Workshop 2.1 14.5 Release
ANSYS Aqwa Hydrodynamic Hydrodynamic Diffraction
The Aqwa Simulation Process
CAD
DesignModeler
Hydrodynamic Diffraction
Creating geometry ready for import to WB HD HD uses imported geometry from DesignModeler. This is used to create the majority of the Aqwa model •
Hull definition
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Morison (TUBE) elements
For Aqwa specific geometry this is input via the HD user interface •
Point masses
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DISC elements
The main requirements for WB HD can be summarized as follows: •
Each vessel (or structure) is represented as a single part, as generated in DesignModeler.
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The panel model must be such that the mesh is up to the waterline.
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The water line defines the global vertical origin for the analysis.
Vessel/Structure definition Each vessel or structure may consist of one or more bodies, but MUST be contained within a Part Here we have a ship and a pier. •
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The ship consists of two bodies, one above the water line and a second below the water line. The pier has three bodies, defining above and below water, plus the base, which has special considerations (more on this later).
Global axis system in AqwaWB The water line defines the Aqwa global reference axis (FRA (FRA)) The geometry is divided at this point to allow meshing above and below the water line
Radiation/Diffraction Analysis Stages
WB HD for the diffraction analysis consists of five stages •
Insert HD system and associate geometry
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Add Aqwa specific parameters
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Add Aqwa specific elements
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Mesh
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Analysis and post-processing
Worked Exercises
Three models are going to be considered •
Single ship
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Two ships demonstrating hydrodynamic interaction
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One ship and a pier demonstrating specific requirements of including a fixed structure in the analysis
Insert HD System and associate geometry
Create HD System (as shown in Lecture 1) Import CAD geometry and/or create model directly Repair as necessary Position the model(s) in the correct vertical and horizontal locations required for the analysis Cut the model(s) at the water line to provide the required delineation between above and below water (no element splitting) Combine bodies associated with a single vessel to form one part.
Create HD system •
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Set Units
Add HD system by drag & drop onto WB Project Schematic page.
RMB on Geometry cell, Import Geometry > Browse, and browse to ship.igs
Import CAD geometry Double click on Geometry cell to open DesignModeler Select meters as length unit On details View for the Imported geometry set Simplify Topology to Yes, and Tolerance to Loose. This will repair some geometry issues with the iges file. Clicking on Generate geometry within DM.
will create the
Position model to correct position We can make use of the Body Operation > Translate to position the vessel in the correct position. We select the body by clicking on the image of the ship. Here we are offsetting by -8 meters in the vertical (Z) direction to adjust the draft. Use Direction Definition Coordiantes. Click to complete the operation.
Define the waterline We now need to cut the model at the water line so that we can mesh without elements crossing this line First use Tools > Freeze to enable the slice operation. The model will become transparent (the default action for DM is to merge bodies together, which is not what we require when we cut the model). (no needed from v14.5)
... Define the waterline The Slice operation uses the global XYPlane to slice all bodies. In this example we only have one body, but the same effect may be applied to multiple bodies. Once the Slice operation is inserted click on XYPlane in the Tree Outline to define the Base Plane. Click
to complete the operation.
... Define the waterline We now have two parts, one above and one below the water line
Make each vessel/structure into a single part We can combine bodies by selecting them in the tree and right click > Form New Part Here we have renamed the combined part as ShipHull, and renamed the individual bodies as UpperHull and LowerHull Close DesignModeler
Adding Aqwa Specific Parameters Double-Click on Model Cell on Project Schematic The units will adopt those used in the geometry creation, but may be modified from the Units menu item
Global parameters Selecting the geometry object in the tree provides access to some global data in the details window, such as water depth and density. Set the water depth to 500m. Note that the default import preferences exclude Line Bodies, but include Solid Bodies (which are actually invalid in Aqwa).
Vessel/Structure parameters Selecting a vessel or structure in the tree provides access to data specific to that vessel, such as whether it should be included in the simulation, is free or fixed, whether an internal lid is required, and additional damping information
Bodies within Vessels/Structures Selecting a body within a vessel or structure in the tree provides access to data specific to that body, such as whether it should be included in the simulation and whether it is diffracting or non-diffracting. When first imported any body below the water line is assumed diffracting (Surface Type Program Controlled).
