Solution-Verification in CFD – Why is it important and who performs it?

This FAQ first provides an overview of the difference between verification and then addresses a crucial aspect of quality assurance in CFD: solution verification. It is an essential step to ensure the accuracy and reliability of CFD calculations. Below, we will explain the key aspects of solution verification in CFD based on the WTG Merkblatt M3 "Numerische Simulation von Windströmungen" and provide practical advice for your work.

A. Validation vs. Verification

First, it is important to distinguish between validation and verification. Validation shows that the correct equations are solved, meaning the simulation solves the given task accurately enough with the chosen model. For CFD applications, a distinction must be made between "program verification" and "solution verification". Program verification is meant to prove that a software program calculates correctly within its conditions, solving the equations correctly. The review of the calculation, i.e., solution verification, aims to ensure that the calculation is internally consistent, meaning a stable solution has been reached where the expected effects occur and no longer depend significantly on the model used. While program verification is conducted by the software manufacturer, solution verification is always performed by the user.

B. Why is Solution Verification so Important?

1. CFD models are always approximations of reality. They involve simplifications and assumptions that can lead to deviations.
2. The underlying mathematical equations are often not exactly solvable and require numerical methods.
3. The responsibility for the quality of the results lies with the user. The user must ensure that the correct model is used for the specific task.

In contrast to solution verification, program verification is carried out and documented by the software manufacturer, such as Dlubal Software GmbH. For this, we refer to our extensive verification examples, knowledge base articles, and FAQs on RWIND.

C. Concrete Steps and Checklist for Solution Verification

The WTG Merkblatt M3 "Numerische Simulation von Windströmungen" provides a concrete checklist for solution verification in section 5.2:

Modeling

• Are the desired effects represented by the chosen model?
• Has the correct scale been used?
• Is the study area chosen sufficiently large? (see 4.1.3)
• Is the blockage ratio sufficiently low? (see 4.1.3)
• Have the boundary conditions been correctly formulated?
• Are the input values sufficiently described (bandwidth, variability)?

Mesh Quality

• Is the grid fine enough at critical points?
• Is the influence of the grid on the solution known?
• Does the grid work equally well for different flow directions?

Numerical Parameters

• Has convergence of the desired target values been achieved?
• Is the time resolution fine enough for the expected phenomena?

Plausibility

• Does the flow occur in the correct direction?
• Are the separation points of the flow plausible?
• Are the pressure and suction coefficients plausible?
• Are the fluctuations in wind speeds plausible?
• Is the distribution of turbulence parameters appropriate?

This list provides the minimum criteria for verifying the various aspects of a CFD solution and can be individually extended.

D. Summary and Outlook

Solution-verification is an important aspect of CFD on the user side to ensure the reliability and accuracy of simulations. Through the systematic evaluation of modeling, mesh quality, numerical parameters, and plausibility, engineers can verify and confirm that the implemented simulations provide realistic and trustworthy results. The verification effort is largely determined by the complexity of the model, which should be justified by the requirements of the investigation problem. Special attention must be given to the necessary care in conducting solution-verification, as it requires a balanced relationship between effort and depth of investigation.

Sources

• Windtechnologische Gesellschaft WTG e.V. (2023). WTG-Merkblatt M1 – Wind Tunnel Experiments in Building Aerodynamics. Aachen: WTG.
• Windtechnologische Gesellschaft WTG e.V. (2023). WTG-Merkblatt M3 – Numerical Simulation of Wind Flows. Aachen: WTG.
• VDI e.V. (2015). VDI Guideline 6201 – Software-Supported Structural Analysis. Düsseldorf: VDI.