Multi Level Meshing

Multi-Level Meshing for a Hanging Node Geometry using ANSYS - Icepak:

Objective: The aim of the project is to lessen the mesh count and improve the mesh resolution by using the multi-level parameters for the components in the CAD model.

Problem Description:

The Icepak model is given for this assignment. Open and unpack the given hanging_node.tzr model.

Without any modifications, the model results in about 700,000 elements. Note that this mesh count results from using the non-conformal meshing technique, which reduces mesh bleeding and mesh count.

You can reduce the mesh count and improve the mesh resolution on and around the CAD objects by using the multi-level meshing technique.

This procedure starts with a coarse background mesh and resolves fine level features through a series of successive mesh refinements. 

By using multi-level meshing and the uniform mesh parameters feature, you can reduce the mesh count to approximately 500,000 elements and improve mesh resolution.

Geometry:

• The below picture the given CAD Model to generate the mesh using the multi-level parameters.
• The Geometry contains Fan, Capacitors, solid blocks, heat sink, and printed circuit boards, etc.

Meshing:

• For the first time, the mesh is generated for the Mesh assemblies separately. Which means a non-conformal mesh is performed.
• A non-conformal mesh, when there are a partial or zero matchings of nodes at the interface. This means that one, few, or every node on one side of the interface doesn’t have a match on the other side of the interface.

• For a general non-conformal mesh the number elements produced are 759498 and the number of nodes is 975478 with coarse mesh is in use.

Use mesh separately option with slack values:

• When the Mesh separately options for an assembly is turned on, the offset added to Assembly in respective direction is called Slack Values.
• By default, the slack values are zero in all directions. However, in order to use non-conformal mesh, the slack values should always be a positive number except it can be zero when the non-conformal boundary touches a hollow object or cabinet boundary.
• The Slack values should be specified in such a way that cells on both sides of the interface are either fluid or solid, with the same material, and it does not violate any of the "Non-Conformal Assembly Rules".

Use object parameters:

• Icepak offers the capability of assigning a parameter to almost any input such as power value, object size, flowrate, object active or inactive, etc.
• These parameters can be defined using the Parameters and Optimization in Icepak (under Solve and select Run Optimization).
• To assign a parameter to an input in your Icepak project, enter the “$” symbol before the name you wish to call your parameter (for example, $fin_count, for a number of fins of a heat sink).
• For each parameter, when assigned for the first time, you will need to provide an initial value for that parameter.

Use set uniform parameters and use average on 1 assembly, in multi-level meshing tab allow multi-level meshing up to level 2 for the objects.:

• To set the uniform parameters, we need to combine the given components in the CAD Model that are represented in blocks.
• Here to combine the components, we need to use the option called Assembly.
• After assembling the components we need to give the multi-level meshing at once. It automatically takes for entire components inside the assembly.

• Without any manual effort, automatically all the components inside the assembly, it's  multi-level change to 2.

Use the same technique of 1:

• By using the same technique, we need to assemble the remaining components and give a name to that assembly.
• The process is the same as we did for the other assembly above.

• Here in this assembly also we have used the multi-level 2.

Generate mesh:

• Now by allowing the multi-level meshing of max-level 2, the mesh is generated.
• What is multi-level meshing means, it subdivides the geometric into no. of parts that consume a large number of elements.
• While meshing a lengthy or a huge element it takes a number of elements and nodes to generate. These elements also have an effect on the neighboring elements in the domain.
• This causes the mesh bleeding phenomenon and leads to incorrect results.
• So, applying the multi-level meshing is benefiting in case of computational time and giving accurate results.

• We can see the mesh bleeding in the above picture. This mesh domain is before implementing the multi-level meshing.

• By observing the above picture we can come to the clarification that what is the effect of multi-level meshing and what it makes to the mesh bleeding.

• It is the cut plane of the given CAD Model in the Z-direction.
• The above picture shows the behavior of elements on one of the CAD components call fan case and heat sink.

• This is the total meshing of the components that are present in the given CAD model. 
• Here we can see the meshing dine for components like a heat sink, grills, some solid components like cylinders, rectangles, etc. Also meshing for the heat sources.
• A solid figure of where the mesh is applied to the CAD model.

Observe in mesh reduction:

• As we implemented the multi-level meshing in the mesh set up the number of elements and nodes are gets reduces and the quality of the meshing is also increased.
• The quality of the mesh is improved. We come to the conclusion on this point by seeing the mesh applied by the solvers. 
• The mesh bleeding, if we compare the meshes that are before multi-level and after multi-level, the mesh bleeding is fully reduced. This is called the improvement of the mesh quality.

• The number of elements generated is 670106 and the number of nodes is 837045.
• By implementing the multi-level meshing technique, as a result, we reduced more than 100,000 elements and nodes. 
• At the same time, the quality of the mesh is also improved.
• The number of elements is even more reduced and comes close to 500,000 when we use the multi-level meshing with max. level increased to 4. 

Do visual inspection:

• It is an analyst's responsibility to evaluate the quality of your finite element mesh and to determine if its results can be used to make engineering decisions.
• This section introduces two of the tools that Ansys provides for this purpose: element shape testing and energy error estimates.
• It also presents some guidelines that can be used to evaluate the quality of a mesh qualitatively by visual inspection and quantitatively through mesh convergence.
• Visually we can clearly say that the number of elements and nodes is reduced using a multi-level meshing technique. The quality of mesh, mesh bleeding also eliminated.

Check for quality:

• The quality of the mesh plays a significant role in the accuracy and stability of the numerical computation.
• The attributes associated with mesh quality are face alignment, volume, and skewness.
• Regardless of the type of mesh used in your domain, checking the quality of your mesh is essential.

Face alignment:

The face alignment is a measure of mesh quality defined by the face-alignment index bar (C0.C1).bar(f)

• Where C0 and C1 are the centroids of the two adjacent elements and is the normal vector to the face between the two elements.
• Range from 0 (bad) to 1 (good).
• Face alignment  >0.1 (preferred), >0.5 (must), 0.15> (best).

Volume:

• Volume is, the preferable volume of mesh to be greater than 1e-13 for single-precision solver.
• Volume >1e-15 for double precision solver as long as the total magnitude of the model is <12(i.e. 1e-15 - 1e-13).
• Max/min cell volume should be <10^7 for a single-precision solver.

Skewness:

• It is one of the primary quality measures for a mesh. According to the definition of skewness, a value of 0 indicates an equilateral cell (best) and a value of 1 indicates a completely degenerate cell (worst).
• Skewness is defined as the difference between the shape of the cell and the shape of an equilateral cell of equivalent volume.
• A general rule is that the maximum skewness for a triangular/tetrahedral mesh in most flows should be kept below 0.95, with an average value that is less than 0.33.

Conclusion:

A multi-level mesh technique is implemented for the given CAD model to reduce the number of elements and nodes and at the same time to improve the mesh quality. As we mentioned in the object, the number of elements is reduced to 100,000 and the same for a number of nodes also. By implementing the multi-level meshing max. level to 4, we can achieve the number of elements and nodes approximate to the 500,000. 

Another important point here is mesh bleeding, the mesh bleeding is significantly reduced compared to the non-multi-level mesh model. This gives that the quality of the mesh is improved while removing the elements.