Provide additional Aqwa elements DesignModeler can be used to create models consisting of panels and Morison type elements, such as TUBEs and STUBs. Some additional element types can be added directly within HD •
Point masses PMAS
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Point buoyancy PBOY
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DISCs
As a minimum we must provide a point mass to describe the mass matrix for the vessel
...Provide additional Aqwa elements
For each vessel/structure defined provide a point mass element This is inserted in the tree by selecting the required vessel, and right clicking on Add > Point Mass
Point Mass Properties The program can compute the mass based upon the displacement of the vessel, or this can be defined directly in the details window. The mass inertia matrix must ALWAYS be defined, either via Radius of Gyration or direct input Values required are highlighted in yellow
Point Mass Input If Mass Definition is program controlled the mass will equal the displacement. Set kXX, kYY and kZZ to standard default values (beam is 40m, length 200m) For a regular ship; •
kXX = 0.34*Beam
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kYY = 0.25*Length
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kZZ = 0.26*Length
Set the VCG (Z coordinate) to 8.5 m for this exercise
Meshing The vessel(s)/structure(s) may now be meshed. When Mesh is selected in the tree two additional toolbar items appear The first invokes the meshing tool using the parameters defined in the details window. The second item provides additional control for localized parts of the model. Both these items can also be accessed by RMB on the Mesh object in the tree.
Mesh Control In the details window the element size and meshing tool may be selected If Program Controlled meshing is chosen the program will use a surface mesher for vessels only containing surfaces (panels), and a combined mesher if the vessel also contains line elements.
... Meshing When Generate Mesh is selected the meshing tool is instigated and a mesh using the parameters defined is created. Mesh information is given in the details panel.
... Meshing Change the frequency unit to rad/s from Hz Adjust the Max element size to 2.5 and tolerance to 0.5 Generate Mesh You will notice that the maximum allowed wave frequency now becomes in the order of 2 rad/s, however number of elements are more than tripled (which affects the solver time). For a quick solution set the Max element size back to 5 Generate Mesh
Analyze
The model is now ready to be analyzed. The following additional items will be seen in the tree •
Analysis Settings
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Gravity
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Structure Selection
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Wave Directions
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Wave Frequencies
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Solution
Analysis Settings The options control how the analysis is to proceed, and what types of results are to be reported and stored. They relate directly to the options used in the Aqwa analysis that are described in the Aqwa Reference Manual. The Sea Grid Factor is used in determining the size of the diffracted water surface plot in the visualization window with respect to the size of the model. Set Ignore modelling rule violations to “Yes” (more on this in Aqwa suite lectures) Set Calculate Full QTF Matrix to “No”
Set Generate Internal Lid Option in Details of ShipHull to “Yes”
Structure selection and gravity Structure Selection enables the definition of interacting structure groups (for multi-body problems). By default all vessels are assumed interacting. Gravity enables the definition of gravity for this analysis.
Wave directions and frequencies These objects permit wave directions and frequencies to be defined either as a range or with individual values. The initial maximum frequency is determined by the mesh size; attempting to change this to a higher frequency will produce an error. If higher frequencies are required the mesh size will need to be reduced. Frequencies may be program controlled (with equal frequency or period interval) or manually defined. Choose program controlled. Reduce no. of frequencies to 10
Save Project Save the project from the Workbench Project Page, File > Save Browse to the training working directory and save the project as ship.wbpj. The title on the Project Page will reflect this change
... Analyze Two Analysis options are available: •
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Calculate hydrostatics only (compute displacement and small angle stability parameters) Compute full hydrodynamic properties and results
These are available thru the toolbar menu item or using the context sensitive menu on the Analysis object (RMB click)
... Analyze If a full analysis is chosen the basic hydrostatic solution is internally run first to ensure consistent mass and displacement. Whilst a solve is being processed a progress bar appears to indicate how far the analysis has reached.
Single Ship Model Solve full analysis Insert results •
Hydrostatic Table
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Hydrodynamic Graphs
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Pressures and Motions
Evaluate All Results must be selected for the results to be presented/updated
Review Results Results can be added when the Solution object is selected. This can be done before or after an analysis has been undertaken. Available sets of results are: Hydrostatic, Hydrodynamic, Pressures and Motions
Hydrostatics When selected these results appear on the Properties tab.
Hydrodynamic graphs Graphs allow computed parameters to be plotted. Multiple graphs may appear on a single plot, and multiple plots may be requested by inserting additional Hydrodynamic Graph objects in the tree.
