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• Non-uniform heating of building

walls and ground in an urban street

canyon induces thermally-driven

airflow

• Important topic of interest because

of its implications for air quality,

building heating & A/C requirements,

and emergency response planning

• Mainly been studied using wind-

tunnel experiments and numerical

models

• Only a few field-scale experiments

have been performed – leaving a

need to verify models and lab

experiments

Thermally Driven Flow in a Mock Street Canyon

A. Dallman1, S. Magnusson2, L. Norford2, H.J.S. Fernando1,

D. Entekhabi2, R. Britter2, S. Pan3

Introduction / Motivation

Experimental Setup

Flow Visualization Data Classification

Critical Buoyancy Parameter

Acknowledgements / References

• A field experiment

was carried out on

the campus of

Nanyang Technical

University in

Singapore

• Consisted of an

idealized ‘street

canyon’

• Constructed with

two rows of shipping

containers (total

length 12.2m)

aligned in the North-

South direction

1Environmental Fluid Dynamics Laboratories

Department of Civil and Environmental Engineering,

University of Notre Dame, South Bend, IN

2Center for Environmental Sensing and Modeling

Singapore-MIT Alliance for Research and Technology and

Massachusetts Institute of Technology, Boston, MA

• Fernando HJS, Zajic D, Di Sabatino S, Dimitrova R, Hedquist B, Dallman A (2010) Flow, turbulence, and pollutant

dispersion in urban atmospheres. Phys Fluids., 22:051301-20

• Li X-X., Britter RE, Norford LK, Koh T-Y, Entekhabi D (2012) Flow and Pollutant Transport in Urban Street Canyons of

Different Aspect Ratios with Ground Heating: Large-Eddy Simulation. Bound-Layer Meteorol 142:289-304

• Qu Y, Milliez M, Musson-Genon L, Carissimo B (2012) Numerical study of the thermal effects of buildings on low-speed

airflow taking into account 3D atmospheric radiation in urban canopy. J Wind Eng Ind Aerodyn 104-106:474-483

The canyon was instrumented with:

• sonic anemometers (wind

speed and direction and

virtual temperature)

• weather stations (wind

speed and direction,

temperature, relative

humidity, pressure, and rain

fall)

• thermocouples (surface

temperature of the

‘buildings’)

Figure 2. Experiment setup, with aspect ratio shown. The view is looking

towards north.

Consider flow inertially induced in the

canyon by overlying flow 𝑢0 (Figure

7a)

Assuming skimming or transitional

(between skimming & wake-

interference) flow regime:

𝑤𝑚 𝐻~ 𝑢𝑚 𝐿 ~ 𝑢0 𝐿 (1)

If 𝑢0 is weak, but walls of canyon are

differentially heated (𝑇2 > 𝑇1), thermally driven circulation is induced

(Figure 7b)

𝑢

𝑢0~

𝑢𝑚𝑢0≈ 𝛽1 𝐵 < 𝐵𝑐

𝑢𝑚 + 𝑢𝑡𝑢0≈ 𝛽1 + 𝛽2 𝐵

1 2 𝐵~𝐵𝑐

𝑢𝑡𝑢0≈ 𝛽3 𝐵

1 2 𝐵 > 𝐵𝑐

Theoretical Considerations

3Center for Environmental Sensing and Modeling

Singapore-MIT Alliance for Research and Technology and

Nanyang Technical University, Singapore

Figure 3. Close-up view of experiment setup, showing sonic

anemometers and weather stations in the centerline of the canyon.

Figure 4. Instrument layout at the experimental site.

Figure 7. (a) Inertially driven flow and (b) thermally driven

flow in a canyon with aspect ratio 𝐻 𝐿 < 1.

