Grid Independence Test in ANSYS Fluent

In Computational Fluid Dynamics (CFD), the accuracy and reliability of simulation results are significantly influenced by the computational mesh. The Grid independence test is a crucial step in ensuring that numerical results are independent of mesh resolution, optimizing computational efficiency without compromising accuracy. This article explores the concept of mesh independence, as well as a step-by-step implementation of the Mesh Independence Test in ANSYS Fluent is provided, along with a case study demonstrating best practices. By the end of this article, readers will have a clear understanding of how to achieve grid-independent solutions in CFD simulations, ensuring precise and reliable results.

Basic Concepts of Meshing in ANSYS Fluent

Meshing is the process of dividing a computational domain into discrete elements where the governing equations are solved. In ANSYS Fluent, different types of meshes can be used, including (see Fig.1):

• Structured Mesh: Organized in a grid format, providing better control over element quality.
• Unstructured Mesh: More flexible and can conform to complex geometries, though it may require more computational resources.
• Hybrid Mesh: Combines both structured and unstructured elements, offering a balance between accuracy and efficiency.

Understanding these concepts is crucial for conducting a successful grid independence study.

Figure 1: Different types of meshes in ANSYS Fluent [2]

What Is The Grid Independence Test?

The Grid Independence Test is a systematic process used in CFD simulations to determine the optimal mesh resolution where further refinement does not significantly affect key simulation results. By gradually increasing the mesh density and analyzing the corresponding changes in critical parameters (such as velocity, pressure, drag coefficient, or heat transfer rates), engineers can identify a point where the results converge and become independent of further mesh refinement. Therefore, choosing the right mesh resolution is key to achieving accurate velocity profiles while maintaining computational efficiency.

Figure 2 [2] demonstrates the importance of mesh resolution in CFD simulations. As the resolution increases from 4 to 32, both the velocity contours and vector fields reveal progressively more detailed and accurate representations of the flow. This highlights the necessity of conducting a grid independence test to ensure that the chosen mesh resolution is sufficient to capture the relevant flow features without excessive computational costs. The results suggest that at higher resolutions, the simulations are likely converging toward a more reliable and valid representation of the fluid dynamics in the given geometry.

Figure 2- Optimal mesh resolution and its impact on the accuracy of the velocity distribution (a) and velocity vector (b)

How Is Grid Independence Study Conducted in ANSYS Fluent?

In ANSYS Fluent, a Grid Independence Study is performed by generating multiple meshes with different levels of refinement and comparing the results of a key variable across these meshes. The goal is to ensure that the solution is not dependent on the grid resolution. The process generally follows these steps:

1. Define the Simulation Objective: Identify the key physical phenomenon being studied (e.g., heat transfer, aerodynamics, fluid flow).
2. Select a Key Variable: Choose a critical parameter such as temperature, velocity, pressure, drag coefficient, or Nusselt number based on the objective.
3. Generate Multiple Meshes: Create at least three different meshes: coarse, medium, and fine.
4. Run Simulations for Each Mesh: Solve the governing equations for each grid and extract the values of the selected key variable.
5. Compare Results: Analyze how the key variable changes across different meshes.
6. Assess Grid Independence: If the changes between successive mesh refinements become negligible (typically less than 1–2%), the results are considered grid-independent.
7. Select the Optimal Mesh: Choose the smallest mesh that provides accurate and stable results to balance computational cost and accuracy.

The figure 3 illustrates a flowchart that outlines the steps involved in a computational fluid dynamics (CFD) simulation process, with a particular focus on the Grid Independence Test, highlighted in the red box. In the provided flowchart, the red box highlights the “Grid Independence Test,” which is a critical decision point in CFD simulation process. This diamond-shaped element signifies a test that determines whether the results of the simulation are independent of the mesh resolution. Within the box, there are two pathways: “Success” and “Fail.” If the test is successful, the process continues to the “Analysis Data” stage, indicating that the results are reliable and can be further analyzed. Conversely, if the test fails, it suggests that the simulation results are still dependent on the grid size, necessitating further refinement of the mesh and a re-evaluation of the simulation process. This step is essential for ensuring the accuracy and credibility of the CFD analysis before concluding the study.

How Much Should We Refine The Computational Mesh?

