Pre-processing of an intake manifold component
AIM:
The aim of this project is to create a solid mesh for an intake manifold model using tetra mesh elements post to topology cleanup. This meshed finite element model is then checked for volume mesh quality criteria and if required refined to generate more appropriate mesh elements that could pass the required quality criteria.
PROCEDURE:
1. Geometry cleanup
Prior to checking the model for geometrical errors, the hot points in the model are refined to a nodal spacing (average target length) of 3 for both the perimeter and macro part of the model
Then a topology cleanup is performed on the model using the topo operation in the FACES block of the TOPO module.
Finally, the model is checked for geometrical errors in the check manager. If any errors in triple/single cons arise, ANSA in most cases would be able to clear it using the FIX command. IF the model passes for topology cleanup the following image can be then visualized.
2. MESH PARAMETERS AND QUALITY CRITERIA
Prior to meshing the user has to define the required mesh parameters. ANSA tries to follow this input data to give a more precise mesh. However, in some cases, these mesh characteristics of the generated mesh may deviate from the user-defined parameters. Hence, to obtain a more accurate mesh we set up the quality criteria to check for those elements that fail the defined parameters.
These failing elements can then be reconstructed using ANSA's efficient mesh refinement algorithms in the MESH module.
In this project, a mesh average target length of 3 is set up with the min and max target length being 1 and 5 respectively in the mesh parameters tab. The type of mesh to be modeled are trias and ortho-trias. These can be switched in between in the mesh generation block.
To check for deviating elements, the quality criteria for min and max length are chosen and its values are 1 and 5 respectively. Similarly, the tet collapse of the to-be generated volume mesh is also selected for error checks with a value of 0.2. Hence, any mesh that doesn't fall in the above-defined shell or solid mesh criteria can then be refined using any one of ANSA's inbuilt features.
3. SHELL / SURFACE MESHING
To generate the mesh we make use of the spot mesh generation feature in the MESH module. This feature allows us to capture certain regions or surfaces of the component that are enclosed within the cons. This helps us generate both trias and ortho trias at required surfaces thereby the whole components don't necessarily have to be modeled just by one.
The reason for selecting unstructured mesh elements like trias is because they offer a more flexible pattern compared to a hexa / brick element or a quad. Moreover obtaining accuracy or smooth mesh flow pattern (structured-like) while simultaneously implementing the flexible feature of the tetra elements we can make use of the ortho tria elements. These elements having orthogonal edges can be useful to generate a smooth mesh pattern if modeled correctly. Hence these elements are a more preferable alternative to brick elements.
These ortho-trias since they give a great convergence result, they are modeled at critical places where most of the load acts and at curves and fillets since normal trias which are equilateral wouldn't be able to capture the detailed surface. Hence these trias are used at straight surfaces where the required field variable magnitude is very minuscule.
MESHING
- Prior to meshing curved surfaces using ortho tria elements the cons in the model has to remodeled since having a clear pattern of cons in the model would be much easier for ANSA to generate a mesh with a smooth flow
- Cons in fillets that are very close to each other may fail in aspect ratio or min/max length since the spacing between hot points in parallel cons would eventually fail for the required target length
These cons can be eliminated by Macros -> Cut
- The recommended way to capture fillets and curved surfaces would be only using ortho tria elements. This can be achieved using spot mesh since spot mesh helps us capture surfaces within enclosed cons. However, an ortho mesh generation using spot mesh would not give a smooth pattern based on the model feature. Hence we can utilize the ANSA reconstruction algorithm to generate a more fine mesh
- It is also recommendable to create straight cons on fillets that discretize the fillet or curve to a certain layer. This helps us achieve a smooth mesh flow compared to creating a mesh on a fillet with no discretized con feature and it would probably need to be reconstructed. However, reconstruction wouldn't be required in the former method
- Similarly, other fillets can be discretized to follow a smooth pattern.
- Similarly, curves that fail to form a perfect pattern can be discretized using cons (Macros -> cut) to focus less strain on ANSA
- If basic mesh generation using spot mesh is able to capture features at the desired pattern, it is better to leave it be rather than reconstructing it since a few cases, this reconstruction could increase the number of elements thereby causing a strain on your hardware during computation.
- It is also preferable to reconstruct as a whole since each element will affect its neighboring one (if selected) thereby giving a smooth pattern overall
However, we should also make sure that we don't reconstruct a mesh that doesn't look random. This macro area can be excluded during reconstruction
- It is advisable to avoid reconstruction of mesh elements as much as possible (unless it's absolutely required) since reconstruction would often increase the number of mesh elements thereby increasing the computational cost
- While generating tria mesh at flat surfaces, regeneration of mesh should be applied only if there is a major deviation of the pattern since unstructured mesh elements would always be random and since it is performed at less crucial areas, reconstructing a flat tria mesh surface should be less prioritized.
- The reconstruction tool can also be used to eliminate any kinks or dips that may arise in the curved surface areas.
- It is advisable to have a mid-con at fillets that create a layer on the fillet for a smooth mesh pattern (as shown below)
- Creating tria elements at flat regions may always produce unorganized (unstructured) mesh flow hence it is always recommendable to perform a reconstruction or a smooth operation on the mesh to orient it to the best possible way.
- While modeling holes always make sure that the number of nodal elements present is even and in case if the number is redundant compared to the hole dia then to achieve accurate capturing we can increase the no of nodal points in the perimeter of the hole
- Use the reconstruction operation only on those surfaces that seem to be unstructured
- If you find it hard to cut cons inside the manifold it is recommended to use a cutting or sectional plane as shown below,
- Another important process, as a continuation to a previous point i.e fillets should have more than one layer should be carried out at each fillet.
Below shows what happens in case these layers are not created.
We can see that the fillet that is unlayered with a mid con has kinks, dips and even unstructured mesh compared to the adjacent row with a mid con. Hence each fillet at the maifold must be split into layers through the following mentioned process:
- To create a mid con, we can make use of the Middle command in the curves tab in the TOPO module to create a mid curve between two opposite selected pair of cons (as shown below)
This curve however fails to fall on the surface rather it just lies between both cons in the 3D space.
To orient or project it on the surface we use to Project command in the CONS tab to create a con on the mid of the two con pairs selection on the required face as shown below
NOTE that if this mid con is way too close with its adjacent cons it may cause min length error. In such cases it is advisable to remove these mid cons
MESH REFINEMENT
- Errors involving max length can be cleared by creating a cut at the middle over the entire row
- Other elements that fail for much similar quality can be joined with the adjacent one that fails to create a more appropriate aspect ratio in the element OR alternatively by performing a cut or split within the failing element.
- Few cases may fail because of unwanted cons that lie relatively close to each other
These can be eliminated by removing these interfering cons using macros -> cut
However, a few intricate features that are very small were not able to be captured using a target length of 3 and ranging from 1 to 5. These features can be captured only using a small target length (<1) which would be invalid in our case.
VOLUME MESHING
To create a solid mesh using the ANSA inbuilt unstructured mesh command, a volume has to be defined prior to mesh generation. The basic process by which a mesh is generated across the volume is by mapping the surface meshed elements on either side of the component.
- First, the volume has to be defined manually
- Then using any of the algorithm Tetra Rapid or Tetra FEM, we can generate the solid mesh as shown below,
Learning outcomes
Gained knowledge on,
- creating a surface mesh of either tria or otho tria elements using spot mesh
- more ways to refine the surface mesh
- creating volume mesh using tetra elements and checking for quality using tet collapse
- possible errors (volume intersection) during volume mesh of intricate geometries
- Eradicate tet collapse errors