OpenFOAM for Air QualityErnst Meijer and Ivo Kalkman

First Dutch OpenFOAM SeminarDelft, 4 november 2010

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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

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Air Quality Issues

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European guidelines for air qualitySpecies Exceedence level

Nitrogendioxide (NO2)

Annual average 40 μg/m³

Hourly average max. 18 time/yr > 200 μg/m³

Particulate Matter (PM10)

Annual average 40 μg/m³

Diurnal average max. 35 times/yr > 50 μg/m³

Primary concern are health effectsHowever allowed PM10 levels are still ~ 104 times too highIn Netherlands air quality is connected to new building plans

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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,

…)

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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, …

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Example study: air quality near a tunnel exit

• Establishing annual mean NO2 and PM10 concentrations (2015)• Evaluating measures to reduce concentration

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Set up calculationsAnsys 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

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observations‘raw’ CFD resultscalibrated CFD results

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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

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Test 1: Comparison Fluent & OpenFOAM

• After Blocken et al. (2007)• RANS standard k-ε model• 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

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Velocity

2 4 6 8 10 12 14 16 18 200

50

100

150

200

250

300

350

400

450

500

Velocity [m/s]

Hei

ght

[m]

Velocity for distances 0-10000 meters along the ground

OpenFOAM

Fluent

0 m1000 m10000 m

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Turbulent Kinetic Energy

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 30

50

100

150

200

250

300

350

400

450

500

Turbulent Kinetic Energy [m2/s2]

Hei

ght

[m]

Turbulent Kinetic Energy for distances 0-1000 meters along the ground

OpenFOAM

Fluent

0 m1000 m10000 m

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Turbulent Dissipation Rate

0 0.5 1 1.5 2 2.5 3 3.5 40

50

100

150

200

250

300

350

400

450

500

Turbulent Dissipation Rate [m2/s3]

Hei

ght

[m]

Turbulent Dissipation Rate for distances 0-10000 meters along the ground

OpenFOAM

Fluent

0.05 0.1 0.15 0.2 0.25 0.3

0

5

10

15

20

25

30

35

40

Turbulent Dissipation Rate [m2/s3]

Hei

ght

[m]

Turbulent Dissipation Rate for distances 0-10000 meters along the ground

OpenFOAM

Fluent

0 m1000 m10000 m

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Test 2: airflow in a street canyon

• RANS standard k-ε model• 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

Actual velocity profile known from measurements: *

0

0

( ) lnABL z d zuU z

z

→ Determine z0, d and u*ABL for different geometries

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Scaled velocity

0

100

200

300

400

500

600

-2 -1 0 1 2 3 4 5 6

Velocity [m/s]

He

igh

t [m

]

2.5 m/s scaled

5 m/s

10 m/s scaled

15 m/s scaled

25 m/s scaled

Scaled velocity ratios

0

100

200

300

400

500

600

0,9999 0,99995 1 1,00005 1,0001 1,00015 1,0002 1,00025 1,0003

Velocity ratio [-]

He

igh

t [m

] 2.5 m/s ratio

10 m/s ratio

15 m/s ratio

25 m/s ratio

Wind speed independence

Velocity

0

100

200

300

400

500

600

-10 -5 0 5 10 15 20 25 30

Velocity [m/s]

He

igh

t [m

]

2.5 m/s

5 m/s

10 m/s

15 m/s

25 m/s

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0

100

200

300

400

500

600

-2 -1 0 1 2 3 4 5 6 7

Calculated

Fitted

Building separation

0

100

200

300

400

500

600

-2 -1 0 1 2 3 4 5 6 7

Velocity [m/s]

He

igh

t [m

]

5 m

10 m

15 m

30 m

50 m

100 m

Effect of separation

5 meters10 meters15 meters

30 meters

50 meters

100 meters

30 meters separation: d = 14,2 m, z0 = 0,20 m, u*ABL=0,74 m/sSeparation [m] d [m] z0 [m] u*

ABL [m/s]

5 14,4 0,06 0,63

10 14,2 0,10 0,67

15 14,3 0,07 0,64

30 14,2 0,2 0,74

50 8,0 13,0 1,81

100 10,0 20,0 2,10

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Building height

0

100

200

300

400

500

600

-2 -1 0 1 2 3 4 5 6 7

Velocity [m/s]

He

igh

t [m

]

5 m

10 m

15 m

30 m

50 m

100 m

Effect of height

15 meters100 meters

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Effect of width

Building width

0

100

200

300

400

500

600

-2 -1 0 1 2 3 4 5 6

Velocity [m/s]

He

igh

t [m

]

5 m

10 m

15 m

30 m

50 m

100 m

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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

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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

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Thank you for your attention!

Dutch OpenFOAM User Group