Typically, the test begins with a coarse mesh, and the number of elements is gradually increased while monitoring the changes in the results. Initially, as the mesh is refined, the output values may fluctuate significantly, indicating that the solution is sensitive to the grid size. However, as the mesh continues to be refined, a point is reached where further increases in the number of elements lead to minimal changes in the results—this is where grid independence test is achieved.

A Practical Example: The Table below illustrates a grid independence test, where the pressure drop & filtration efficiency are monitored Particle Filtration DPM CFD Simulation With Brownian Force and UDF | ANSYS Fluent Training. Initially, for coarse meshes (239k- 434k elements), the pressure drop & filtration efficiency fluctuates significantly, indicating numerical instability and insufficient resolution. As the mesh is refined (532k elements and beyond), the results start to stabilize, suggesting reduced grid dependency. Beyond approximately 532k elements, further refinement leads to negligible changes in parameters, confirming grid independence. This indicates that an optimal balance between computational cost and accuracy is achieved around 532k elements. Overall, a well-executed mesh independence study in CFD not only enhances the credibility of the simulation by minimizing the numerical errors but also optimizes the balance between accuracy and computational efficiency.

Table 1: grid independence test where the pressure drop & filtration efficiency are monitored

Particle Filtration DPM CFD Simulation With Brownian Force and UDF | ANSYS Fluent Training

Conclusion

The Grid Independence Test is a critical step in CFD simulations, ensuring that numerical results are accurate, reliable, and independent of mesh resolution. By systematically refining the computational mesh and analyzing key parameters such as velocity, pressure, and drag coefficient, engineers can determine the optimal mesh resolution that balances accuracy and computational efficiency. In ANSYS Fluent, performing a grid independence study involves generating multiple mesh levels, running simulations, and comparing results to identify the point at which further refinement no longer significantly affects the solution. A well-executed grid independence study enhances the credibility of CFD analyses by minimizing numerical errors while avoiding excessive computational costs. This approach is essential for applications ranging from aerodynamics and heat transfer to multiphase flows and turbulence modeling. By adopting best practices in meshing and convergence analysis, engineers can achieve highly accurate and efficient CFD simulations.

FAQs

1. What is the purpose of a Grid Independence Test in CFD?

The Grid Independence Test ensures that simulation results are independent of mesh resolution, allowing for accurate predictions without excessive computational costs.

2. How do I determine if my grid is independent?

A grid is considered independent when further refinement results in negligible changes (typically less than 1–2%) in key parameters such as velocity, pressure, or drag coefficient.

3. How many mesh refinements are needed for a Grid Independence Study?

At least three mesh levels (coarse, medium, and fine) are recommended, but more may be needed depending on the complexity of the flow and convergence behavior.

4. What key parameters should I monitor during a Grid Independence Test?

Common parameters include velocity, pressure, temperature, drag coefficient, lift coefficient, and Nusselt number, depending on the simulation objective.

5. Should I always use the finest mesh possible?

No, using the finest mesh increases computational cost without necessarily improving accuracy beyond a certain point. The goal is to find an optimal balance between accuracy and efficiency.

6. How does mesh type (structured, unstructured, hybrid) affect grid independence?

Structured meshes generally offer better accuracy and convergence properties, while unstructured meshes provide flexibility for complex geometries. Hybrid meshes combine both advantages.

7. What is the typical process for conducting a Grid Independence Test in ANSYS Fluent?

The process involves defining the simulation objective, selecting a key variable, generating multiple meshes with varying resolutions, running simulations for each mesh, comparing results, and assessing grid independence based on the stability of the key variable.

8. How do I choose the appropriate mesh resolution for my simulation?

Start with a coarse mesh and gradually refine it while monitoring key parameters. The goal is to identify the point at which further refinement yields negligible changes in results, indicating that an optimal balance between computational cost and accuracy has been achieved.

9. What are the consequences of not performing a Grid Independence Test?

Failing to conduct a Grid Independence Test can lead to inaccurate results, misinterpretation of flow behavior, and potentially flawed design decisions. It may also result in wasted computational resources due to unnecessary mesh refinement.

Reference

[1] https://www.ssg-aero.com/news/structured-meshes-the-old-pipes-gives-the-sweetest-smoke

[2] Lee, M., Park, G., Park, C. and Kim, C., 2020. Improvement of grid independence test for computational fluid dynamics model of building based on grid resolution. Advances in Civil Engineering, 2020(1), p.8827936.