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.