... Hydrodynamic graphs
Hydrodynamic pressures and motions This object presents plots of wave contours and hull pressures
...Hydrodynamic pressures and motions These are used to select the wave frequency and direction to utilize in the plot. Choose higher frequencies for more interesting plots Result type may be Amplitude/Cyclic/Minimum/Maximum •
Choose Cyclic if specific wave position required
If Cyclic chosen then Wave Position may be 0/90/Specified/Range •
Choose Range if animation required
If Range chosen then No. of steps specifies how many wave positions to display in the animation
...Hydrodynamic pressures and motions Hull pressures may be interpolated or individual panel results. Can also show resultant (RAO) displacement. This dims or shows the above water part of the vessel(s) Choose either head of water or pressure for the hull contours Wave contour can be displayed or disabled Include or exclude wave components Change Incident Wave Amplitude to see the effect of Second Order Terms
Worked Exercises
Three models are going to be considered •
Single ship
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Two ships demonstrating hydrodynamic interaction
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One ship and a pier demonstrating specific requirements of including a fixed structure in the analysis
Two Ship Model This model uses two instances of the ship used in the first exercise Set up the geometry •
Open Workbench and select Component Systems > Geometry
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Right click on the Geometry cell and select Import Geometry > Browse
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Browse to ship2.agdb. This is a DesignModeler database that was created from an imported IGES file
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Double click on the Geometry cell to open up DesignModeler and view the vessel
Modelling – Generate the second ship Generate a second copy of the ship using Create > Body Operation • •
Set Type to Translate Select All bodies by clicking on the two bodies in the graphics window
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Select Direction Definition as Coordinates
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Set Preserve Bodies to Yes
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Set Y Offset as 45.0
Click on Generate
Modelling – Generate the second ship Combine bodies to form two ships Close DesignModeler and Save Project as TwoShips
Import geometry to WB HD
Create HD system and share geometry
... Import geometry to WB HD Open HD system by double clicking the Model Cell Add a point mass to each ship and define inertia values by radius of gyration. Use the same values as for the single ship model. Note that the program assumes hydrodynamic interaction for multi-body analyses.
… Manually specify wave frequencies For the previous single ship model Program Controlled Wave Frequencies was adopted. For the two ship model select Manual Definition for the Range, then enter 4 single periods at 6, 7, 8 and 9 seconds
Note that this can also be achieved using range definitions, rather than single values
Two Ship Model Mesh (with 5m element size) and Solve. Try comparing RAOs between the two ships (for instance, Y RAO for the 90 degree wave case)
Worked Exercises
Three models are going to be considered •
Single ship
•
Two ships demonstrating hydrodynamic interaction
•
One ship and a pier demonstrating specific requirements of including a fixed structure in the analysis
Ship Pier and Model
For this model we model a ship alongside a pier. This is a bottom founded structure that requires special treatment. Since the pier sits on the seabed we need to make the underside of the pier non-diffracting.
Modelling – set up geometry Open Workbench and select Component Systems > Geometry Right click on the Geometry cell and select Import Geometry > Browse Browse to ship3.agdb. This is a DesignModeler database that was created from an imported IGES file Double click on the Geometry cell to open up DesignModeler and view the vessel
Modelling – Create the Pier Use Create > Primitives > Box •
Set Base Plane to XYPlane
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Box Type From Two Points
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Point 1 and Point 2 coordinates as shown
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As Thin/Surface No (otherwise generates an open ended box)
Click on
Modelling – Define the waterline We need to cut the model at the water line as before. First we must make the pier into a surface model (currently solid) Set Selection Type to Bodies Only Select the pier as the geometry Set Thickness to 0 Now slice both ship and pier using XYPlane; remember to Freeze first
Modelling – Separate the Pier base The base needs to be separated from the remainder of the pier •
Use the Create > Slice object as before
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Set Slice Type to Slice By Surface
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Choose the pier base surface as the Target Face Select the Pier body as the Slice Targets
Form two parts, one for Ship and one for Pier. Rename bodies for easy identification Save Project as ..\Ship&Pier
Import Model to WB HD Create H-D system and share geometry Add a point mass to ship and pier and define inertia values by radius of gyration. Use the same values as for the single ship model. The pier model can use dummy values since it will be fixed. Set water depth to 60m Set Pier to Structure is fixed in place Set Pier Base to be Non-Diffracting
Ship and Pier Model Mesh (choose an element size of 7m), select wave periods of 5.5 and 8 seconds, and solve
