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 lAtmosphere Habitee Laboratoire de Mecanique des Fluides Ecole Centrale de Nantes, France

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

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.

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.

H = 20 m U ref = 1.0 – 2.6 m/s T ref = 293 K T w = 298 K Modelled full-scale case Temp. diff. = 5 o C, 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

Definition of Froude Number Fr = U ref 2 / ( g H ( w – T ref ) / ref ) U ref = freestream velocity g = acceleration due to gravity (9.81 m/s 2 ) H = height of the cavity (H = W = 285 mm) ref = absolute ambient temperature T w = absolute temperature of windward wall

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 heated Boundary layer: Neutral. = 1m (3.5H), U ref = m/s, Re = 1 – 3 x10 4, Fr = , z 0 = mm, d = 0mm, u * /Uref = – Work Conducted: LDA measurements of mean velocity (± 3.5%) and turbulence (± 4%) and temperature by Thermocouples (± 1.2 o C) and Platinum Resistance Thermometers (± 0.5 o C)

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

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

Boundary layer mean velocity profiles for different wind speeds and locations

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

Spanwise variation of mean velocity at two different heights (Z/H = 0.19 and 0.98) in the b.l. at two different speeds (U ref = 0.5 and 1.5 m/s) Data within ±5% 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 (U ref = 0.5 and 1.5 m/s) Data within ±10% over central ±20% of span

Mean velocity and temperature in canyon, W/H=1 Fr = U ref = 1m/s, T w = T ref Fr = 2.03 U ref = 1m/s, T w = 80 o C Vectors reduced here in all other cases Significant shift of vortex and weakening of downwash

Mean velocity and temperature in canyon, W/H=1 Fr = 0.73 U ref = 0.8m/s, T w = 120 o C Fr = 1.17 U ref = 1m/s, T w = 120 o C Strengthening of downwash Strengthening of secondary vortex

Mean velocity and temperature in canyon Fr = 0.27 U ref = 0.5m/s, T w = 120 o C

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

Fr = 2.03Fr = 1.17 Detail of temperature distribution near heated wall T max at Z/H = T max at Z/H = In all cases the thermal boundary layer thickness at height of the maximum temperature is 0.2 W

Fr = 0.73 Fr = 0.27 Detail of temperature distribution near heated wall T max at Z/H = T max at Z/H = Largest transition in the location of maximum temperature occurs at Fr between 1.17 and 0.73

Fr = Fr = 2.03 Distribution of turbulent kinetic energy (k / U ref 2 ) within the canyon As Fr changes, the TKE in the upwind half of the canyon remains largely unchanged

Fr = 1.17 Fr = 0.73 Distribution of turbulent kinetic energy (k / U ref 2 ) within the canyon

Fr = 0.27 Distribution of turbulent kinetic energy (k / U ref 2 ) within the canyon 0.11 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.

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

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

Full-scale observations Temperature drops of 18 o C 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.

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.