Outline

1. OpenFOAM for Air QualityErnst Meijer and Ivo KalkmanFirst Dutch OpenFOAM SeminarDelft, 4 november 2010

2. Outline • Introduction to air quality • Application of CFD to air quality problems • Example case study • OpenFoam versus Fluent • OpenFoam 2D test case for urban wind profiles • Discussion and conclusions First Dutch OpenFOAM Seminar

3. Air Quality Issues First Dutch OpenFOAM Seminar

4. European guidelines for air quality Primary concern are health effects However allowed PM10 levels are still ~ 104 times too high In Netherlands air quality is connected to new building plans First Dutch OpenFOAM Seminar

5. Local Air Quality and Climate Field experiments Wind tunnel Models • Meeting European guidelines (NO2, PM10, PM2.5) • Evaluation of measures • Health assessment; black carbon aerosol • Urban Heat Island • Integrated assessment on environmental impacts (noise, heat, safety, …) First Dutch OpenFOAM Seminar

6. Application of CFD to AQ • Open field: gaussian Urban: wind tunnel • Gaussian approach not suitable for urban environment • Windtunnel has ‘real turbulence’, but limited capacity • Windtunnel gives limited number of information (‘scaled’ field exp) • CFD offers capacity • CFD gives full 3D, t information • CFD allows for chemistry, depositon, multi-phase, heat exchange, … First Dutch OpenFOAM Seminar

7. Example study: air quality near a tunnel exit • Establishing annual mean NO2 and PM10 concentrations (2015) • Evaluating measures to reduce concentration First Dutch OpenFOAM Seminar

8. Set up calculations Ansys Fluent • RANS simulations with k-ε RNG • Computational domain 500m x 300m x 90m • Logarithmic wind/turbulence profiles with z0 = 2m • Traffic induced momentum • 4 tunnel ventilations (0.1 m/s, 1.25 m/s, 3.0 m/s, 4.0 m/s) • Stationary flow calculations for 12 wind directions • Tracer dispersion calculations per source (tunnel exit, streets) Post processing to annual mean concentrations, based on: • Wind statistics (KNMI) • Background concentrations (RIVM) • Traffic data (#vehicles, emission factors) Calibrating the CFD results • Passive NO2 observations for a 8 weeks period • Adjust tunnel ventilations speed for best fit with measurements First Dutch OpenFOAM Seminar

9. First Dutch OpenFOAM Seminar

10. observations ‘raw’ CFD results calibrated CFD results First Dutch OpenFOAM Seminar

11. From Fluent to OpenFoam • Practical • Costs • AQ require large domains and many computions (48 in example) • Specific for atmospheric flows and AQ • Surface layer is important (concentrations at 1.5 m) • Non-neutral conditions, i.e. stratification, thermal inversions, convective ABL • Tool development • Data assimilation • Coupling of regional, urban, street scale models First Dutch OpenFOAM Seminar

12. Test 1: Comparison Fluent & OpenFOAM • After Blocken et al. (2007) • RANS standard k-εmodel • 2D domain, 500 m high, 10 km long • Hexagonal grid, cell density graded towards ground. Smallest cells 50 cm high & 10 m long • 2nd order discretization & interpolation schemes • Logarithmic ABL velocity profile at inlet (airspeed of 18.5 m/s at top of domain) • Ground roughness height 0.012 m First Dutch OpenFOAM Seminar

13. 0 m 1000 m 10000 m Velocity First Dutch OpenFOAM Seminar

14. 0 m 1000 m 10000 m Turbulent Kinetic Energy First Dutch OpenFOAM Seminar

15. 0 m 1000 m 10000 m Turbulent Dissipation Rate First Dutch OpenFOAM Seminar

16. Actual velocity profile known from measurements: Test 2: airflow in a street canyon • RANS standard k-εmodel • 2D domain, 500 m high, hexagonal grid, 0.5 x 0.5 m sized cells near ground • Periodic boundary conditions • 2nd order discretization & interpolation schemes • Building reference geometry: 15 m high, 10 m wide, 30 m separation • Average airspeed of 5 m/s over inlet • Building & ground roughness height 0.01 m → Determine z0, d and u*ABL for different geometries First Dutch OpenFOAM Seminar

17. Wind speed independence First Dutch OpenFOAM Seminar

18. Effect of separation 30 meters separation: d = 14,2 m, z0= 0,20 m, u*ABL=0,74 m/s 50 meters 100 meters 15 meters 10 meters 30 meters 5 meters First Dutch OpenFOAM Seminar

19. Effect of height 15 meters First Dutch OpenFOAM Seminar 100 meters

20. Effect of width First Dutch OpenFOAM Seminar

21. Limitations • Solving on a coarse grid and mapping solution onto a finer grid often necessary • Test 2: • Excessively large number of iterations needed; typically 600,000 • Spurious problems with numerical stability, even after optimization of stability parameters • Possibly connected with the average speed BC on inlet First Dutch OpenFOAM Seminar

22. Conclusions • Test 1: Good match between OpenFOAM and Fluent results! • Test 2: Calculated wind speed profiles match known velocity profiles • Values of derived parameters mainly depend on the presence of large-scale recirculation zones between the buildings (present when height/separation >≈ 0,3 • Velocity at ground level highest when height/separation ≈ 1 • Results are in agreement with findings of other studies • OpenFOAM is applicable for AQ and has many advantages • Still lots to be done… • Unstable/stable atmospheric boundary layers • Tracer dispersion (OpenFOAM mesh and volume sources?) • Moving from RANS to LES First Dutch OpenFOAM Seminar

23. Thank you for your attention! Dutch OpenFOAM User Group First Dutch OpenFOAM Seminar