Creating an AQWA model using Workbench DesignModeler

1 Introduction When using the Hydrodynamic Diffraction system in Workbench (WB-HD) it is intended that nearly all geometry modelling will be carried out using the Workbench modelling application DesignModeler (DM). This document gives some guidance on the process of creating an AQWA model in DM, but does not give detailed instructions for all the capabilities in DM. Using DM it is possible to import a geometry file in various formats, create a model from scratch, or import offset data to define the hull shape.

2 Initial Geometry Creation Importing a geometry file is straightforward using File | Import External Geometry File. The file formats supported depend on your licence.

Creation of a model from scratch requires familiarity with the DM modelling tools and is not covered in this document. Creating a model from offset data is discussed further in section 4.

Here is an example of a ship geometry imported from an IGES file.

3 Draught and Trim Once the basic geometry is created it is necessary to position the vessel at the correct draught and trim. It is not possible to vary these in the WB-HD application. It is also necessary to split the model at the waterline so that the meshing application will put nodes at the waterline. It is possible to parameterise the draught and trim, so that additional models may be easily generated. The following shows the basic steps to set the model in readiness for importing into WB-HD.

3.1 Create the Draft Line and Global Water Axes

AQWA uses a global axis system known as the Fixed Reference Axis. This system has the Z origin on the water surface, with Z going vertically upwards (out of the water). To simplify matters we rename the XY Plane in the geometry tree as GlobalWaterAxes, and we then create a new plane that is parallel to this that represents the draft line. To achieve this select Create New Plane, and choose a transform to offset Z by the amount required to give the desired draft. In our case the local origin is at the keel of the ship, so the offset represents the draft value. For the example a 2m offset is chosen. The new plane is renamed DraftLine.

It is now necessary to shift the model vertically so that the desired draft line is at the Fixed Reference Axis. This is achieved by using the Create Body Operation. DesignModeler undertakes body moves by defining a source plane and a target plane. In this case the source plane is the DraftLine, and the target plane is the GlobalWaterAxes i.e. we are shifting the draft line plane to be at the global water axes, as required. You can check that the transformation has been undertaken correctly by viewing the model side on (click on the green Y axis in the triad on the lower right hand corner). In our case the model moves down so that the GlobalWaterAxes are now cutting the vessel.

AQWA requires that elements must not cut the water surface; we require diffracting elements below the water line and non-diffracting above. To facilitate the required meshing in WB-HD it is required to split the model at the water surface. This is achieved by first “freezing” the model with Tools Freeze, then using a Create Slice to undertake the physical separation of the top and bottom sections of the vessel. The freeze does little to the visual representation other than making the model transparent.

The Slice operation uses the GlobalWaterAxes plane to slice all bodies, in this case there is only one body, but the same effect may be applied to multiple bodies.

Each vessel needs to be represented as a part in WB-HD. At the moment it can be seen that there are two parts, representing the upper and lower hull. To make this one part the two bodies (parts) and right hand mouse button select Form New Part. This has been renamed in this case as ShipHull

This completes the process for setting up the geometry in readiness for import to WB-HD. If multiple vessels are being created (for hydrodynamic interaction in AQWA) then this process is simply extended and will result in a number of “Bodies” within DM, typically (if there are two ships) Body Description 1 Ship A, below water 2 Ship A, above water 3 Ship B, below water 4 Ship B, above water These bodies must then be grouped into “Parts”, typically Part Name Body no. Body name

1 Ship A 1 Body 1 2 Body 2

2 Ship B 3 Body 3 4 Body 4

4 Hull definition using offset data NOTE This is a beta facility. Any comments which will help to make it more usable will be welcomed. Once it is formally released, it will be more difficult to make changes because it will be necessary to maintain backwards compatibility (i.e. the ability to read old versions of the file). The AQWA Graphical Supervisor (AGS) has a facility for creating a mesh from “lines plan” data, stored as x,y,z coordinates in a text file with a .LIN extension. In version 12.0 of DM there is a beta version of a similar facility to create a model from offset data. DesignModeler can also reads offset data from a text file, but the format is different from the .LIN file. This has been done to allow greater reliability and flexibility in defining the model (for example, specifying certain lines as “hard” lines, defining appendages and transom sterns). After some development it has been realised that it may be possible to accept data in the format of the old .lin file, allowing re-use of old files. However, this capability is not yet available, and at present files need to be in the new format. The new format is illustrated in Appendix A. This file can be imported using File | Import External Geometry File in DesignModeler. All the tools in DM can then be used to create the model. It is likely that some sections of the hull will be missing and will have to be repaired.

APPENDIX A - Format of text file for import of offset data to Designmodeler / Comment lines begin with ‘/’ / Blank lines are not permitted Title 200.0 50.0 25.0 17.0 (Length, beam, depth, draught) HULL 21 (keyword for main hull data, no of sections) 0.0 10 (x position of section 1, no. of points) 0.0 5.0 10.0 20.0 21.0 22.0 ….. (y coordinates) 1.0 1.5 1.6 2.0 5.0 7.0 …… (z coordinates) 2.5 12 (x position of section 2, no. of points) ... / repeat for all sections / NB coordinates may be on multiple lines, read until N reached CENTER (or CENTRE) 15 (keyword for centreline, no of points) 0.0 5.0 (x coordinates) 2.0 4.0 (z coordinates) Future keywords (not yet implemented) may include Deck_Edge Hard_Line Transom_Stern Appendage