Building Trust in CFD: Validation, Verification, and Calibration for Wind Simulation Acceptance

Introduction

As computational methods continue to evolve, computational fluid dynamics (CFD) has become a promising alternative or supplement to wind tunnel testing in structural wind engineering. However, for CFD to gain widespread acceptance among engineers, authorities, and reviewers, its reliability must be ensured through rigorous verification and calibration procedures. This article outlines the key steps and principles behind these processes.

According to Section 1.5 of EN 1991-1-4, numerical simulations may be used as a supplement to calculations and physical wind tunnel tests, as long as they are proven and/or properly validated. This enables engineers to obtain reliable load and response information using accurate models of both the structure and the natural wind environment. Similarly, ASCE 7-22, through reference to ASCE 49, recognizes that while CFD is increasingly applied in wind engineering, its use must be carefully controlled. Since there is currently no dedicated standard detailing the full procedures for CFD in this context, ASCE emphasizes that any application of CFD to determine wind loads on the Main Wind Force Resisting System (MWFRS), Components and Cladding (C&C), or other structures must undergo peer review and a Verification and Validation (V&V) study to ensure the accuracy and reliability of the results.

1. Why Verification and Calibration Are Necessary

CFD simulations are highly sensitive to numerous factors, including:

• Turbulence models

• Inflow boundary conditions

• Mesh quality and resolution

• Solver settings and numerical schemes

Without proper verification and calibration, CFD results may look visually convincing but can be misleading or non-conservative in real-world structural applications. Both EN 1991-1-4 and ASCE 7-22 acknowledge the potential of numerical methods, such as CFD, for determining wind loads, provided that these methods are properly validated and verified.

2. Verification vs. Validation vs. Calibration: Definitions

It is essential to distinguish between these terms, which are commonly used in place of one another:

• Verification ensures that the CFD model is solving the equations correctly (i.e., the code and numerical setup are free from errors).

• Validation assesses whether the model accurately represents the physical behavior of the real-world system (typically through comparison with wind tunnel or full-scale data).

• How to Conduct Validation Example in RWIND

• Calibration involves adjusting model parameters to align CFD results with known or measured data.

3. Verification Procedure

Verification involves checking that:

• The mesh is sufficiently refined (mesh sensitivity study)

• The time step and numerical scheme are appropriate

• The boundary conditions are implemented correctly

• The solver converges consistently

This includes:

• Grid Convergence Index (GCI) analysis

• Residual monitoring and time-averaged stability checks

• Code-to-code comparison (benchmarking against trusted solvers)

• Applying Wind Loads in RWIND for Accurate Structural Analysis

4. Validation with Experimental Data and Standards

The most critical aspect of CFD acceptance is validation against physical test results, such as:

• Wind tunnel measurements

• Full-scale field monitoring (e.g., pressure taps, anemometers)

Key steps include:

• Reproducing the test setup: Geometry, terrain roughness, and inflow turbulence must match the experiment.

• Comparing quantities of interest: Mean and peak pressure coefficients, force/moment coefficients, or flow field characteristics.

• Statistical analysis: Use of RMS error, correlation coefficients, or normalized deviation metrics like:

• WTG Manual Examples

Validation should be structure-specific, especially for both typical and unusual geometries like:

• Antenna examples in collaboration with RWTH Aachen University

• Single Sharp-Edged Antenna Validation Model from RWTH Aachen University

• Three Sharp-Edged Antenna Validation Model from RWTH Aachen University

• Six Sharp-Edged Antenna Validation Model from RWTH Aachen University

• Validation Example of the Kathrein Antenna Model 80010804 in Collaboration with RWTH Aachen University

• Based on typical shapes defined in design codes (EN 1991-1-4, ASCE/SEI 7, NBC 2020) and experimental studies (TPU, AIJ)

• Validation Example According to Standards (EN 1991-1-4, ASCE 7-22, etc.)

5. Calibration Strategy

If minor deviations exist after validation, calibration can be performed by adjusting:

• Inflow turbulence intensity and length scales

• Turbulence model constants (with caution)

• Surface roughness and wall functions

However, over-calibration must be avoided, as it can lead to a model that is tailored to one case but unreliable elsewhere.

6. Documentation and Traceability

For CFD results to be accepted by building authorities or certifying engineers, the process must be:

• Transparent: All input parameters, solver settings, and assumptions documented

• Repeatable: Other experts should be able to reproduce the results

• Traceable: Validation cases must be linked to published benchmarks or experimental references

7. Integration into Structural Design Workflow

Finally, for CFD to be accepted in structural load determination:

• The CFD outputs (e.g., pressure distributions) must be transferred to FEM software (e.g., RFEM)

• Load combinations must follow LRFD or EN design combinations

• The pressure data must represent statistically valid wind actions (e.g., 50-year return period)

Conclusion

CFD holds great promise for the future of wind engineering, offering cost-effective, flexible, and detailed insights into aerodynamic behavior. However, its acceptance depends entirely on disciplined verification, robust validation, and careful calibration. When performed systematically, CFD can become a trusted component of a “design by analysis” approach, supported by standards and engineering best practices.