Week 3

2D MESHING OF HOOD MODEL OF A CAR UNDER THE GIVEN ELEMENT QUALITY CRITERIA USING ANSA PREPROCESSOR.

• AIM

1. Perform 2D Meshing on the given hood model of a car under the required element quality criteria and assign thickness to the component after meshing.
2. Obtain a good mesh flow for the component and OFF elements after giving quality criteria must be zero, also eliminate rotating quads and Opposite trias present in the model. Trias near to free edge or holes or on the fillets should be eliminated.

Following are the steps to proceed,

• Geometry Cleanup
• Mid-surface extraction
• 2D Meshing of the parts
• Element quality checkup
• Remesh- Quality Improvement
• Assigning thickness to the model

• PROCEDURE

The given component is a hood model which is also called bonnet of a car. since this part is a sheet metal part and thickness is very low compare to the length of the component we opt for 2D Meshing ( Mid-surface extraction and assign thickness ).

The hood is having 4 separate parts, which are hinge reinforcement, Latch reinforcement, an inner panel, and an outer panel. So we need to extract the mid surface of each of these 4 parts and do meshing under the given element quality criteria.

• Detailed steps

1) open ANSA software and import the CAD file of hood model using file -> open.

following are the 4 parts of the model before geometric cleanup.

fig 1 - Hinge reinforcement.

fig. 2 - Latch reinforcement

fig.3- outer panel

fig. 4 - Inner Panel

• GEOMETRY CLEANUP

2) Have to perform geometry cleanup for each part separately before mid-surface extraction to make sure geometry is clean and don't have any errors.

For this go to, Tools -> checks -> geometry. geometry checks window pops up then select the errors you want to check and click on execute,

ANSA will show the geometric errors in the model in red colour.

fig. 5- Geometry checks on Inner panel

As you can see in the picture above there are many geometric errors in the model like, unchecked faces, Needle face, single cones, triple cones etc. most of the errors ANSA will clear automatically to fix the errors select all the errors by left mouseclick drag or by Cntrl+ A and right click on the selection and select fix in the pop-up window. if ANSA fix all the errors when we re-run the geometry checks window will show green bar with 100 % sign.

fig. 6 - Inner panel after fixing geometric errors.

If some errors are not fixed automatically, we have to fix them manually. For example triple cons ( cyan colour) error, the most common case triple cons arrives is when degenerate face exists (faces have their opposite cons coincident) this problem can be solved by deleting degenerate face, by using Topo module-> Faces-> delete function.

• MID-SURFACE EXTRACTION

3) After fixing all geometrical errors we need to proceed with the mid-surface extraction of all 4 parts. For manual mid-surface extraction method, go to Topo module-> Faces-> Offset then select the face, the selected face will offset towards the arrow direction according to the offset value we input in the window. a negative value would offset the face in the opposite direction indicated by the arrow.

4) since the part ( here inner panel) has many features it is difficult to select each and every entities separately, so we use feature area selection tool (indicated in red circle in fig. 7), if we select one element all neighboring elements get selected, provided angle formed by normals of neighboring elements are less than feature angle value specified.

fig. 7 - selecting the entire face using Feature area selection tool

5) After selecting the face, input the offset value as half of thickness of the component, (here the arrow direction of selected face is positive, so we need to put a -ve sign inorder to offset the face in opposite direction.)

fig. 8- inputting offset value of selected face

6) After giving offset value, a mid-surface is created (you can see in fig. 9, a red colour line through the middle of the component, red colour because outer perimeter of mid-surface has single cons because it is free edge ) . so we have to isolate the mid-surface using !Not operation -> select the mid-surface and give a middle mouse confirmation -> selected mid-surface is isolated, use Neighbour-> All option to bring all the mid-surface visible, thus the mid-surface is extracted.

fig.9- mid-surface is created and has to isolate using !Not function.

7) Orient the mid-surface by Topo Module-> Faces-> Orient then assign a new property for the mid-surface by using Topo module-> Faces ->set PID select the entire mid-surface and confirm with middle mouse button. property window opens, right-click on the window to assign new property, usually for NASTRAN deck PSHELL property card is selected and also input thickness.

