Challenges in Meshing of Thermal Simulations
Mesh independence is crucial because it ensures that the results of a simulation do not change with different mesh densities. This is important for obtaining accurate and reliable results. A mesh-independent solution means that the results remain consistent even as the mesh is refined. This is achieved by specifying an appropriate element size and using meshing controls to define the element size accurately. For instance, by reducing the element size in high temperature gradient regions, you can achieve a more mesh-independent solution. Additionally, a good mesh captures all the important geometric details and avoids convergence difficulties, which can lead to errors. The accuracy of results is highly dependent on mesh quality, and a solution is considered mesh-independent when the important geometric details are well captured, and the mesh quality criteria are met.
AEDT Icepak Solution to Meshing
AEDT Icepak offers two different approaches to meshing:
1. A one-click Slider Bar method is available that is automated and has five different levels of mesh resolution. This approach is suggested at the first attempt at meshing. It is found to work for most of the practical geometries, and provides a good quality starting mesh. Once the model starts to generate reasonable results, a mesh dependency should be conducted. This can either be done by increasing the mesh resolution in the slider bar method or using the advanced meshing techniques.
2. Advanced meshing techniques. These approaches are used to keep mesh count low and/or for complex geometries with small features.
a. Mesh Regions: Subregions within the computational domain may be defined using a bounding box. For each mesh region "Manual Mesh Controls" may be used for better control of mesh size.
b. Multi-Level Meshing (MLM): When this option is used, the mesh automatically subdivided into two as it approaches smaller features. User can also specify number of levels to this subdivision. It is suggested when the CAD geometry has curvatures.
Engineering Application
Let's define a fairly simple electronics cooling problem: A heat generating chip package with a heat sink on top that also has convection air cooling as shown below:
Fig. 1 Thermal problem schematic
The Slider Bar meshing tool in AEDT Icepak generates a good quality 99k hexahedral mesh with a single mouse-click (at level-3 resolution) as shown below.
Fig. 2 Top view of mesh with heat sink region automatically resolved
If we further increase mesh resolution with the Slider Bar method, we do not get any further significant mesh refinement. The temperature field on the bottom surface of the heat sink is compared at level-3 and level-5 show very similar contours with a maximum variation of 0.2 C.
Fig. 3 Temperature contours: Level-3 versus Level-5 Slider Bar meshing
Similarity of these two solutions now gives us more confidence about the mesh and convergence. Let's improve our mesh in the heat sink region which has fine features and is the area of interest. For this purpose, we will add a Mesh Region around the heat sink. This can be easily achieved by selecting heat sink solid objects, right-mouse-click and selecting "Assign Mesh Region". In Fig. 1, the thin gray outlines for the Mesh Region is visible. We will use "Advanced" tab for defining the mesh size and use the settings below:
Fig. 4 Mesh Region settings: No MLM meshing
A nine minute youtube video is available here with step-by-step meshing instructions.
These Mesh Region settings along with level-3 global mesh settings will produce only a 152k hexahedral cell (see figure below). The mesh in the heat sink region is finer with 2 cells for each fin and 5 cells between fins. Care was taken to ensure that non-conformal interfaces do not have more than 1-to-4 connection.
Fig.5 Top view of mesh with Mesh Region colored green
When the heat sink region is resolved with finer mesh, we observe the peak temperatures go up by about 0.7 C. It seems that we are very close to to a mesh independent solution.
Fig. 6 Temperature contours: Level-3 Slider Bar meshing vs Refined meshing in heat sink region
Let's further reduce the maximum cells size in the Mesh Region (see Fig.4) of in directions of x to 0.1 and z to 0.1 mm. The 345k mesh in the heat sink region is finer with 3 cells for each fin and 9 cells between fins. Fig. 7 compares the two Mesh Region solutions where the mesh resolution has been doubled. The peak temperature is within 0.1 C that indicates a mesh independent solution.
Fig. 7 Temperature contours: Mesh Region method mesh vs Doubled resolution mesh
Please note that we have not covered Multi Level Meshing technique in this blog. The heat sink geometry (due to its squared edges and rectangular structure) will be meshed very similarly with or without the MLM approach.
Ansys Solution Benefits
Customers of Ansys Icepak are able to:
1. Quickly predict a reasonably accurate thermal solution with one-click meshing.
2. Improve upon the mesh by easy to use advanced meshing techniques to achieve mesh independent solutions.
Ozen Engineering Expertise
Ozen leverages its extensive consulting expertise in CFD, FEA, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges like multiphase flows, erosion modeling, and channel flows using Ansys software.
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- Ozen Engineering Inc. Blog: Cold Plate Modeling with Ansys AEDT Icepak
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- Ansys Blog: Ansys Electronics Reliability
