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Presented at NATO ASI May 2004

Solar-induced thermal effects on the flow in a street canyon. Eric Savory Advanced Fluids Mechanics Research Group Dept of Mechanical and Materials Engineering University of Western Ontario, Canada Jean-Francois Sini Equipe Dynamique de l’Atmosphere Habitee

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Presented at NATO ASI May 2004

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  1. Solar-induced thermal effects on the flow in a street canyon Eric Savory Advanced Fluids Mechanics Research Group Dept of Mechanical and Materials Engineering University of Western Ontario, Canada Jean-Francois Sini Equipe Dynamique de l’Atmosphere Habitee Laboratoire de Mecanique des Fluides Ecole Centrale de Nantes, France Presented at NATO ASI May 2004

  2. Contents Background Objective Experimental details Wind tunnel and canyon model Boundary layer profiles Results Discussion Including full-scale and CFD data Concluding remarks

  3. Background Under conditions of low wind speed, the effect of wall heating in street canyons, due to solar radiation incident on one or more walls during the course of a day, may be important. Previous numerical predictions, Mestayer et al. (1995), suggest that the buoyancy forces may be large enough to disrupt the dominant canyon vortex and give another flow regime with adverse consequences for local dispersion characteristics.

  4. Objective To determine whether there are any threshold Froude numbers at which solar-induced heating of the windward facing wall of a canyon causes changes to occur in the canyon flow regime. To carry out a wind tunnel investigation, modelling different cases of buoyancy by different temperatures and velocities giving various test Froude numbers.

  5. Uref = 1.0 – 2.6 m/s Tref = 293 K Tw = 298 K H = 20 m Temp. diff. = 5oC, wind speed range = 1.0 – 2.6 m/s giving Froude numbers = 0.27 – 2.00 Related to “urban dome” case. Approx 1:70 geometrical scale Modelled full-scale case

  6. Definition of Froude Number Fr = Uref2 / ( g H (Tw – Tref) / Tref ) Uref = freestream velocity g = acceleration due to gravity (9.81 m/s2) H = height of the cavity (H = W = 285 mm) Tref = absolute ambient temperature Tw = absolute temperature of windward wall

  7. Wind tunnel:working section WxHxL: 3.5m x 1.5m x 20m, max. vel. 4m/s, for neutral, stable and convective layers.Cavity:nominally 2-D street canyon, uniform height, perpendicular to oncoming flow, W/H=1 (0.285m x 0.285m), L/W=8.8, windward wall heatedBoundary layer:Neutral. = 1m (3.5H), Uref = 0.5 - 1.5m/s, Re = 1 – 3 x104, Fr = 0.27 - 2.03, z0 = 1 - 1.6mm, d = 0mm, u*/Uref = 0.064 – 0.070Work Conducted: LDA measurements of mean velocity (± 3.5%) and turbulence (± 4%) and temperature by Thermocouples (± 1.2oC) and Platinum Resistance Thermometers (± 0.5oC)

  8. End plates: H= 0.8m, Lus = 4.5 m, Lds = 2.4m Flow measurement:LDA ? Canyon model for studies of effect of solar-induced heating of the windward-facing wall

  9. Flow Heated wall X/W = 0.07 X/W = 0.28 X/W = 0.50 X/W = 0.72 X/W = 0.93 Location of flow measurement profiles

  10. Boundary layer mean velocity profiles for different wind speeds and locations

  11. Boundary layer turbulent kinetic energy profiles for different wind speeds and locations

  12. Data within ±5% over central ±20% of span Spanwise variation of mean velocity at two different heights (Z/H = 0.19 and 0.98) in the b.l. at two different speeds (Uref = 0.5 and 1.5 m/s)

  13. Data within ±10% over central ±20% of span Spanwise variation of turbulent k.e. at two different heights (Z/H = 0.19 and 0.98) in the b.l. at two different speeds (Uref = 0.5 and 1.5 m/s)

  14. Significant shift of vortex and weakening of downwash Vectors reduced here in all other cases Fr =  Uref = 1m/s, Tw = Tref Fr = 2.03 Uref = 1m/s, Tw = 80oC Mean velocity and temperature in canyon, W/H=1

  15. Strengthening of secondary vortex Strengthening of downwash Fr = 1.17 Uref = 1m/s, Tw = 120oC Fr = 0.73 Uref = 0.8m/s, Tw = 120oC Mean velocity and temperature in canyon, W/H=1

  16. Fr = 0.27 Uref = 0.5m/s, Tw = 120oC Mean velocity and temperature in canyon

  17. Variation with height (Z) of vertical velocity component (W) in canyon near heated wall (X/H=0.93) for the different cases

  18. Tmax at Z/H = -0.60 Tmax at Z/H = -0.53 Fr = 2.03 Fr = 1.17 In all cases the thermal boundary layer thickness at height of the maximum temperature is  0.2 W Detail of temperature distribution near heated wall

  19. Tmax at Z/H = -0.18 Tmax at Z/H = -0.25 Fr = 0.73 Fr = 0.27 Largest transition in the location of maximum temperature occurs at Fr between 1.17 and 0.73 Detail of temperature distribution near heated wall

  20. 0.01 0.03 0.01 0.005 Fr =  Fr = 2.03 As Fr changes, the TKE in the upwind half of the canyon remains largely unchanged Distribution of turbulent kinetic energy (k / Uref2) within the canyon

  21. 0.03 0.03 0.05 0.05 Fr = 1.17 Fr = 0.73 Distribution of turbulent kinetic energy (k / Uref2) within the canyon

  22. The increase in maximum TKE near the heated wall with decreasing Fr is consistent and far in excess of the experimental uncertainty. Changing from the neutral case to Fr = 0.73 leads to an order of magnitude increase in TKE near the wall. 0.11 Fr = 0.27 Distribution of turbulent kinetic energy (k / Uref2) within the canyon

  23. Variation of turbulence intensities with height in canyon near heated wall (X/H=0.93) for the different cases

  24. Fr =  Fr 0.14 Full-scale measurements and CFD predictions (CHENSI) for Rue de Strasbourg, Nantes, France Louka et al (2002)

  25. Full-scale observations Temperature drops of 18oC observed at 1.5m (0.1W) from the heated wall. Measured temperature gradients strongest at 0.02m from the wall but still strong at 0.2m (0.014W) from the wall. CHENSI Predictions CHENSI overpredicts the effects of the heating, due to the near-wall temperature function used. Thermal gradients in the boundary layer are very large and the layer very thin such that the wall model is used in grid cells that are outside this layer.

  26. Concluding Remarks • The wall heating does appear to have some influence on the generation of a very weak secondary flow close to the ground of the canyon at very low Fr. • No evidence that buoyancy forces induce a widespread upward motion, except in a thin layer near the heated wall, as noted from field experiments in Nantes, France. • Hence, not possible to clearly state that effects of wall heating will be significant in terms of the canyon flow field & the motion and dispersion of pollutants.

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