Reviewing CFD Modelling Reports for Car Park Ventilation: Key Considerations

CFD simulation visualising jet fan airflow patterns in a car park.

Why Reviewing CFD Modelling Reports for Car Park Ventilation is Crucial?

Car parks are complex environments with varying airflow dynamics, from mechanical ventilation systems using jet fans and ducts to natural ventilation through ramps and openings. Proper ventilation is essential for controlling CO emissions and ensuring that air quality remains safe for users. CFD models simulate these ventilation systems and their effectiveness in a car park setting. However, reviewing these reports involves checking if the modelling approach, assumptions, and results align with the actual conditions and regulatory standards.

Jet Fan airflow simulation with CFD

Key Points to Check When Reviewing CFD Modelling Reports for Car Park Ventilation

1. Accuracy of Pollutant Source Representation

• Vehicle Emissions: The CFD model should realistically represent vehicle emissions, particularly CO. Check if the emission rates are based on actual vehicle types, engine types, and traffic patterns. Models that use generic or unrealistic emission rates may produce unreliable results.

• Pollutant Dispersion: Ensure the model accurately simulates how pollutants, especially CO, are transported through the car park. This includes verifying the airflow paths and pollutant concentration levels at various heights (750 mm to 1800 mm above the floor) where human exposure is critical.

2. Ventilation System Configuration and Effectiveness

• Inlet and Outlet Placement: Confirm that the air inlets and outlets are located and modelled according to the car park's actual design. If jet fans or ducts are used, verify that their placement and operational settings are correctly represented in the model.

• Flow Distribution: Review the model to ensure that airflow is distributed effectively throughout the car park, especially in areas with high vehicle density or near potential CO hot spots.

  
  

3. Carbon Monoxide (CO) Concentration Analysis

• Average and Peak CO Levels: Check that the report provides both average and peak CO concentrations at different levels of the car park. Pay particular attention to the 750 mm to 1800 mm height range, as this is where people are most likely to be exposed to harmful concentrations.

• Worst-Case Scenarios: Ensure that the model considers worst-case scenarios, such as high traffic volumes, equipment failure, or extreme environmental conditions, to test whether the ventilation system can handle these situations.

4. Mesh Quality and Resolution

• Mesh Refinement: Review the mesh used in the CFD model. The mesh should be refined enough to accurately capture high-speed airflow and turbulent regions, especially around jet fans and mechanical ducts. Poor mesh quality can lead to inaccurate simulation results, particularly in critical areas.

• Grid Independence: Check if the model has been validated for grid independence, meaning that further refinement of the mesh does not significantly change the results.

5. Turbulence Model Selection

• Appropriate Turbulence Models: Verify that the right turbulence models have been used for the CFD simulation. For complex airflow dynamics in car parks, turbulence models like k-epsilon or k-omega are typically employed. The model should be validated to ensure that it accurately captures turbulence, especially in high-speed flow areas near jet fans.

6. Compliance with Regulatory Standards

• Regulatory Compliance: Ensure that the CFD modeling report adheres to relevant standards such as AS1668.2 and the National Construction Code (NCC). These standards outline requirements for ventilation performance, CO concentration limits, and air quality, and should guide the CFD simulation and the design of the ventilation system.

7. Validation of Simulation Results

• Realism of Results: Examine the CFD results for consistency with real-world expectations. If possible, compare the simulation results with real-world measurements (such as air quality data or ventilation system performance) to ensure the model is accurately reflecting the car park's conditions.

• Sensitivity Analysis: Check whether a sensitivity analysis was performed to see how changes in assumptions (e.g., traffic flow, emission rates) affect the results. This can help identify the most critical variables in the system and ensure robustness in the design.

3D model of a car park showcasing structural geometry, including supporting columns and vehicle spaces.

Challenges to Look Out for in CFD Modelling for Car Parks

1. Turbulent and High-Speed Flow: Mechanical ventilation systems, such as jet fans, create highly turbulent and high-speed airflow. Accurately capturing these dynamics is crucial, but challenging. Poor modelling of turbulence can lead to an ineffective ventilation design, resulting in areas with poor air circulation and CO buildup.

2. Inaccurate Emission Modelling: Inaccurate assumptions about vehicle emissions can lead to misleading CO concentration results. Ensure that the emission rates are reflective of the actual operating conditions of the car park, taking into account vehicle types, traffic patterns, and car park usage.

3. Software Limitations: Not all CFD software is suitable for modelling complex car park ventilation systems. For instance, FDS (Fire Dynamics Simulator), though widely used for fire scenarios, may not be the most appropriate tool for simulating general airflow and pollutant dispersion in car parks. It's essential to assess whether the selected software is capable of handling the unique challenges of car park ventilation.

4. Complexity in Validation: CFD models should always be validated against real-world conditions. Without physical testing or clear benchmarks, it can be difficult to assess whether the simulation results are accurate. Therefore, CFD reports should include a comprehensive assessment of the simulation’s validity, particularly for extreme conditions.

   
   
   
   
   
   

   

   Carbon Monoxide Concentration dispersion visualisation.

   
   
   
   

Conclusion: Key Considerations for Reviewing Car Park CFD Modelling Reports

When reviewing a CFD modelling report for car park ventilation, it is crucial to ensure that the model accurately represents the real-world conditions of the car park, from pollutant emissions to airflow dynamics. The key points to check include:

• Pollutant Source Representation: Ensure that vehicle emissions are modelled realistically and accurately.

• Ventilation System Configuration: Confirm the accurate placement and operation of air inlets, outlets, and mechanical systems.

• CO Concentration Analysis: Focus on both average and peak CO levels, especially between 750 mm and 1800 mm above the floor.

• Mesh Quality: Verify that the mesh is sufficiently refined to capture complex airflow dynamics and turbulent regions, particularly around jet fans.

• Turbulence Model Selection: Check that the appropriate turbulence model is used, especially for high-speed airflow situations.

• Regulatory Compliance: Ensure that the report complies with local and national standards, such as AS1668.2 and the NCC.

• Validation and Sensitivity: Review whether the model has been validated against real-world conditions, and sensitivity analysis has been performed.

By focusing on these critical aspects, reviewers can ensure that the CFD report provides reliable, accurate insights into the performance of the ventilation system, helping to avoid unsafe designs and ensuring the car park's safety and operational efficiency.

References

1. AS 1668.2: 2012 - The Use of Ventilation and Air Conditioning in Buildings - Part 2: Mechanical Ventilation in Buildings.

2. AS 1668.2: 2024 - The Use of Ventilation and Air Conditioning in Buildings - Part 2: Mechanical Ventilation in Buildings.

3. National Construction Code (NCC) 2023.