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

2. Robins et al., 1977a,b Ogawa et al., 1983 Li et al., 1983 Oikawa 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

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. 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,1997 Mavroidis,2001

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 ･ 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 …

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

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. (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 inflow turbulenceuniform inflow

9. 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

10. Governing equations and LES model 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, C d ： model constant in scalar field, C c ： Navier-Stokes ： scalar conservation ：

11. 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

12. 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

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

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

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

16. In the case without a normal plateIn 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

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 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 km

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

20. Kasumigaseki Kanda Roughness density,λ : 0.2794 0.4490 Normalized 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

21. NNW wind above 0.3m/s above 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

22. (a)Generation of inflow turbulence (a)DRIVER UNIT for Spatially-Developing Boundary Layer Size: 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 1.25H To simulate urban boundary layer Computational model for urban dispersion 1.25H H Kasumigaseki area Kanda area

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

24. 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 Log-log profiles of mean velocity in urban areas ABCDEABCDE (obtained by using Counihan’s equation) KasumigasekiKanda

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

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

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

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

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

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

33. Point source ② Time series of concentration fluctuation in Kasumigaseki area Kasumigaseki area ①② ① inside the wakeoutside the wake

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

35. 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.

36. 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.

37. (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

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