close to 180 degrees will merge few faces.
Each time two faces are merged, the new face gets an ID (the ID is incremented one by one). After that, the plugin will assign specific values according to the name of the face: hull, deck, bow, aft, etc. These are the examples of keywords recognized by the plugin.
See the FINE™/Marine user manual for the C-Wizard mode calculations.
• Fluid properties for viscous layers insertion: The given kinematic viscosity by the user is used
to compute the first layer thickness as an information, printed in the shell.
Viscous layers are then defined and computed for solid faces only and if the face is not called “DECK” or “Deck” since there is usually no need to insert viscous layers on the deck of a ship (viscous effects from the air part are negligible).
The script assumes an isotropic mesh is used on the solid walls. Hence, the height of the first Euler cell on the solid walls can be computed by the script according to the number of refine- ment: size_euler_cell = Initial cell size / 2**N, with N the Number of refinement on the face. The number of viscous layers needed to completely fill the Euler cell is computed with inflation technique activated and a stretching ratio of 1.2.
• Triangulation density: defines the density desired for the triangulation.
• User-defined Y+: estimates the Y+ through the equation shown below. However, user can also
write its own value.
yplus = max (yplus_min, min (30+(Reynolds-1.0e6)/1.0e9 * 270, yplus_max))
where:
yplus_max=300 yplus_min=30
The buffer insertion of Type II is activated by default for the edges on the Mirror plane during the snapping step. However, HEXPRESS™ will try to respect this input but it is not guaranteed that it will be the case after snapping. If not, a warning is raised in the .rep file or in the report file.
After specification of the inputs, domain is constructed, boundary conditions are also defined automatically, different internal surfaces are created to allow different zone of refinements and all mesh settings are setup. The user is invited to check the whole mesh wizard before generat- ing the mesh.
The domain creation will not work in case the symmetry plane is not perfectly aligned on the Y plane.
Domain creation is not yet implemented for 2d case.
The geometry should be aligned with X-axis initially. Rotation can be done with CAD manipulation before if necessary. Z-axis should be the gravity axis.
The geometry should not have a bounding box already included since the script will create it automatically.
Y+ is based on the supposition that wall function will be used during the computation.
The number of viscous layers is computed based on a theoretical formula and does not take into account the influence from patches nearby. It is advised to let the inflation method active or to check the number of viscous layers to insert after the optimization step.b) Domhydro
This plugin allows the "domhydro" tool to be launched inside HEXPRESS™ interface. "Domhy- dro" serves as a tool which can solve the hydrostatic problem of a body using the domain and boundary conditions files from HEXPRESS™.
b.1)Assumptions
—Horizontal positions of center of gravity and the center of buoyancy are equal. The vertical position of the center of gravity is approximated considering that the mass is equally distrib- uted on the shell below the free surface (equal distribution of the mass on the wetted sur- face). The same distribution of mass is used to approximate the different inertia.
For instance, this should be taken into account especially for superstructures where the mass in the air part can be important.—This tool provides parameters based on geometric considerations. Hence, it is advised to compare the results with real values if they are known (given by the naval architect for instance).
—In 3D, the body is assumed to be aligned along the X-axis. If it is not the case (for instance the body has an initial yaw angle), one should specify it in the Cardan angles.
—In 2D, the roll motion of a boat section is defined by the rotation around z0, i.e. the "yaw Rz0" rotation.
—When using an additional external effort, only the final Z-axis resultant and the moment along X and Y-axis are taken into account the equilibrium position (the others components cannot be counterbalanced by hydrostatic forces).
—The body should not be in a bucket shape (the body is not close by a flat deck and it is "empty" inside), otherwise if its ground is below the free surface location, water will be con- sidered inside.
FIGURE 2.3.7-2 Illustration of a problematic case where water is considered inside the body