OpenFOAM for Air Quality
Ernst Meijer and Ivo Kalkman
First Dutch OpenFOAM Seminar
Delft, 4 november 2010
Outline
2
•
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
4 November 2010
Air Quality Issues
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European guidelines for air quality
Species
Nitrogendioxide
(NO2)
Particulate Matter
(PM10)
Exceedence level
Annual average
40 μg/m³
Hourly average
max. 18 time/yr
> 200 μg/m³
Annual average
40 μg/m³
Diurnal average
max. 35 times/yr >
50 μg/m³
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
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Local Air Quality and Climate
Field experiments
•
•
•
•
•
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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
4 November 2010
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 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
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observations
‘raw’ CFD results
calibrated 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
Velocity for distances 0-10000 meters along the ground
500
OpenFOAM
Fluent
0m
1000 m
10000 m
450
400
350
Height [m]
300
250
200
150
100
50
0
13
2
4
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8
10
12
Velocity [m/s]
14
16
18
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4 November 2010
Turbulent Kinetic Energy
Turbulent Kinetic Energy for distances 0-1000 meters along the ground
500
OpenFOAM
Fluent
0m
1000 m
10000 m
450
400
350
Height [m]
300
250
200
150
100
50
0
14
1.4
First Dutch OpenFOAM Seminar
1.6
1.8
2
2.2
Turbulent Kinetic Energy [m 2/s 2]
2.4
2.6
2.8
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4 November 2010
Turbulent Dissipation Rate
Turbulent Dissipation Rate for distances 0-10000 meters along the ground
500
OpenFOAM
Fluent
Turbulent Dissipation Rate for distances 0-10000 meters along the ground
450
OpenFOAM
Fluent
40
35
0m
1000 m
10000 m
400
30
350
Height [m]
25
Height [m]
300
250
20
15
10
200
5
150
0
0.05
0.1
0.15
0.2
0.25
0.3
Turbulent Dissipation Rate [m 2/s 3]
100
50
0
15
0
0.5
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1.5
2
Turbulent Dissipation Rate [m 2/s 3]
2.5
3
3.5
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4 November 2010
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: U ( z ) 
 z  d  z0 
ln 


z
0


u*ABL
→ Determine z0, d and u*ABL for different geometries
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Wind speed independence
Velocity
Scaled
velocity
ratios
Scaled
velocity
Height [m]
Height
Height[m]
[m]
600
-2
17
600
600
500
500
500
400
400
400
2.5 m/s scaled
2.52.5
m/s
m/s ratio
5 m/s
5 m/s
10 m/s ratio
300
300
1015
m/s10
m/sm/s
ratioscaled
300
15 m/s15 m/s scaled
25 m/s ratio
25 m/s25 m/s scaled
200
200
200
100
100
100
0
0,9999
-10
0
-1
0,99995
-5
0
First Dutch OpenFOAM Seminar
0
1
0
1
1,00005
5
2 1,0001
10
Velocity ratio [-]
Velocity
[m/s]
Velocity
[m/s]
1,00015
315
1,0002
4
20
1,00025
25 5
1,0003 6
30
4 November 2010
Effect of separation
600
Building separation
d [m]
z0 [m]
u*ABL [m/s]
*
30 meters separation: d = 14,2 m, z0 = 0,20 m, u ABL=0,74 m/s
Separation [m]
600
5
500
14,4
0,06
0,63
14,2
0,10
0,67
14,3
0,07
0,64
500
10
400
400
15
5m
Calculated
Height [m]
300
Fitted
10 m
15 m
300
30 m
30
14,2
0,2
0,74
8,0
50 meters 13,0
100 meters
1,81
10,0
20,0
2,10
50 m
100 m
200
200
50
100100
100
-2
18
-2
-1
-1
0
0
0
0
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1
2
15 10
meters
30meters
meters
5 meters
2
3
Velocity [m/s]
3
4
4
5
5
6
6
7
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Effect of height
Building height
600
500
400
Height [m]
5m
10 m
15 m
300
30 m
50 m
100 m
200
100
0
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-1
1
15Velocity
meters
[m/s]
100 meters
2
3
4
5
6
4 November7 2010
Effect of width
Building width
600
500
400
5m
Height [m]
10 m
15 m
300
30 m
50 m
100 m
200
100
0
-2
20
-1
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0
1
2
Velocity [m/s]
3
4
5
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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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Dutch OpenFOAM User Group
Thank you for
your attention!
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