Thermal circulation velocities 𝑢𝑡 = 𝑢𝑡 , 𝑤𝑡 can be calculated using vorticity

generation, 𝐷𝜔 𝐷𝑡 ~ 𝛻 × 𝑏 as:

𝑢𝑡 ∙ 𝛻𝜔𝑦 ~ −𝜕𝑏

𝜕𝑥(2)

where

𝜔𝑦 ~ − 𝜕𝑢 𝜕𝑧 − 𝜕𝑤 𝜕𝑥

~ − 𝑢𝑡 𝐻 − 𝑢𝑡 𝐻 𝐻 𝐿2

~ − 𝑢𝑡 𝐻 (3)

For 𝐻 𝐿 ≲ 1, (2) becomes, with 𝑏 = 𝑔 𝛼 ∆𝑇 and 𝑢𝑡 𝐿 ~ 𝑤𝑡 𝐻,

𝑢𝑡𝜕𝜔𝑦

𝜕𝑥+ 𝑤𝑡𝜕𝜔𝑦

𝜕𝑧= −𝜕𝑏

𝜕𝑥(4)

−𝑢𝑡

𝐿

𝑢𝑡

𝐻−𝑤𝑡

𝐻

𝑢𝑡

𝐻~ −𝑔 𝛼 ∆𝑇

𝐿

Thus the scale for thermal circulation is:

𝑢𝑡 ~ 𝑔 𝛼 ∆𝑇 𝐻 1 2 (5)

When a mean flow and thermal circulation are both present, the latter could

dominate when 𝑢𝑡 ≫ 𝑢𝑚 or

𝐵 =𝑔 𝛼 ∆𝑇 𝐻

𝑢02 > 𝐵𝑐 (6)

where 𝐵𝑐 is a critical value of the buoyancy parameter, 𝐵

• A fog machine was used to visualize flow in the canyon

• Below are still shots from a video taken on July 2 at

12:27 with little buoyant activity (cloudy)

Figure 5. Visualization shows inertially driven flow forming a vortex,

which is confirmed by concurrent data (pictured on the right).

Figure 6. Visualization shows a vortex being advected through the

canyon due to a change in background wind direction, which is

confirmed by concurrent data (pictured on the right).

Author Contact Information: adallman@nd.edu

Research supported in part by the Singapore National Research Foundation through the Singapore-MIT Alliance for

Research and Technology’s Center for Environmental Sensing and Modeling and by the University of Notre Dame

Professional Development Fund

Figure 1. Schematic of (a) even and (b) differential

heating in a street canyon.

𝑎

𝑏

Do scaled RMS velocities

depend on 𝑢0?

For B < 𝐵𝑐 :

𝜎𝑢

𝑢0~𝑐𝑜𝑛𝑠𝑡

𝐵 is not important

For B > 𝐵𝑐 :

𝜎𝑢

𝑢0~𝑔 𝛼 ∆𝑇 𝐻

𝑢02

1 2

𝑢0 is not important

Conclusions

A dimensionless number, the buoyancy parameter 𝐵, was proposed to

determine the dominance of thermal circulation in a street canyon

The critical buoyancy parameter was found to be 𝐵𝑐 ~ 0.06 − 0.2, where thermal circulation dominates for B > 𝐵𝑐

Scaling laws were proposed for RMS velocities for B <

𝐵𝑐 ( 𝜎𝑢 𝑢0~ 𝑐𝑜𝑛𝑠𝑡) and B > 𝐵𝑐 ( 𝜎𝑢 𝑢0~𝐵 1 2)

Currently running CFD simulations to compare with measured field data

Figure 8a. Westerly flow with either same wall temperatures

or west wall warmer.

Figure 10. Scaled canyon RMS velocity vs. the buoyancy parameter.

Based on (1), the scaled

velocity in the canyon for

inertially driven flow

should be a constant

For canyon flow

dominated by thermal

circulation, the scaled

velocity should be a

function of B

An intermediate zone can

be defined where the flow

is dependent on both the

overlying flow and the

differential heating

Figure 9. Scaled canyon velocity vs. the buoyancy parameter.

Consecutive periods (>=30sec) of 10 sec averages were considered for overlying flow

perpendicular to the canyon

Only cases where inertial and thermal effects aided circulation were considered

Figure 8b. Easterly flow with either same wall temperatures

or east wall warmer.