This document summarizes a study comparing wind tunnel and computational fluid dynamics (CFD) modelling for near-field dispersion from an exhaust stack. The study found that while CFD and wind tunnel results agreed for downwind receptors, CFD showed lower dilution and was inconsistent for receptors not directly downstream. Key takeaways are that complex flows challenge steady-state CFD, guidelines for exhaust assessments using CFD are needed as the field matures, and future work should consider multiple wind directions.

1. COMPARISON OF A WIND
TUNNEL AND CFD MODELLING
FOR NEARFIELD DISPERSION
FROM AN EXHAUST STACK
Ruth Shilston MEng CEng MIMechE
IAQM DMUG 19th April 2018

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3. Study
Motivation

7. K-Omega Realizable K-Epsilon

8. RWDI study
approach

9. Source
R1 R2
Source
R1 R2
Modelling Approach

12. Parameter
Mesh size YES
Turbulence model YES
ABL treatment YES
Building Roughness
Steady State YES
Unsteady
LES
10H
5H
5H
K-Omega Realizable K-Epsilon

13. 0.27 Million Cells
1.78 Million Cells
13.2 Million Cells

14. Study Results

15. R1 R2
Source
North

16. K-omega K-epsilon RK-epsilon
Mesh 1
Mesh 2
Mesh 3

18. Results: CFD vs Wind
Tunnel North Wind
R1
R2
Wind
Tunnel

19. Results: CFD vs Wind
Tunnel South Wind
Wind
Tunnel
R1
R2

20. Results: CFD vs Wind Tunnel South Wind
Mesh 1 – k-Omega Mesh 2 – k-Omega

21. Final Thoughts

23. Key Takeaways
Complex flow features challenged the steady state CFD.
Downwind receptors registered “hits” in both CFD and
wind tunnel however CFD results had lower dilution.
CFD was inconsistent when receptors were not
downstream of source.
In a real project, we would consider 36 wind directions
to give a full assessment of the exhaust performance.
As research matures guidelines should be established.

24. Questions