○ H. NAKAYAMA T. TAMURA (Tokyo Institute of Technology, JAPAN) LES ANALYSIS ON FLUCTUATING...
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Transcript of ○ H. NAKAYAMA T. TAMURA (Tokyo Institute of Technology, JAPAN) LES ANALYSIS ON FLUCTUATING...
○H. NAKAYAMA
T. TAMURA (Tokyo Institute of Technology, JAPAN)
LES ANALYSIS ON FLUCTUATING DISPERSION
IN ACTUAL URBAN CANOPY
Robins et al., 1977a,bOgawa et al., 1983Li et al., 1983Oikawa et al., 1998
In the case of accidental release of toxic and flammable gas from industrial facilities, it is necessary to estimate not only mean but also fluctuating concentrations around buildings.
The characteristics of fluctuating concentrations around a building model have been investigated by wind tunnel experiments.
BACKGROUND and MOTIVATIONS
Plume dispersion in the wake of a building model
・ The structure of concentration fluctuation field in the wake of a building model・ The relationship between peak and mean concentrations・ Prediction of peak concentration based on probability density functions
On the other hand, as a potential problem, it can be assumed that the accidental spillage for the transportation and storage of hazard materials or poison gas dispersion by terrorist occur even in urban area. Therefore, flow and plume dispersion in regular arrays of cubes as urban model have been studied by wind tunnel and field experiments.
Plume dispersion in the typical urban model
・ The lateral concentrations profiles of a plume emitted from the point source located at the half height of building model are Gaussian.・ In a gap between two cubes instantaneous high concentrations frequently occur and downwind of a cube fluctuations of concentration become continuous and smooth.
Macdonald,1997Mavroidis,2001
However, taking into consideration aspects of actual urban canopy, the height and width of buildings, their arrangement, the spacings between them and the width of the streets change variously.
Therefore, aspects of actual urban roughness are so complicated that characteristics of plume dispersion in surface layer are very different from those of plume dispersion in typical urban model arrayed regularly.
OBJECTIVES
・ Investigate the characteristics of flow and plume dispersion in actual urban canopy ・ Estimate peak concentrations based on various kinds of roughness elements for safety analysis
We carry out Large-Eddy Simulation for plume dispersion in Actual Urban Area.
We have focused on …
A normal plate : height,H side length,2HPoint source : height,H 2H upstream of a plate
Model of dispersion field around a normal plate in turbulent boundary layer
z
y
x
turbulent boundary l ayer
point source
Ue
H2H
o2H
H
Numerical validation
・ Compare LES results with experimental data
A free-stream velocity
tracer gas
C2H4
turbulence grid
roughness elementswith L-shaped cross sections
fast-response flame ionization detector
plume
turbulent boundary layer
point source
Ue (=2m/s)
Air
normal plate
Experimental model for plume dispersion around a normal plate
Schematic of wind tunnel experimental arrangement
(at Central Research Institute of Electric Power Industry)
(a)Generation of inflow turbulence (b)Normal plate in turbulent boundary layer (a)DRIVER UNIT for Spatially-Developing Boundary Layer (Size: 26.7H×20H×6.7H Grid points:200×240×100)
3 bars of type A (0.4H×20H×0.5H)1 bar of type B (0.4H×20H×0.7H)40 cubes of type C (0.4H×0.4H×0.4H)
(b)MAIN COMPUTATIONAL UNIT for Plume Dispersion over A Normal Plate (Size: 30H×20H×6.7H Grid points:360×240×100)
A normal plate: height: H, side length: 2H Blockage: less than 5%
Numerical model for plume dispersion around a normal plate
To develop a thick boundary layer
To induce sufficient velocity fluctuation
At the entrance; uniform inflow is imposed
At the entrance; inflow turbulence obtained at the exit in driver unit is imposed
6.7H
20H 26.7H14.7H
H2H
6.7H
20H10H
20H
inflow turbulenceuniform inflow
Coupling algorithm: MAC method Time integration: Adams-Bashforth scheme Time step: Δ t =0.001 The Re number: 5000 (=UeH/ν) Flow field: Spatial discretization: a fourth-order accurate central difference
Concentration field: Spatial discretization: Convection term; CIP scheme Diffusion term; A fourth-order accurate central difference
Numerical discretization and algorithm
Governing equations and LES model
0
i
i
x
u
ijijjij
jii Sxx
p
x
uu
t
u
21
jjj
j hxx
uc
t
c
continuity equation :
Navier-Stokes :
scalar conservation :
The incompressible Navier-Stokes, scalar conservation and continuity equations are presented by the following grid-filtered form:
In this study, we employ model constants in flow and scalar fields, Cd, Cc, respectively, by Dynamic procedure. The incompressible Navier-Stokes and scalar conservation equations are presented by the following test-filtered form:
model constant in flow field, Cd :
model constant in scalar field, Cc :
)~~~
(2 22ijijdij SSSSCL
~
)~~~
( 22
jjcj x
cS
x
cSCP
~
ijijjij
jii STxx
p
x
uu
t
u ~2
~1
~~~
jjj
j Hxx
uc
t
c
~~~
Navier-Stokes :
scalar conservation :
ijij
ijijd MM
MLC
jj
jjc QQ
QPC
)~~~
(2 22ijijij SSSSM
jjj x
cS
x
cSQ
22~~~
Vertical profiles of mean velocity and turbulence intensity at the position of point source
Point source(z/δ=0.27)
Characteristics of inflow turbulence and dispersion plume(Results obtained by wind tunnel experiment and LES)
Turbulence intensity obtained by LES is smaller in the upper part than that obtained by wind tunnel experiment
δ: thick of turbulent boundary layer
Power spectrum of velocity fluctuation at the position of point source
Vertical spreading rates of a plume(Pasquill-Gifford chart)
Characteristics of inflow turbulence and dispersion plume(Results obtained by wind tunnel experiment and LES)
Both of vertical spreading rates obtained by LES and wind tunnel experiment correspond to values between stability conditions C and D.
