○H. NAKAYAMA T. TAMURA (Tokyo Institute of Technology, JAPAN)

1. LES ANALYSIS ON FLUCTUATING DISPERSION IN ACTUAL URBAN CANOPY ○H. NAKAYAMA T. TAMURA (Tokyo Institute of Technology, JAPAN)

2. BACKGROUND and MOTIVATIONS 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. ･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 Robins et al., 1977a,b Ogawa et al., 1983 Li et al., 1983 Oikawa et al., 1998 Plume dispersion in the wake of a building model

3. 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. ･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,1997 Mavroidis,2001 Plume dispersion in the typical urban model

4. 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.

5. OBJECTIVES We carry out Large-Eddy Simulation for plume dispersion in Actual Urban Area. We have focused on … ･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

6. Numericalvalidation Model of dispersion field around a normal plate in turbulent boundary layer ･Compare LES results with experimental data A normal plate： height,H side length,2H Point source ： height,H 2H upstream of a plate

7. Experimental model for plume dispersion around a normal plate Schematic of wind tunnel experimental arrangement (at Central Research Institute of Electric Power Industry)

8. To develop a thick boundary layer To induce sufficient velocity fluctuation Numerical model for plume dispersion around a normal plate uniform inflow inflow turbulence (b)Normal plate in turbulent boundary layer (a)Generation of inflow turbulence (a)DRIVER UNITfor Spatially-Developing Boundary Layer (Size: 26.7H×20H×6.7H Grid points:200×240×100) At the entrance; uniform inflow is imposed 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) At the entrance; inflow turbulence obtained at the exit in driver unit is imposed A normal plate: height: H, side length: 2H Blockage: less than 5%

9. Numerical discretization and algorithm Coupling algorithm: MAC method Time integration: Adams-Bashforth scheme Time step: Δｔ=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

10. continuity equation： Navier-Stokes： scalar conservation： Governing equations and LES model 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: Navier-Stokes： scalar conservation： model constant in flow field, Cd： model constant in scalar field, Cc：

11. Turbulence intensity obtained by LES is smaller in the upper part than that obtained by wind tunnel experiment Characteristics of inflow turbulence and dispersion plume (Results obtained by wind tunnel experiment and LES) Point source (z/δ=0.27) δ: thick of turbulent boundary layer Vertical profiles of mean velocity and turbulence intensity at the position of point source

12. 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 Characteristics of inflow turbulence and dispersion plume (Results obtained by wind tunnel experiment and LES) Power spectrum of velocity fluctuation at the position of point source Vertical spreading rates of a plume (Pasquill-Gifford chart)

13. The animation of plume dispersion in the case with and without a normal plate The case without a normal plate Point source The case with a normal plate Concentration field Point source Vorticity

14. Numerical validation for concentration fields in the case without a normal plate Mean concentration R.m.s concentration

15. Numerical validation for concentration fields in the case with a normal plate Mean concentration R.m.s concentration

16. Numerical validation for time series of concentration fluctuation at the position, x/H=6 Wind tunnel experiment LES Wind tunnel experiment LES In the case without a normal plate In the case with a normal plate

17. Numerical validation for concentration fields around a normal plate Application to plume dispersion in actual urban area

18. Numerical model for plume dispersion in actual urban areas In this study, we carry out LES for Kasumigaseki and Kanda areas of Tokyo as actual urban areas. 1 km 1 km Kasumigaseki area (Government office quarter) Kanda area (Commercial area) Aspect of surface roughness ･Large groups of high-rise buildings ･Street canyon embedded into a dense built-up area ･Green area with few trees Aspect of surface roughness ・Large groups of massive buildings ･Green area with few trees

19. Profiles of roughness height and density Kasumigaseki area Kanda area Two sites in Tokyo Profile of p.d.f of roughness height Profile of roughness density p:probability density function of roughness height δ: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 Kanda Roughness density,λ: 0.2794 0.4490 Af: the total frontal area of the obstacles Af: the total area covered by the obstacles

20. Kasumigaseki area Kasumigaseki area Kanda area Kanda area Profiles of roughness density and length Zo:Roughness length h:Roughness height Kasumigaseki Kanda Roughness density,λ : 0.2794 0.4490 Normalized roughness length,Zo/h : 0.060 0.026 (obtained by using Raupach’s curve)

21. above 0.3m/s above 0.5m/s NNW wind NNW wind The profiles of wind direction in Tokyo Kasumigaseki area Kanda area In central area of Tokyo, the frequency of the NNW wind is dominant. Therefore, we report the results for NNW wind direction.

22. To simulate urban boundary layer To develop a thick boundary layer To induce sufficient velocity fluctuation Computational model for urban dispersion (a)DRIVER UNIT for Spatially-Developing Boundary Layer Size: 5H×1.25H×H Grid points:250×250×100 H 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) 5H 1.25H (a)Generation of inflow turbulence Kasumigaseki area Kanda area H 1.25H 1.25H (b) MAIN COMPUTATIONAL UNIT for Urban Dispersion (Size: 1.25H×1.25H×H Grid points:250×250×100)

23. Characteristics of inflow turbulence in driver unit Uniform inflow Inflow turbulence Vertical profile of mean velocity Vertical profile of turbulence intensity Power spectrum of velocity fluctuation

24. A A B B C C D D E E Log-log profiles of mean velocity in urban areas Kasumigaseki Kanda Kasumigaseki area Kanda area Average building height : 10.46m 14.69m Roughness height,h : 10.46m 14.69m The zero-place displacement,d : 9.92m 11.75m Roughness length,Zo : 1.255m 0.76m (obtained by Raupach’s curve) The exponent in the power law,α : 0.22 0.20 (obtained by using Counihan’s equation)

25. Air flow Flow field near the ground surface in Kasumigaseki Red: strong wind Yellow:weak wind Blue:revised flow The range of velocity

26. Air flow Flow field near the ground surface in Kanda Red: strong wind Yellow:weak wind Blue:revised flow The range of velocity

27. Air flow Point source Dispersion field near the ground surface in Kasumigaseki The position of point source: above the main street The range of concentration

28. Air flow Dispersion field near the ground surface in Kanda Point source The range of concentration

29. Flow and Dispersion fields near the ground surface in Kasumigaseki area

30. Flow and Dispersion fields near the ground surface in Kanda area

31. Dispersion fields near the ground surface in Kasumigaseki area Large groups of massive buildings Green area with few trees The range of concentration Point source Point source Point source R.m.s. concentration field Maximum concentration field Mean concentration field

32. Dispersion fields near the ground surface in Kanda area Large groups of high-rise buildings The range of concentration Dense built-up area Point source Point source Point source R.m.s. concentration field Maximum concentration field Mean concentration field

33. Time series of concentration fluctuation in Kasumigaseki area Point source ① ② ① ② inside the wake outside the wake Kasumigaseki area

34. Time series of concentration fluctuation in Kanda area Point source ② ① ① ② in street canyon inside the wake Kanda area

35. Conclusions 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. 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.

36. 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. • 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.

37. 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 (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 Strong negative concentration flux due to a roll-up of the separated shear layer (c)Vertical concentration flux

38. Dispersion fields at the height of urban canopy Kasumigaseki area Kanda area