8) Mid-surface has to be checked for any geometric errors by Tools -> checks -> geometry. Measure the distance between nodes and cons by using Utilities-> Measure to check if all areas are passing minimum element length criteria if not, perform defeaturing on features which do not satisfy the required minimum element length criteria, by using Topo Module-> Faces-> cut option, right-click to remove any cut and left-click to make any new cut on the surface.

fig. 10- Measurement made at the fillet area does not satisfy minimum element length criteria of 3 mm. so defeaturing should be done by removing unnecessary cuts also remove the unwanted cuts in the flat surfaces of the part.

• Auto mid-surface extraction

9) Automatic mid-surface can be extracted by using Topo module-> Faces-> mid-surface->skin, select the entire mid-surface by left-click box selection and give a middle mouse confirmation. ANSA will automatically select the faces, by default outer face in green colour, inner face in red and thickness in blue colour. when give ok mid-surface is generated. Parent surface will be deleted in auto mid-surface extraction.

fig.11- Automatic mid-surface, faces selection of outer panel.

Fig. 12 - Mid-surface of inner panel.

Fig. 13- outer panel mid-surface.

Fig.14- Hinge reinforcement mid-surface.

Fig.15- latch reinforcement mid-surface.

• 2D MESHING

10) As we successfully extracted the mid-surface of all parts, we can now move to Mesh module. First input the element quality criteria required, by pressing F11 in the keyboard and enter the given values under shell tab, click on apply-> ok.

fig. 16- Inputting element quality criteria by clicking F11 in the keyboard

Fig. 17-Also input meshing parameters by using utilities-> Mesh parameter, enter mesh type, element type, average target length, minimum and maximum target length of element.

11) Now we can mesh the part by using, Mesh module-> Mesh generation-> best mesh /spot mesh, select the face we want to create the mesh, and confirm the selection with middle mouse click, thus mesh will be generated on the selected face. Each mesh generation function is based on preset algorithms in ANSA.

• ELEMENT QUALITY CHECKUP

12) To check if all elements in the mesh will pass the element quality criteria, we have to switch to Hidden mode( fig.18 see the orange circle at bottom toolbar). so we can see total number of off elements in the mesh and also in which property the element failed is indicated in different colours.

fig. 18- Indication of elements failed to satisfy the quality criteria

• REMESH-QUALITY IMPROVEMENT

13) we can improve the quality of element by reconstructing the mesh, Mesh module-> shell mesh-> Reconstruct, select the required area of mesh by ENT selection or the entire mesh by Macro area selection and confirm the selection with middle mouse click. ANSA will automatically reconstruct the mesh.

14) To avoid trias near to free edge, holes or on the fillet and to eliminate rotating quads, opposite trias formed we can use Mesh module-> Elements-> split, Swap, Join option , also Grids-> Paste option. After operation use Mesh module-> Shell mesh -> smooth option to smooth the mesh flow in selected areas.

• ASSIGNING THICKNESS TO THE MODEL

15) After successfully meshed all parts we can assign thickness to the material by pressing F11 and under presentation parameters tab select Draw shell as solid->Apply->ok.

• CONCLUSION

• completed Geometry cleanup of the model and extracted mid-surface of all 4 parts
• Successfully meshed all parts of hood model under required element quality criteria and assigned thickness to the model.
• OFF elements in the mesh pattern created is zero, and trias near to free edge, holes and on the fillets are eliminated.
• Also rotating quads and opposite trias formed in the model is removed.

fig. 19- Meshed inner plate before assigning thickness

fig.20- Meshed Inner plate after assigning thickness

fig.21- Meshed outer plate before assigning thickness

fig. 22- Meshed outer plate after assigning thickness

fig.23- Meshed Hinge part without assigning thickness.

fig.24- Meshed Hinge part after assigning thickness

fig. 25- Meshed Latch part before assigning thickness

fig.26- Meshed Latch part after assigning thickness.

Fig. 27- Final assembly of hood model

• Learning Outcomes

• Learned about how to fix geometrical errors like triple cons manually and perform geometry cleanup of the model.
• learned how to isolate certain parts of the model using focus functions ( or, And, Not, !Not etc.) to find out errors easily.
• learned to extract mid-surface both manually and by automatic method and isolating mid-surface using !Not function, how to take measurements of the parts between nodes, cons.
• learned about how to assign new PID to mid-surface and to perform defeaturing on mid-surface.
• learned to perform 2D meshing on the part under required element quality criteria.
• learned how to perform quality improvement of mesh generated and how to eliminate opposite trias, rotating quads in the mesh also to avoid trias near free edge, holes or on the fillet area.