Preliminary Plan of Numerical Simulations of Three Dimensional Flow-Field in Street Canyons

With rapid development in computer hardwar-e and numeric algorithms, computational fluid dynamics (CFD)techniques are widely utilized to study the wind field in urban street canyon, and two-dimensional models usually are employed in recent years. This paper developes a three-dimensional numerical model, which based on Reynolds-averaged Navier-Stokes equations equipped with the realizable k-ε turbulence model by CFD. In the past, no one has done similar work, so this paper investigates a simple ideal model. Because the model is in line with the actual situation, this three-dimensional model can further simulate the real flow-filed in street canyons.


Computational model and boundary conditions 2.1 Computational model
The three-dimensional computional domain consists of two parallel arranged in a string of street in the streamwise direction. The origin of coordinates is located in the center of the bottom of the buildings. (Fig.1). The flow properties of the street canyon of the domain are presented in the following discussion. h and b denotes the height and the width of the street canyon, respectively. The height of the street canyon of aspect ratio (H=h=2b) and the freestream inflow speed U are considered as the reference length and velocity scales, respectively. The Reynolds number is prescribed at 9.0X10 5 . The flow is treated as an incompressible, isothermal, and pseudo steady-state turbulence [2] .

Boundary conditions
In the three-dimensional computional domain, the solid boundaries at the bottom and around the building are assumed as no-slip wall conditions. The inlet boundary condition in www.intechopen.com the upstream direction: uU = ,0 υ = ,0 ω = . The outlet boundary condition in the downstream direction: . Other boundaries are considered as symmetry conditions: uU = ,0 υ = ,0 ω = . (Fig.1)

Equations
In the use of software "Fluent" to model calculation, the first step needs to build a variety of Street Canyon models and a reasonable mesh in pre-processor Gambit, and then the definition of the boundary conditions and the physical model can be used to solve the model in fluent software. [3] The three-dimensional computional domain chooses hex and wedge grids. The totle numbers of the grids is 2,755,206.The precision ε takes10 -6 . This simulation carries on by the parallel computer in Donghua University. The coming velocity: 2.0 / Um s = . In this paper the results are based on the numerical solution to the governing fluid flow and transport equations, which are derived from basic conservation principles as follows: www.intechopen.com The mass conservation equation [4] : (1) ( i u mean velocity component in the (,,) xyz directions 1 () ms − ) The momentum conservation (Navier-Stokes) equation [4] : The realizable k ε − equations [4] : β mean the thermal expansion coefcient)      As shown in Fig. 7, some strong vorticities are found in the every target streets. The first street in the direction of wind has a large vorticity, in the first two streets, there are respectively two vorticities (a large vorticity, a small vorticity), and the vorticity can not be found in the last street. But some vorticities can be found on the lower part of the building in the leeward direction at the bottom of the streets. As described in Fig. 8, some strong vorticities are found in the every target streets. The number of vorticity in the direction of the wind is more than that in the leeward direction. some large recent oval vorticity can be found in the back of the buildings in the direction of the wind. It is found that the wind is difficult to go through the streets. Fig. 9. Velocity distribution in x-y plane at z = 2.0 and a Reynolds number is 9.0 × 10 5

Conclusion
As described in Fig. 9, some strong vorticities are found in the every target streets. With the high degree increasing, there is a clear vorticity on the back part of the building in the leeward direction. In addition, flow lines are basically as the same as the profile of the streets.
As described in Fig. 10, none of strong vorticity is found on this cross-section. The contours of x velocities are not found above the streets, so the flow of this part, to the basic, is unchanged. Three-dimensional flow-field is investigated in street canyons at a Reynodes number of 9.0 × 10 5 using a realizable k ε − model, and we get the results (depicted in the front). The present model can exactly describe the flow-field in street canyons and we can get the results in any directions. The results of the present numerical model show that X velocity in the upstream directions is as same as that outside the street canyon, but X velocity is lower in most parts of inside the street canyons, and the air inside and outside streets is difficult to be exchanged In this paper only the velocities in X direction could be shown, other results are not mentioned. The pollution in the streets does not considered. So, More works need to be done to perfect our research. This book will interest researchers, scientists, engineers and graduate students in many disciplines, who make use of mathematical modeling and computer simulation. Although it represents only a small sample of the research activity on numerical simulations, the book will certainly serve as a valuable tool for researchers interested in getting involved in this multidisciplinary ï¬ eld. It will be useful to encourage further experimental and theoretical researches in the above mentioned areas of numerical simulation.

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