Every time a CFD problem is solved, no influence from the mesh configuration is desired in the final results. During our study, different ways of constructing meshes for a heat transfer of a car interior were analyzed in an attempt to find an optimal model that could allow an acceptable precision when comparing the results with the ones obtained from experimentation. Having the intention of understanding the influence of the mesh over the results, all possible ways of CFD meshes construction were analyzed, as well as the variables that could be modified.

1. MESH DEPENDENCY
Ivan Dario Arroyave Zuluaga
Automotive Engineering Research Center
(CIMA)
Tecnológico de Monterrey Campus Toluca
México

2. Opportunity Statement / Expected Outcome
Current State
In a OEM, Cabin & Thermal models are meshed for analysis using tetrahedral mesh
with generic requirements recommended by the Software Vendor. CFD engineers
have encountered significant differences in the simulations when the grow rate and
type of element is changed.
Desired State
Find the required mesh type and mesh requirements for each analysis creating Best
practices.

3. Robust Design
• We are looking for a new general methodology to
build grids for CFD problems; we would like to do
our mesh design robust for different types of
elements. Hence we selected robust design to
carry out our analysis.
• This is the first approach to mesh dependency
problems; we want to understand not only how
to optimize the mesh construction, but also
figure out if Robust Design is a good tool to do
this optimization.

4. Background

5. Background

6. Background

7. Background

8. Opportunity Statement / Expected Outcome
Defroster Side Windows Defroster Windshield Physical
Physical test patterns test patterns

9. Opportunity Statement / Expected Outcome
Expected Outcome (Specific physics test):
• Understand sensitivity of meshing characteristics of the determined factors in the event
of thermal analysis
Constrains:
• Software capability, computational cost

10. Develop Concept: Current Best Practices

11. Develop Concept
The following parameters were considered critical for mesh construction
using Hypermesh
A. Tetra Number of uniform layers B. Tetra Growth rate C. Number of BL layers
4 1.6
1 1.1
D. First layer thickness F. BL Growth rate:
1mm 1.4
0.6mm 6
1 0
Noise/Tetra to Polyhedral
• YES
• NO
C,D,F
A
B

12. Parameter Diagram
Control Factors:
Tetra Number of uniform layers
Tetra Growth rate
Number of BL layers
First layer thickness
BL Growth rate
System
Input: Outputs:
2D Mesh, Boundary Fluid Mesh process fit against
condition physical test
results multiple
response
Noise Factors:
(R1,R2,R3)
(e.g.) Tetra to polyhedral
Noise Factor 1
Noise Factor 2
Symptoms:
Noise Factor 3
Meshing time
Solve time
Quality Mesh

13. Control Factor Strategy
A - Tetra Number of uniform
layers TNUL1 1 Layers
• Tetra Number of uniform layers TNUL2 2 Layers
TNUL3 3 Layers
levels comprehend benchmark TNUL4 4 Layers
observed typical values
B -Tetra Growth rate TGR1 1.1 rate
TGR2 1.2 rate
• Tetra Growth rate levels is TGR3
TGR4
1.4
1.6
rate
rate
selected for its current best
practices and the lower limit that C -Number of BL layers NBL1 0 Layers
NBL2 2 Layers
allows software. NBL3 4 Layers
NBL4 6 Layers
D– First layer thickness FLT1 0.6 mm
• Number of BL layers , First layer FLT2 0.8 mm
thickness And BL Growth rate is FLT3 0.9 mm
FLT4 1 mm
chosen to explore different ways to
achieve fill out space without F - BL Growth rate BGR1 1. rate
BGR2 1.2 rate
interference between one surface BGR3 1.3 rate
and its opposite. BGR4 1.4 rate

14. Noise Factor Strategy
Full factorial, two cases
• First case one factor:
– Tetra to Polyhedral (2 levels)
• Second case Three Factors
– Noise factor 1 (2 levels)
– Noise factor 2 (3 levels)
– Noise factor 3 (3 levels)
Response Strategy
Separated Analysis
• R1. Numerical-fit physical results
– % Defroster area 25 minutes.
– % Defroster area 35 minutes.
• R2. Qualitative-fit physical results
• R3. Solving time

15. Optimization Details response 1
Noise factor polyhedral
Due to the parameters and parameters levels, an L16 orthogonal array was
chose. The objective is to fit the response to experimental value, the
nominal is best formulation is selected.

16. Response Plots for Means

17. Response Plots for S/N

18. Results for one Response
• Optimal levels and factors for Means
– Factor A Level 3
– Factor B Level 4
– Factor C Level 1
• Optimal level and factors for S/N
– Factor A level 2
– Factor B level 2
– Factor C level 1

19. Conclusions
• Hypermesh is versatile enough to carry out an experimental mesh
dependency for CFD Thermal Analysis.
• This analysis should be carried out for each phenomena.
• Variables (e.g. iteration convergence) from the specific solver should
be considered as a response, in order to get general mesh
construction rules.
• This study is the first step to create rules for optimal mesh
generation process.
• Robust Design could be a useful tool to analysis Mesh Dependency
when the quality of mesh does not interfere with the convergence
speed.

20. Thank you