Velocity and Concentration Field in Crossroad Area

WDS'07 Proceedings of Contributed Papers, Part III, 161–168, 2007.

ISBN 978-80-7378-025-8 © MATFYZPRESS

Velocity and Concentration Field in Crossroad Area R. Kellnerová1,2 and Z. Jaˇ nour1 1

Institute of Termomechanics, Academy of Science of the Czech Republic, Prague, Czech Republic. 2 Department of Meteorology and Environment Protection, Charles University, Prague, Czech Republic.

Abstract. The aim of this project is to investigate the influence of intersection configuration on spreading of contaminants from the vehicle traffic. Experiment was performed by physical modeling method. For crossroad in the shape of perpendicular X, we have measured the mean velocity and concentration field. The flow investigation has confirmed that within the area of corner vortexes the largest accumulation occurs. Further in the canyon the concentration field is affected by developed vertical vortex and accumulation appears mostly on the leeward side.

Introduction Large portion of frequented municipal roadways are situated in close vicinity of densely populated areas. Due to increasing volume of vehicle traffic, lot of European cities have difficulties with high abundance of traffic exhalations. During the stationary meteorological conditions, some places indicate higher values of mean concentration than another ones. Physical modeling can offer a proper method for studying these situations.

Experimental set-up The project has focused on spreading of air pollution inside the idealised urban area. The idealised model simulated the typical inner-city area with 20 m high apartment houses with saddle roofs. The houses form the regular blocks divided by intersections (see scheme on Figure 1). Model has been scaled down to 1 : 200. The dispersion and transportation of traffic emission inside the intersection area are main objective of this contribution.

Figure 1. Scheme of building, X-shaped intersection, and photograph from experiment. Street aspect ratio was S/H =1, where S is width of the street and H is the height of buildings. For the depth of buildings B is valid B/H=1/2.

The experiment was conducted in low-speed open-circuit aerodynamic tunnel IT AS CR in Nov´ y Kn´ın. Tunnel has 20.5 m long generating section with roughness elements and spires.

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´ AND JANOUR: ˇ KELLNEROVA VELOCITY AND CONCENTRATION FIELD IN CROSSROAD

With the air flowing over the section, the fully turbulent boundary layer can be developed. On the beginning of working section the thickness of turbulent layer attained more than 70 cm. The flow measurements were carried out using two-dimensional fibre-optic laser Doppler anemometry (LDA). In order to measure air-particle velocity it was necessary to add the glycerine droplets in the main flow, because the droplets can be traced by laser. Droplets with mean radius approximately 1 µm passively flow in the air with sufficiently low sedimentation velocity. Concentration was measured using slow-response flame ionisation detector (FID) with ethane used as sample gas. Ethane is passive and non-reactive gas with its own density close to density of the air. Traffic exhalations were simulated by a continual double line source. The source was constituted from 2 500 parallel needles with inner diameter 0.1 mm (red lines in Figure 1). At first, we have to verify that internal boundary layer or at least its lower part called roughness layer, is fully developed above measuring position. We checked the sufficient distance from the model front that is the required fetch for flow inside the roughness layer to be in equilibrium with the surface beneath. Under neutrally stratified conditions, the vertical profiles of turbulent characteristics were measured in the central and the symmetrically upstream and downstream intersection. Two examples are depicted in Figure 2.

Figure 2. Vertical velocity profiles in the intersections. The positions of profiles are shown in the sketch. U0 means the reference velocity. The strong similarity of profiles suggests that internal boundary layer above these positions has very close turbulent characteristics. Requirements for equilibrium of canopy sublayer, e.g. layer below the roof level, are weaker. Hence the requirements for fully turbulent developing of urban canopy above the measuring position are satisfied. The spatially averaged velocity data were fitted with the logarithmic and power law. Roughness length and displacement were obtained from the velocity profile via shear stress determined from stress values within the inertial sublayer (for more information see Cheng and Castro (vol. 104, 2002). The exponent α was determined by fitting the power law: Roughness length z0 = 3,96 mm Displacement d0 = 17 mm Power law exponent α = 0,24 The roughness length equals to z0 =0.8 m in a full scale. This corresponds according to 162


