Large-Eddy Simulation of plume dispersion within various actual urban areas

Hiromasa Nakayama*, Klara Jurcakova** and Haruyasu Nagai*

*Japan Atomic Energy Agency, Japan**Academy of Sciences of the Czech Republic, Prague

Local-scale atmospheric dispersion models for emergency response

Atmospheric dispersion problems within urban areas•Accidental release of chemical materials•Intentional release of hazardous materials by terrorist attack

Prediction of spatial extent of contaminated regions using building-resolving CFD models•M2UE (Danish Meteorological Institute)•FAST3D-CT (U.S. Naval Research Laboratory)•FEM3MP (U.S. National Atmospheric Release Advisory Center)•MSS (French Atomic Energy Commission)•LOHDIM-LES (Japan Atomic Energy Agency)

These urban dispersion CFD models can provide detailed information on turbulent flow patterns and spatial distributions of plume concentration within urban areas.

What is the spatial extent of distribution patterns of concentration influenced by urban surface geometry?

Schematic of wind flow in urban area

• Urban areas consisting of buildings with variable height and obstacle density• Strong three-dimensionality of the turbulent flows within urban canopy• Complex spatial distribution patterns of plume concentration

Objective•Carry out a series of LESs of plume dispersion in urban areas with a wide range of building height variability and obstacle density•Clarify the distribution patterns of plume concentration influenced by urban surface geometry by comparative analysis

(Ratti et al. 2002 and Nakayama et al. 2012)

Recycling only fluctuating components of wind velocity

Tripping Fence

Spatially-developing rough-wall boundary layer flow

Fully-developed urban boundary layer flow

Urban area

Turbulent Inflow

Turbulent Flow

Roughness Blocks

Model structure of LOHDIM-LES developed at JAEA

Driver region Main analysis regionSchematic diagram

Procedure of calculating•Generate basic boundary layer flow by recycling method(Kataoka&Mizuno,2002)•Produce active turbulent fluctuations by various roughness obstacles•Carry out LESs of turbulent flow and plume dispersion in urban areasBasic equations•Spatially-filtered continuity equation, Navier-Stokes equation, and Scalar conservation equationSGS turbulent and scalar effects•For flow field, the standard Smagorinsky model(1963) with the constant of 0.1•For dispersion filed, the standard Smagorinsky model with the turbulent Schmidt number of 0.5Building effects•Immersed boundary method by Goldstein et al. (1998)

Computational conditionsComputational

modelDriver region Main analysis

regionResolved actual

urban area

Mesh number 460×250×90 475×250×90 375×250×90

Domain size 5.5km×1km×1km 2.0km×1km×1km 1.5km×1km×1km

Grid resolution 4.0m-20.0m×4.0m×1.3m-53m

4.0m×4.0m×1.3m-53m

4.0m×4.0m×1.3m-53m

Boundary Flow field Dispersion field

InletDriver region

Recycling technique (Kataoka & Mizuno, 2002)

Main regionTurbulent inflow generated in the driver region

Exit Conventional convective type

Top

Ground

Side Periodic

Building effectsExternal force(Goldstein et al.,1993)

0x

c

0x

c

0,0,0

wy

v

z

u0

z

c

0 wvu 0z

c

0y

c

000

z

c,

y

c,

x

c

Cases of comparative analysis for wind tunnel experiments （ Bezpalcova,2008 ） and LESs

Surface geometry typeAverage building height[m]

Building height variability[-]

Obstacle density[-]

Idealized urban canopy(Bezpalcova,2008)

Cubic buildings array 42.0 0.0 0.16

Cubic buildings array 42.0 0.0 0.25

Cubic buildings array 42.0 0.0 0.33

Actual urban site in Central Tokyo (present LESs)

Low-rise buildings area 9.6 0.60 0.39

Street-canyon area 22.7 0.71 0.56

Complex of high-rise and low-rise buildings area

26.5 0.85 0.52

High-rise buildings area 34.1 0.94 0.52

Building heights distributions of Central Tokyo used in LESs100m

0m

(a)Low-rise buildings area (b)Street-canyon area

(d)High-rise buildings area(c)Complex of high-rise and low-rise buildings area

100m

0m

NNW

NNW

1km

1km

Tripping fenceRoughness blocks

Characteristics of approach flow generated in driver region

Turbulence intensity for u-component

Turbulence intensity for v-component

Turbulence intensity for w-component

Mean velocity

0.5km 5.0km

10-1

10-6

Spatial distributions of mean concentration at ground level

(a)Low-rise buildings area (b)Street-canyon area

(d)High-rise buildings area(c)Complex of high-rise and low-rise buildings area

NNW

NNW

Cmean/Cinit

10-1

10-6

Cmean/Cinit

Source location

Source location

Source location

Source location

Spatial distributions of r.m.s. concentration at ground level

(a)Low-rise buildings area (b)Street-canyon area

(d)High-rise buildings area(c)Complex of high-rise and low-rise buildings area

NNW

NNW

10-1

10-6

Cr.m.s./Cinit

10-1

10-6

Cr.m.s./Cinit

Source location

Source location

Source location

Source location

Mean concentration distributions along wind direction from the point source

R.m.s. concentration distributions along wind direction from the point source

Conclusion

We investigated spatial extent of the distribution patterns of plume concentrations within urban canopy.

•Mean concentration distributions are nearly the same among various urban areas at a downwind distance of the point source greater than 1.0km. •The difference of r.m.s. concentration distributions among various urban areas also become small at a downwind distance of the point source greater than 1.0km.•In order to capture distribution patterns of plume concentrations within urban canopy, computational domain size should be at least 1.0km along wind direction from the point source.