Feature 2 | CAD/CAM UPDATE NAPA adds mesh maturity
NAPA reckons to have improved its Finite Element Model (FEM) creation by upgrading meshing quality with a new unstructured meshing algorithm introduced into the meshing task.
F
EM is a state-of-the-art FE pre-processor converting 3D ship product models into Finite Element
Models. The main idea is to idealise an as-built product model by removing unnecessary details and simplify geometry suitable for the finite element analysis. The idealisation process in NAPA is
realised in several steps that are completely under the user’s control. Te NAPA FEM pre-processor allows users to produce various kinds of a good quality FE mesh for different needs by modifying the control parameters and options during the idealisation steps. Te control options are selected based
on the final target of the FEM mesh. A final requirement for a FE model depends on things like:
• Ship type: bulk carrier, containership, passengership, tanker, navy
• Analysis type: rule requirement, stress analysis, vibration, fatigue ...
model, hotspot analysis, etc. Te goal of the FEM creation process is
to support design process by producing a good quality FEM mesh for a further Finite Element Analysis . As the idealisation control information is stored as part of the 3D product model, a new FE mesh can be updated whenever the structure model in NAPA Steel is changed or updated: a new mesh can be produced automatically within a few minutes with predefined parameters from the Steel model. Te major part of users’ work is done only once when these options and parameters are defined when the meshing is made for the first time. Once the control data is defined for a ship
project, these definitions can be used both for updating the current model, and also in the following similar ship projects, enabling very efficient design processes applying advanced structural analyses techniques.
44 Mesh Mesh Surface meshing Surface meshing 3D model 3D model Topology Topology Idealization Idealization
1D meshing
1D meshing
Figure 1 Mesh creation process in NAPA from as-build 3D model to FE mesh.
Figure 1 Mesh creation process in NAPA from as-build 3D model to FE mesh. Figure 1 Mesh creation process in NAPA from as-build 3D model to FE mesh.
• Level of details: global model, three cargo hold model, Local model, Fine mesh
Figure 2 Example of a Global FE model of a Ropax Vessel.
Figure 2 Example of a Global FE model of a Ropax Vessel. Figure 2 Example of a Global FE model of a Ropax Vessel.
Based on the same initial user input,
NAPA continues automatically the FE model creation processes:
• Te topological connection of a NAPA Steel model
• Te idealisation of geometry • 1D meshing
• Surface meshing Tere are many reasons why the user may
want to restrict the number of details in the FE model. NAPA says that NAPA Steel allows modelling the as-built steel structures to a very detailed and realistic level, but normally, the FE model for stress analysis is an approximation of the structures. Typical reasons for compromising the true
structures are ship type related instructions from classification societies, the required result accuracy and limitations in FE solver resources. One way for approximations to take place
is just to prevent some structural items to be included in the FE model. A straightforward way is to use the coordinate limits when creating the FE model, but normally this is not an adequate method. The more advanced ways are possible through the NAPA Steel user interface allowing detailed structure and structure type dependent choice of structures. Te NAPA Steel user interface controls
several parameters related to the idealisation and meshing of the model. Tey mainly affect the following items:
The Naval Architect July/August 2009
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