Post on 12-Jan-2016
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.