Power spectra obtained by LES are a little smaller than the Karman type in lower and higher frequency sides
The case without a normal plate
The case with a normal plateConcentration field
Vorticity
Point source
Point source
The animation of plume dispersion in the case with and without a normal plate
Mean concentration
Numerical validation for concentration fields in the case without a normal plate
R.m.s concentration
Mean concentration
R.m.s concentration
Numerical validation for concentration fields in the case with a normal plate
In the case without a normal plate In the case with a normal plate
Numerical validation for time series of concentration fluctuation at the position, x/H=6
Wind tunnel experiment
Wind tunnel experiment
LES LES
Numerical validation for concentration fields around a normal plate
Application to plume dispersion in actual urban area
Numerical model for plume dispersion in actual urban areas
Kasumigaseki area(Government office quarter)
Kanda area(Commercial area)
In this study, we carry out LES for Kasumigaseki and Kanda areas of Tokyo as actual urban areas.
Aspect of surface roughness・ Large groups of massive buildings ・ Green area with few trees
Aspect of surface roughness・ Large groups of high-rise buildings・ Street canyon embedded into a dense built-up area ・ Green area with few trees
1 km1 km
Profiles of roughness height and density
Kasumigaseki KandaRoughness density,λ: 0.2794 0.4490
p:probability density function of roughness heightδ:thickness of turbulence boundary layer(=500m)λ:roughness density defined as the ratio of the total frontal area of the obstacles to the lot area of the obstacles
Kasumigaseki area
Kanda area
Profile of p.d.f of roughness height
Profile of roughness density
d
f
A
A Af: the total frontal area of the obstacles
Af: the total area covered by the obstacles
Two sites in Tokyo
Kasumigaseki KandaRoughness density,λ : 0.2794 0.4490Normalized roughness length,Zo/h : 0.060 0.026
Zo:Roughness length h:Roughness height
Profiles of roughness density and length
Kasumigaseki area
Kanda area
(obtained by using Raupach’s curve)
Kasumigaseki area
Kanda area
NNW wind
0
10N
NNENE
ENE
E
ESE
SESSE
SSSW
SW
WSW
W
WNW
NWNNW
above 0.3m/sabove 0.5m/s
In central area of Tokyo, the frequency of the NNW wind is dominant.Therefore, we report the results for NNW wind direction.
