PRELIMINARY STUDY OF COMPUTATIONAL SETUP FOR URBAN STREET CANYONS by MUHAMMAD NOOR AFIQ WITRI, M.Eng 1
CONTENTS 1.Introduction 2.Building Configuration 3.Boundary Condition 4.Previous Works 5.Summary 2
Intro: Scientific Evaluation Model information required for setting-up and running of the model in CFD include: Deciding on level of detail to be used for the modelling: Spatial (micro, meso, macro) and temporal (minutes, hours, days, years) scale Boundary conditions Choice of turbulence model Numerical schemes Grid and meshing techniques 3
Intro: Urban boundary layer Grimmond & Oke, 2002. The flow are horizontally homogeneous but can vary in the vertical. The flow is horizontally heterogeneous, determined by local length scales such as the height of the roughness elements (buildings), breadth or separation The flow is highly heterogeneous spatially and subjected to form drag. 4
Building Configuration Isolated building Simple city blocks Building layout scales Building complexes with simple building shapes in actual urban area 5
Building Configuration (cont.) The open country - an isolated street canyon. The urban roughness - an approximation of the urban fabric and is obtained by replicating similar street canyons, parallel to the test canyon, upstream and downstream. 6
Building Configuration (cont.) For an isolated building model: Top boundary: 5H above buildings (Y. Tominaga, 2008) Outflow boundary: 10H behind buildings For an urban area, needs additional street canyons which are located either upstream or downstream of the target street canyons (Meroney et. al., 1997). Hoydysh et al.,1974 determined that an upwind fetch of 8 to 10 street canyons is required. 7
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Boundary Condition (BC) With regards to the evaluation models some boundary conditions are quite important since they could lead to significant numerical errors. Boundary condition corresponding to the urban street canyon geometry include: Inlet BC Top BC Outflow BC Wall BC 9
BC- Inlet Velocity Profile (cont.) Generally, there are two types of wind speed profile based on field measurement and wind tunnel study namely: 1) Power law profile (Y. Tominaga et al, 2008) U(Z) Z = ( ) U( Z ) Z ref ref U(Z) is the wind speed at a height Z above the ground, Z ref is a 10m meteorological reference height α is the power law exponent (1/7 for neutral stability value) Consistent with the wind load estimation method in Japan (Architectural Institute of Japan, 2004) D.A. Bechrakis and P.D Sparis, 2000 10
BC- Inlet Velocity Profile (cont.) 2) Logarithmic profile (Richards & Hoxey, 1993) U(Z) 1 Z = ln ( ) U k Zo * U * = friction velocity, Zo = aerodynamic roughness length k = 0.4 = von Karman constant Assume that the height of the computational domain is much lower than the atmospheric boundary layer height because the assumption of constant shear stresses is only valid in the lower part of the atmospheric boundary layer. J.E. Cermak et. al., 1995 11
BC- Inlet Velocity Profile (cont.) Davenport et al., 2000. 12
BC- Inlet Velocity Profile (cont.) A wind tunnel of velocity profile results also can be used to allow numerical results to be compared. An example, CODASC projects which have been done in Laboratory of Building and Environmental Aerodynamics, Karlsruhe Institute of Technology (KIT). A boundary layer flow with mean velocity, u(z) profile exponent = 0.30 according to the power law formula were reproduced after regression analysis in the test section. CODASC, 2008 13
BC- Inlet Velocity Profile (cont.) A field data analysis by M.L. Ray et al., 2006 found that there is not a significant difference in performance between the log and power laws. Either method gives an accurate prediction are low and using either of these shear models may result in significantly large mean wind speed estimation errors. 14
BC-Near-wall Treatment Near the wall, the viscosity-affected region is produced and are made up of roughly three zones (with their corresponding wall y+): FLUENT, 2005a Blocken et al, 2007 15
BC-Near-wall Treatment (cont.) FLUENT offers two approaches to modelling the near-wall regions. 1) the viscosity-affected inner region (viscous sublayer and buffer layer) is not resolved. Instead, semiempirical formulas called wall-functions are used to bridge the viscosity-affected region between the wall and the log-law region. 2) Turbulence models are modified to enable the viscosity-affected region to be resolved with a mesh all the way to the wall, including the viscous sublayer, and are termed near-wall modelling. 16
BC-Near-wall Treatment (cont.) FLUENT, 2005 17
BC-Near-wall Treatment (cont.) The k-ε and RSM models are primarily valid for turbulent core flows (somewhat far from the walls) and hence are coupled with Wall-Functions Model to bridge them with the solution variables in the viscosity-affected region (Launder & Spalding, 1974). Spalart-Allmaras and k-ω are applicable Near-Wall Model mesh with proper fine resolution (FLUENT, 2005a, Versteeg and Malalasekera, 2007). 18
BC- Top Boundary condition The choice of the top boundary condition is very important for sustaining equilibrium boundary layer profiles. Usually Symmetry boundary condition was imposed which prescribe zero normal derivatives for all other flow variables and should be used only if the domain top is outside the boundary layer (Y. Tominaga et al, 2008). If the computations are to be compared with wind tunnel measurements, then the top boundary located at the position of the wind tunnel s top wall should be treated as a solid wall (COST ACTION 732). 19
BC- Outflow Boundary cond. An Outflow boundary condition is imposed at the outlet so that the derivatives of all flow variables are forced to vanish, corresponding to a fully developed flow (Y. Tominaga et al, 2008). Therefore this boundary should be ideally far enough away from the built area to not have any fluid entering into the computational domain through this boundary (COST Action 732). 20
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Previous Works: Idealized street canyon Logarithmic profile T.L. Chan, 2002 Jeong & Andrews, 2002 Power-law profile Kim & Baik, 1999 Uniform profile Banerjee & Christian, 2011 Logarithmic profile S. Di Sabatino et. al., 2008 Power-law profile Chang & Meroney, 2003 Uniform profile W.C. Cheng et. al. 2008 22
Previous Works: Urban Area 1 Chang & Meroney, 2003 1) Simulate flow-field around a groups of building located in a city. 2) Power law profile based on wind tunnel experiment was adopted. 3) Discusses influence of reproduction range of the additional upstream building (N=1,2,,8). 4) For H/W=1, the open-country roughness cases (N=1,2) have higher concentrations at street level than the urban roughness cases (N=3,8). 5) For H/W=0.5, the results are contrary. 23
Previous Works: Urban Area 2 R. Yoshie et al., 2006 1) Simulate flow-field around a high-rise building located in a city. 2) Power law profile based on wind tunnel experiment was adopted. 3) Influence on the calculation results of the reproduction range of the surrounding urban block is relatively low. 24
Summary Constant inlet velocity is the best suits for idealized street canyons s case. Logarithmic and power-law velocity profile are applicable for complex urban configuration and meteorological prediction study. Additional upstream buildings are required to prescribed an urban roughness airflow. Selection of BC types at top and outflow are according to the literatures recommendations. Wall treatment depends on turbulent model and Re number choices. 25
Future Works Choice of : Case study (idealized/urban street canyon) Turbulence models (RANS/LES), Numerical schemes (1 st /2 nd order upwind etc.) Grid & mesh design (structured/unstructured etc.) Simulation of flow in a street canyons and verification through: Velocity profile tests (Blocken et al., 2007) Grid independence study 26
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