Britter and Hanna (2003) with parameters for skimming regime z0 ≈ 1.0 m in a densely built-up area without much obstacle height variation. On the other hand, parameters normalised by the height of building HB are lower than expected range proposed by Grimmond and Oke (1999) : z0 /HB =0.04 (range from 0.06 to 0.2) d0 /HB =0.17 (range from 0.35 to 0.85) Another important parameter is dimensionless frontal area λf , which provides information about building dimension and spacing. λf is ratio between frontal surface area Af and total surface area AT . In our case λf = Af /AT equals 0.36 and lies in the lower edge of interval for downtown areas. For comparison purpose we have measured the profiles above street canyons corresponding to the intersections. The intersections are open to incoming flow (position red B0 ), whilst canyons are perpendicular to free flow (positions green A1 , B1 and C1 ) - see sketch in Figure 3. In accordance with various authors, two effects at roof level were found in a street canyon (green lines) in comparison with the situation in the intersection (red line): significant velocity retardation accompanied by increase of vertical Reynolds stress < −u0 w0 >.

Figure 3. Vertical velocity profiles in the street canyons. The positions of profiles are marked in the sketch. High absolute stress values indicate the region of enhanced mixing and turbulent kinetic energy, where intensive mixture of canyon air with upper air occurs. Based on theory, large-scale eddies are disturbed to small-scale eddies and high momentum flux causes the fast concentration exchange. As a result, in this important region the traffic pollution quits the canyon area. According to [3], these effects are strengthened by the presence of pitched roofs. Within the urban canopy, inside the central street parallel with incoming flow, the velocity gradually increases with longitudinal distance (Figure 4). On the entry of the model, the velocity grows explosively due to flow convergence and immediately drops down. In agreement with Cheng and Castro (vol. 105, 2002) the stress < −u0 w0 > varies with longitudinal fetch. The variation has wave character with wave length equivalent to the pattern length X/H=3.5. Over the house block the stress descends to minimal value, whereas it ascends to its maximum above the canyon.

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Figure 4. Wave variation of vertical Reynolds stress with longitudinal distance. Z/H=0.75

Results Velocity field The flow inside the canopy is strongly three-dimensional. Velocity in canyons has both horizontal and vertical components and spiral vortexes are formed on the both side of street canyon intersection. A nearly symmetrical velocity field with corner vortexes and slow moving contra flow in the street canyon was found at the horizontal cross-section (Figure 5 – left). Within an intersection area, velocity drops due to divergence. Figure 5 – right shows symmetrical area of inverse values of Reynolds stress < u0 v 0 >. High values of horizontal Reynolds stress occurs close to corner areas. Further in canyon, the sign of Reynolds stress reverses. The absolute value of stress < u0 v 0 > decreases with increasing height (see Figure 6). Above the roof level, Reynolds stress reaches small positive values that are spatially independent. Values of velocity fluctuation can be analysed by the quadrant analysis technique. Depending on positive or negative deviation from the velocity average for both flow components u and v, we can establish four quadrants: • u0 > 0, v 0 > 0

1. quadrant - outward interaction

• u0 > 0, v 0 < 0

2. quadrant - sweeps

• u0 < 0, v 0 < 0

3. quadrant - inward interaction

• u0 < 0, v 0 > 0

4. quadrant - ejection.

The partial contribution from i-th quadrant to the total Reynolds stress < u0 v 0 > was obtained from formula of weighted average: τi =

< u0 v 0 >i .Ni Ntotal

where < u0 v 0 >i means averaged stress within the i-th quadrant, Ni is the number of events belonging to the i-th quadrant and Ntotal is the global number of events recorded during time period. From quadrant analysis it can be concluded that contributions of Reynolds components from first quadrant (outward interaction, u0 > 0, v 0 > 0) and third quadrant (inward interaction, u0 < 0, v 0 < 0) are nearly equal. The same notice is valid for second (sweeps, u0 > 0, v 0 < 0) and 164


Figure 5. Horizontal cross-section at Z/H=0.3. Left: Velocity field with vectors and streamlines. UH means velocity at height of building. Right: Reynolds stress < u0 v 0 > field, black points mark measuring positions.

Figure 6. Horizontal Reynolds stress along the canyon. X/H=0

fourth quadrant (ejection,u0 < 0, v 0 > 0). At the lower level, values from particular quadrants are strongly dependent on horizontal position (not shown). With increasing height values increase and come to be constant with horizontal coordinate Y in all quadrants. Furthermore their global sum decreases. Above the roof level data from all quadrants do not vary with space and fall to small positive values in total sum. The vertical velocity field inside the street canyon depends on the distance from the intersection. At specific fetch from the intersection (Y/H=1.5) the recirculation zone is created (Figure 8 – left). With increasing distance the zone center moves upwards. Inside the lower part of canyon, large absolute values of Reynolds stress rapidly decrease with distance from the corner, whereas the upper canyon stress values are spatially independent (Figure 8 – right).