The profiles of wind direction in Tokyo
NNW wind
Kanda areaKasumigaseki area
(a)Generation of inflow turbulence
(a)DRIVER UNIT for Spatially-Developing Boundary LayerSize: 5H×1.25H×H Grid points:250×250×100
3 bars of type A (0.075H×1.25H×H)1 bar of type B (0.1H×1.25H×H)
15 cubes of type C (0.06H×0.06H×0.06H)
30 cubes of type D (0.05H×0.05H×0.05H)
(b) MAIN COMPUTATIONAL UNIT for Urban Dispersion(Size: 1.25H×1.25H×H Grid points:250×250×100)
To develop a thick boundary layer
To induce sufficient velocity fluctuation 5H
H
1.25HTo simulate urban boundary layer
Computational model for urban dispersion
1.25H
1.25H
H
Kasumigaseki area Kanda area
0.01 0.1 1
U/U∞
0.001
0.01
0.1
1
z/δ
1/4 power lawLES
∞
z/δ
0.01 0.1 1
U/U∞
0.001
0.01
0.1
1
z/δ
1/4 power lawLES
∞
z/δ
∞
z/δ
0 0.1 0.2
uirms/U∞
0
1
2z/δ
urms(LES)vrmswrms
∞
z/δ
0 0.1 0.2
uirms/U∞
0
1
2z/δ
urms(LES)vrmswrms
0.001 0.01 0.1 1 100.001
0.01
0.1
1
Karman typez/δ=0.20z/δ=0.50z/δ=0.90z/δ =0.9z/δ =0.5z/δ =0.2
fLux/UfE
(f)/
u’2
0.001 0.01 0.1 1 100.001
0.01
0.1
1
Karman typez/δ=0.20z/δ=0.50z/δ=0.90z/δ =0.9z/δ =0.5z/δ =0.2
fLux/UfE
(f)/
u’2
Vertical profile of mean velocity
Power spectrum of velocity fluctuation
Vertical profile of turbulence intensity
Characteristics of inflow turbulence in driver unit
Uniform inflow Inflow turbulence
Kasumigaseki area Kanda areaAverage building height : 10.46m 14.69mRoughness height,h : 10.46m 14.69mThe zero-place displacement,d : 9.92m 11.75mRoughness length,Zo : 1.255m 0.76m(obtained by Raupach’s curve)The exponent in the power law,α : 0.22 0.20
Log-log profiles of mean velocity in urban areas
A B C D E A B C D E
21010 )(log016.0)(log096.024.0 oo zz (obtained by using Counihan’s equation)
Kasumigaseki Kanda
Flow field near the ground surface in Kasumigaseki
UU 6.01.0 ~The range of velocity
Red: strong windYellow:weak windBlue:revised flow
Air flow
UU 6.01.0 ~The range of velocity
Red: strong windYellow:weak windBlue:revised flow
Flow field near the ground surface in Kanda
Air flow
Dispersion field near the ground surface in Kasumigaseki
Point source
The range of concentration
initialinitial CeCe 25 0.10.1 ~
Air flow The position of point source: above the main street
Point source
The range of concentration
initialinitial CeCe 25 0.10.1 ~
Dispersion field near the ground surface in Kanda
Air flow
Flow and Dispersion fields near the ground surface in Kasumigaseki area
Flow and Dispersion fields near the ground surface in Kanda area
Mean concentration field R.m.s. concentration field
Dispersion fields near the ground surface in Kasumigaseki area
Maximum concentration field
Point source Point source Point source
The range of concentrationinitialinitial CeCe 25 0.10.1 ~
Large groups of massive buildings
Green area with few trees
Dispersion fields near the ground surface in Kanda area
Mean concentration field R.m.s. concentration field Maximum concentration field
Point source Point source Point source
The range of concentrationinitialinitial CeCe 25 0.10.1 ~
Large groups of high-rise buildings
Dense built-up area
Point source
②
Time series of concentration fluctuation in Kasumigaseki area
Kasumigaseki area
① ②
①
inside the wake outside the wake
①
Kanda area
Point source
①
②
in street canyon
inside the wake
②
Time series of concentration fluctuation in Kanda area
Conclusions
1. The value of roughness height is set to be same as that of average building height and the value of the zero-place displacement is set to be 80% of roughness height, we estimate roughness length obtained by Raupach’s curve. As the results, profiles of the computed mean velocity are almost corresponding to those of the power law obtained by Counihan’s equation except near the ground surface.
2. In sparse array of massive buildings, the effect of plume entrainment by each buildings wake is so large that the spatial distributions of mean and r.m.s fields are distorted and a core with large values of mean and r.m.s concentrations is located also in the buildings wake.
We show numerical validation for the results of dispersion field around a normal plate compared with the results obtained by wind tunnel experiment and carry out LES for flow and plume dispersion in actual urban areas.
2. On the other hand, in dense built-up area, the effect of plume entrainment by each buildings wake is small but the effect of the existence of street canyon is so large that plume is transported easily downstream above main street. Therefore, a core with large values of mean and r.m.s concentrations is located in street canyon.
3. The concentration fluctuates smoothly and continuously inside the buildings wake by active turbulence mixing between air flow and a plume. However, instantaneous high concentrations frequently occur in street canyon.
(a)a shot for upward extension of the separated shear layer
(b)a shot for a roll-up of the separated shear layer
Instantaneous contours for vorticity and concentration
a plume core moves up due to upward extension of the separated shear layer
a plume core is entrained into wake region due to a roll-up of the separated shear layer
(c)Vertical concentration flux
Strong negative concentration flux due to a roll-up of the separated shear layer
Kasumigaseki area Kanda area
Dispersion fields at the height of urban canopy