Project 1 - 2D meshing on the instrumental Panel
AIM :
To mesh the given Dash board sub parts cad model by following these quality criteria.
S.NO Quality Criteria Value
1 Target length 4
2 Minimum length 2
3 Maximum length 6
4 Aspect 3
5 Warpage 15
6 Skewness 45
7 Jacobian 0.7
8 Minimum Quad Angle 45
9 Maximum Quad Angle 135
10 Minimum Tria Angle 30
11 Maximum Tria Angle 120
12 Tria Percentage 15
PROCEDURE :
In order to mesh the given cad model, certain steps should be followed So that better quality mesh is achieved.
First import the cad geometry in the ANSA or any pre-processing software. After importing the geometry there may be a chance of occurring new topological errors, So topo function is to be performed after importing the geometry. This was performed by FACES > TOPO. Next, the cad model orientation issues must be checked by FACES > ORIENT. The entire cad model will be represented in grey and yellow colors in ENT mode.
FIG 1 - Orientation of cad model
After this topological errors must be checked before performing meshing so that accurate results were obtained. This was done by CHECKS MANAGER > GEOMETRY > EXECUTE. The checks manager will show the errors that were present in the cad model. ANSA will provide the option of solving these errors either manually or automatically.
FIG 2 - Topological errors of cad model
In the fig2 by selecting the fix option, the topological errors were automatically fixed. Some of these errors cannot be automatically fixed, they have to be performed manually. In fig 3, the red lines are single cons which indicates the volume is not closed and it is a topological error that has to be fixed manually by creating a new face.
FIG 3 - Topological errors of cad model to be fixed manually
The meshing can be performed in 2D as well as in 3D. 3D meshing is time consuming process as it takes a lot of time to compute the model whereas 2D meshing is performed for those models where thickness of components is less than 5mm. For 2D meshing mid surface must be extracted for the cad model and meshing is performed which gives similar result as of 3D meshing. Next the thickness of the cad model is measured by MEASURE command, as the given cad model is having thickness < 5mm so 2D meshing is performed. In ANSA the mid surface extraction can be done either manually or automatically. Automatic mid surface extraction is done by FACES > MIDSURFACE > SKIN. In the fig 4, as the cad model contains lot of features ANSA GUI is not able to extract the mid surface so manual mid surface extraction is done.
FIG 4 - Automatic mid surface extraction
The base plate of the cad model is having a uniform thickness of 0.6mm, So by FACES > OFFSET mid surface is extracted. The cad model contains a lot of features, in order to extract mid surface for these features a series of steps has to be followed. In fig 5, first create a middle curve that covers the perimeter of the given feature by CURVES > MIDDLE and create a new face enclosing these curves.
FIG 5 - Extracting mid surface for the rib feature
Next, in order to have connectivity extend the extracted feature mid surface to the base plate by FACES > EXTEND as shown in fig 6. As the edge is shared by the 3 faces a triple con is formed.
FIG 6 - Mid surface for the rib feature
In fig 7, as cylinder is not having any thickness, cross patterns were extracted which indicate the midsurface for the cylinder as shown in fig 8.
FIG 7 - Dog house feature
FIG 8 - Extracted mid surface for the Dog house
To define the Property ID to these cross patterns, as cylinder feature s not having any thickness, assign the thickness of the ribs to these cross patterns. After extracting the mid surface for the entire cad geometry, check whether the extracted mid surface had captured all the features of the geometry. This was done by switching on the transparency of the parent geometry and switch to PID Mode from ENT mode to visualize both parent geometry and extracted mid surface simultaneously as shown in fig 9.
FIG 9 - Parent geometry and Mid surface
Next after extracting the mid surface, mesh has to be generated. ANSA will provide automatic as well as manual way for mesh generation. As the cad model contains lot of construction and feature lines, it is preferred to perform BATCH mesh as it automatically defeatures the construction lines where ver it is required and generates the mesh. In order to do BATCH mesh, assign a new part to the extracted mid surface by MODEL BROWSER > SET PART and select the mid surface entities and assign a new part. After assigning a new part, batch mesh is done by BATCH MESH MANAGER and in the contents add the mid surface part and assign quality criteria range in MESH PARAMETERS and QUALITY CRITERIA as shown in fig 10. Fig 11 shows the mesh performed by the BATCH Mesh manager.
FIG 10 - Batch mesh manager
FIG 11 - Mesh obtained from Batch mesh manager
Fig 12 shows the defeaturing done by the batch mesh in order to satisfy quality criteria. The orange lines indicate the defeatured construction and feature lines.
FIG 12 - Defeaturing done by Batch mesh manager
The off elements indicates the number of generated mesh elements that does not satisfy the quality criteria. It is not mandatory to keep the off elements to zero as in some areas like sharp corners, triple conjunctions the elements won't satisfy quality criteria as shown in fig 13. In such areas keep the off elements. It is recommended to keep the off elements less than 1%.
FIG 13 - Off elements at Dog house triple con junction
After this next, release the mesh elements so that mesh associtivity with the cad is removed. This was done by MESH MODULE > ELEMENTS > RELEASE. Next, assign thickness to mesh elements by MESH MODULE > MACROS > SET PID.
After assigning thickness, make sure to enable DRAW SHELL AS SOLID option in quality criteria. Fig 14 shows the representation of mesh elements after enabling DRAW SHELL AS SOLID option, this will convert the extracted 2D mesh to Solid so that analysis can be done.
FIG 14 - 2D mesh to solid mesh
LEARNING OUTCOMES :
1) Fixing the topological issues manually like triple cons, needle faces, unchecked faces, cracks and collapsed cons.
2) Extracting mid surface manually and automatically.
3) Perform BATCH meshing.
4) Manually rearranging the unnecessary patterns like back-to-back trias, rotating quads etc.
5) Bring the tria elements away from the curvature to plane surface and try to eliminate them.
6) Removing OFF elements.
CONCLUSION :
Pre processing the cad model is one of the important task that needs to be done so that accurate results can be achieved by generating the better mesh flow. So using preprocessing software like ANSA, HYPER MESH etc and by following certain set of guide lines better mesh flow is achieved which gives better results. After Pre processing, solving and Post processing has to be done.