Time-evolution of the Volume- Averaged Concentration Within Urban - - PowerPoint PPT Presentation

August 2018 10th ICUC / 14th SUE 1

Time-evolution of the Volume- Averaged Concentration Within Urban Street Canyons

Validation and Parametrization of the One-box Model

• B. Conan, L. Perret,

LHEEA Centrale Nantes – CNRS, Nantes, France

• E. Savory
• Dpt. Mech. Eng, Western University, London, ON, Canada

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Ventilation of urban canopy

–

Urban air quality is a major health issue for our generation and the next,

–

Understanding and modeling mechanisms of turbulent transport

• f passive scalar in urban environment is still a scientifjc

challenge,

–

Simplifjed approaches such as “box models” are used in

• perational conditions,

–

Fine studies of simplifjed street canyon ventilation contributes to understanding the physics and evaluating simplifjed approaches.

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Box models for dispersion modeling

–

Caracteristic time τc depends on:

• Mean fmow
• Canyon geometry
• Inlet conditions ?

dCc(t) dt =−1 τc [CC (t)−Cabl]

Cc volume-averaged canyon concentration (assumes well mixed pollutant in the canyon) Cabl concentration of the incoming ABL

What parameter to evaluate τc?

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Exchange velocity

In many studies, E = α Ue is used In the study of « dead-zones » in river groynes Weitbrecht (2008) used E, the exchange velocity as a parameter for the emptying: Weitbrecht (2008) observed that E/Ue has a linear relation with the hydraulic radius RD:

Image from Weitbrecht (2008).

E (t)= 1 W ∫

−W /2 W/2

|w (x, z=h,t )|dx

dCc(t) dt =−1 τc [CC (t)−Cabl]=−¯ E h [CC(t)−Cabl]

RD= hW (h+W )

W h

(Johnson, 1973; Berkowicz, 2000; Soulhac, 2000 ; Salizzoni, 2009)

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Application to street canyons & infmuence of infmow conditions

Inspired by Weitbrecht (2008), Perret et al. (2017) applied the exchange velocity to urban street canyons to study the infmuence of infmow conditions on E.

Perret (2017). Blackman (2015)

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Application to street canyons & infmuence of infmow conditions

Perret et al. (2017) proposed a modifjed hydraulic radius including d parameter to account for infmow conditions in the estimation of E:

~ RH= (h+h−d)W (h+h−d)+W

Closed symbols: W/h = 1, open symbols: W/h = 3. Upstream roughness: 1 h bars (red squares), 3 h bars (blue circles) and 25% cubes (purple triangles). Data for river groynes of Weitbrecht et

• al. (2008) (brown plus signs) and canonical cavities of Chang et al. (2006) (orange stars).

RH= hW h+W

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Questions

• Does the 3D space-average concentration in a urban

street canyon follow an exponential decay?

• Is the exchange velocity E a wise candidate to

parametrize exponential decay?

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Canyon dispersion simulation

Numerical set-up (LES)

• W = 1 h (x, y, z) => (6h, L = 4h, H = 4h)
• W = 0.5 h (x, y, z) => (4.5h, L = 4h, H = 4h)
• (Δx, Δy, Δz) = (h/48, h/48, h/48) up to z = h/2
• Cyclic boundary conditions in x and y (except for C in x)
• Sc = 0,71
• Re = 6 500
• C = 1 inside one canyon
• C = 0 elsewhere
• 7 to 15 M cells
• Scalar release after 160T

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Canyon dispersion simulation

• fmow validation
• fmow inside the canyon

uκ/u ∗

√(|u' w '|)/u ∗ √(k)/u ∗

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Exponential decay is observed

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Ue parameters works well

for the confjgurations tested

E (t)= 1 W L ∫

−W /2 W /2

∫

− L/2 L/2

|w (x, y ,z=h,t)|dx dy

• Adaptation for 3D
• Results for W=0.5h (black square) and W=1h (black triangle)

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Conclusions

– Time-evolution of the volume-average concentration in a

canyon follows an exponential decay for W/h=0.5h W/h=1 ratio,

– The exchange velocity proposed by Weitbrecht (2008), and

adapted in 3D seems to work well for street canyons of 1h and 0.5h, → is it true for higher W/h ratios?

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Acknowledgement & References

This study was performed with the fjnancial support from the French National Research Agency through research grant URBANTURB no. ANR-14-CE22-0012-01. This work was granted access to the HPC resources of supercomputer LIGER under the allocation 2017-E1703020 from Ecole Centrale de Nantes

Berkowicz, R., Nov 2000. Ospm - a parameterised street pollution model. Environmental Monitoring and Assessment 65 (1), 323–331. Blackman, K., Perret, L., Savory, E., Aug 2015. Efgect of upstream fmow regime on street canyon fmow mean turbulence statistics. Environmental Fluid Mechanics 15 (4), 823–849. Johnson, W., Ludwig, F ., Dabberdt, W., Allen, R., 1973. An urban difgusion simulation model for carbon monoxide. Journal of the Air Pollution Control Association 23 (6), 490–498. Perret, L., Blackman, K., Fernandes, R., Savory, E., 2017. Relating street canyon vertical mass-exchange to upstream fmow regime and canyon geometry. Sustainable Cities and Society 30 (Supplement C), 49 – 57. Salizzoni, P ., Soulhac, L., Mejean, P., 2009. Street canyon ventilation and atmospheric turbulence. Atmospheric Environment 43, 5056– 5067. Soulhac, L., 2000. Modêlisation de la dispersion atmosphêrique á l’intêrieur de la canopêe urbaine. Ph.D. thesis, Ecole Centrale de Lyon. Weitbrecht, V., Socolofsky, S. A., Jirka, G. H., 2008. Experiments on mass exchange between groin fjelds and main stream in rivers. Journal

• f Hydraulic Engineering 134 (2), 173–183.