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Figure 7. Scheme of cross-sections.

Figure 8. Vertical cross-section in Y/H=0.5 (up) and Y/H=1.5 (down) - marked in sketch in figure 7. Left: velocity field with streamlines. Right: Reynolds stress < u0 w0 > field. UH means velocity at height of building.

Concentration field We have found the critical Reynolds building number ReB = uH H/ν, where ν is kinematic viscosity, and verified the requirements for Towsend hypothesis (see Kellnerová, 2005). The experiment was done with Reynolds building number ReB =15 200 that lies on the lower edge of interval for valid Towsend hypothesis. Relevant free stream velocity was 4 m.s−1 . The vehicle traffic was simulated by continual double line source situated on the floor, therefore the height of the source equals zero. The dimensionless concentration was obtained from formula: K∗ =

CuH HLQ . Q

(1)

C means measured concentration, uH means velocity at the height of buildings H, LQ is length of the line source, Q is source flux. A significant accumulation of traffic pollution occurs inside the corner vortexes created in the street canyons (Figure 9). The similar effect, e.g. collecting gas, was found near the leeward side of the buildings due to developing of vertical vortex (vortex rotating in the vertical plane around a horizontal axis). Windward side concentrations depend complexly on model geometry.

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Figure 9. The concentration field, horizontal cross-section at Z/H=0.3 with line-source. Black points mark measurement points.

Figure 10. Concentration field, vertical cross-section at Y/H=0.75 (left) and Y/H=1.5 (right) – positions are shown in the figure above.

Concentration of contaminant achieved maximum in the center of corner vortex (Y/H=0.75), so it reaches extremely high values up to the roof level (Figure 10 – left). Inside the canyon vertical vortex is developed and affects spreading of pollutants. As a result, the concentration with regard to the latter is much lower, nevertheless the leeward side shows notable accumulation of dangerous gas and particles (Figure 10 – right).

Conclusion The wind tunnel experiments described in this paper have provided significant data sets on pollutant diffusion in an urban intersection. The sufficient fetch for urban canopy layer to develop was found to be the second or the third pattern length from the roughness step change. Inside street canyons that are perpendicular to the main stream the velocity retardation and increase of Reynolds stress are well-obvious. Wave-like variation of vertical stress was detected along the central street inside a canopy layer. The variation has the same wavelength as the pattern length. The flow inside the canopy has a strongly three-dimensional character, therefore the recirculation zone is not created immediately behind the entry of the canyon but further inside. In our case the fetch for developing a closed rotation was 1.5 H from the center of intersection. 167


The large horizontal Reynolds stress values near the windward corner area of intersection decrease with high to small positive values. On the contrary, quadrant analysis proved that partial contributions of Reynolds stress increase with height. Since the partial contributions have similar values with inverse sign, their total sum decreases almost to zero. A significant accumulation of traffic pollution occurs inside the corner vortexes created in the street canyons. In this area the concentration reaches its maximum and dangerous values of emissions attain up to the roof level. Similar accumulation of gas, but with lower concentration, was found near to the leeward side of the buildings. Acknowledgments. and COST 732.

This project was supported by Institutional Research Plan AVOZ20760514

References Britter R.E., Hanna S.R.: Flow and Dispersion in Urban Areas. Annu. Rev. Fluid Mech. 35 (2003):469–96. Cheng H., Castro I.P.: Near Wall Flow Over Urban-Like Roughness. Boundary-Layer Meteorology 104 (2002): 229–259. Cheng H., Castro I.P.: Near-Wall Development After a Step Change in Surface Roughness. BoundarLayer Meteorology 105, (2002), 411- 432. Rafailidis S.: Influence of Building Areal Density and Roof Shape on the Wind Characteristics Above a Town. Boundary-Layer Meteorology 85, (1997), 255-271. Grimmond C.S.B., Oke, T.R.: Aerodynamics Properties of Urban Areas Derived from Analysis of Surface Form, J. Appl. Meteorol. 38, 1999, 1262–1292. Kellnerová R.: Odhad rozloˇzen´ı koncentrac´ı od liniového zdroje v pr´ızemn´ı vrstvˇe uvnitˇr mˇesteské zástavby, (2005), diplomov´ a pr´ ace, 64-68.

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