presented at nato asi may 2004 solar-induced thermal effects on the flow in a street canyon eric...
TRANSCRIPT
Presented at NATO ASI May 2004
Solar-induced thermal effects onthe flow in a street canyon
Eric SavoryAdvanced Fluids Mechanics Research Group
Dept of Mechanical and Materials EngineeringUniversity of Western Ontario, Canada
Jean-Francois SiniEquipe Dynamique de l’Atmosphere Habitee
Laboratoire de Mecanique des FluidesEcole Centrale de Nantes, France
Contents
BackgroundObjectiveExperimental details
Wind tunnel and canyon modelBoundary layer profiles
ResultsDiscussion
Including full-scale and CFD dataConcluding 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
Uref = 1.0 – 2.6 m/s
Tref = 293 K
Tw = 298 K
Modelled full-scale case
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
Definition of Froude Number
Fr = Uref2 / ( g H (w – Tref) / ref )
Uref = freestream velocity
g = acceleration due to gravity (9.81 m/s2) H = height of the cavity (H = W = 285 mm)ref = absolute ambient temperature
Tw = 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 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)
Endplate
W=285mm
Effective Canyon length L=2.5m
H=285mm
Flow direction
HeatedWall [Tw]
X
Z
Thermocouple
PLAN
ELEVATION
Canyon
X
Y
Canyon model for studies of effect of solar-induced heating of the windward-facing wall
Flow measurement:LDA
End plates: H= 0.8m,Lus = 4.5 m, Lds = 2.4m
?
X/W = 0.07X/W = 0.28
X/W = 0.50X/W = 0.72X/W = 0.93
Heated wall
Flow
Location of flow measurement profiles
0
0.5
1
1.5
2
0.4 0.6 0.8 1
U / Uref
Z /
HUref = 1m/s, X / W =-3.16Uref = 1m/s, X / W =-0.088Uref = 0.8m/s, X / W= -3.16Uref = 0.8m/s, X / W= -0.088Uref = 0.5m/s, X / W= -3.16Uref = 0.5m/s, X / W= -0.088
Boundary layer mean velocity profiles fordifferent wind speeds and locations
0
0.5
1
1.5
2
0 0.005 0.01 0.015 0.02k / Uref 2
Z / H
Uref = 1m/s, X / W = -3.16
Uref = 1m/s, X / W = -0.088
Uref = 0.8m/s, X / W = -3.16
Uref = 0.8m/s, X / W = -0.088
Uref = 0.5m/s, X / W = -3.16
Uref = 0.5m/s, X / W = -0.088
Boundary layer turbulent kinetic energy profilesfor different wind speeds and locations
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.4 0.6 0.8 1
U / Uref
Y /
L
Uref = 1.5m/s, X / W =-0.088, Z / H = 0.19
Uref = 1.5m/s, X / W =-0.088, Z / H = 0.98
Uref = 0.5m/s, X / W =-0.088, Z / H = 0.19
Uref = 0.5m/s, X / W =-0.088, Z / H = 0.98
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)
Data within ±5% over central ±20% of span
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0 0.005 0.01 0.015 0.02
k / Uref2
Y /
L
Uref = 1.5m/s, X / W = -0.088,Z / H = 0.19
Uref = 1.5m/s, X / W = -0.088,Z / H = 0.98
Uref = 0.5m/s, X / W = -0.088,Z / H = 0.19
Uref = 0.5m/s, X / W = -0.088,Z / H = 0.98
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)
Data within ±10% over central ±20% of span
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X/W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
Reference vector (vel./Uref=1)
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
T [C]80706055504540353020
Reference vector (Vel/Uref=1)
Mean velocity and temperature in canyon, W/H=1
Fr = Uref = 1m/s, Tw = Tref
Fr = 2.03Uref = 1m/s, Tw = 80oC
Vectors reduced here inall other cases
Significant shift of vortex and weakening of downwash
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
T [C]80706055504540353020
Reference vector (Vel/Uref=1)
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
T [C]80706055504540353020
Reference vector (Vel/Uref=1)
Mean velocity and temperature in canyon, W/H=1
Fr = 0.73Uref = 0.8m/s, Tw = 120oC
Fr = 1.17Uref = 1m/s, Tw = 120oC
Strengthening of downwash Strengthening of secondaryvortex
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
T [C]80706055504540353020
Reference vector (Vel/Uref=1)
Mean velocity and temperature in canyon
Fr = 0.27Uref = 0.5m/s, Tw = 120oC
W / Uref
-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10
Z /
H
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6Fr = inf.Fr = 2.03 Fr = 1.17Fr = 0.73Fr = 0.27
Variation with height (Z) of vertical velocity component (W) in canyon near heated wall (X/H=0.93) for the different cases
0.8 0.9 1X / W
-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1
00.10.20.3
Z/H
T [C]50454035302520
0.8 0.9 1X / W
-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1
00.10.20.3
Z/H
T [C]80757065605550454035
Fr = 2.03 Fr = 1.17
Detail of temperature distribution near heated wall
Tmax at Z/H = -0.60 Tmax at Z/H = -0.53
In all cases the thermal boundary layer thickness atheight of the maximum temperature is 0.2 W
0.8 0.9 1X / W
-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1
00.10.20.3
Z/H
T [C]80757065605550454035
0.8 0.9 1X / W
-1-0.9-0.8-0.7-0.6-0.5-0.4-0.3-0.2-0.1
00.10.20.3
Z/H
T [C]80757065605550454035
Fr = 0.73 Fr = 0.27
Detail of temperature distribution near heated wall
Tmax at Z/H = -0.25 Tmax at Z/H = -0.18
Largest transition in the location of maximum temperatureoccurs at Fr between 1.17 and 0.73
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
0.01
0.0
05
0.01
k/Uref**20.090.070.050.030.010.005
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
0.01
0.005
0.01
0.03
0.005
k/Uref**20.090.070.050.030.010.005
Fr = Fr = 2.03
Distribution of turbulent kinetic energy (k / Uref2)
within the canyon
0.01
0.01
0.005
0.03
As Fr changes, the TKE in the upwind halfof the canyon remains largely unchanged
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
0.01
0.01
k/Uref**20.090.070.050.030.010.005
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
Z/H
0.0
5
0.03
k/Uref**20.110.090.070.050.030.010.005
Fr = 1.17 Fr = 0.73
Distribution of turbulent kinetic energy (k / Uref2)
within the canyon
0.03 0.050.03
0.05
-0.2 0 0.2 0.4 0.6 0.8 1 1.2X / W
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3Z
/H 0.05
0.11
0.030.07
k/Uref**20.110.090.070.050.030.010.005
Fr = 0.27
Distribution of turbulent kinetic energy (k / Uref2)
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.
u'2 / Uref2
0.00 0.02 0.04 0.06 0.08 0.10
Z /
H
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6Fr = inf.Fr = 2.03 Fr = 1.17Fr = 0.73Fr = 0.27
w'2 / Uref2
0.00 0.02 0.04 0.06 0.08 0.10
Z /
H-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6Fr = inf.Fr = 2.03 Fr = 1.17Fr = 0.73Fr = 0.27
Variation of turbulence intensities with height in canyon near heated wall (X/H=0.93) for the different cases
Full-scale measurements andCFD predictions (CHENSI) for Rue de Strasbourg, Nantes, France Louka et al (2002)
Fr = Fr 0.14
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.
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.