1. INTRODUCTION

QWIC-URB is a fast response model designedto generate high resolution, 3-dimensional windfields around buildings. The wind fields areproduced using a mass consistent diagnosticwind model based on the work of Röckle (1990,1998) and Kaplan & Dinar (1996).

QWIC-URB has been used for producing windfields around single buildings with variousincident wind angles (Pardyjak and Brown2001). Recently, the model has been expandedto consider two-building, 3D canyon flow. Thatis, two rectangular parallelepipeds of height H,crosswind width W, and length L separated by adistance S.

The purpose of this work is to continue toevaluate the Röckle (1990) model and developimprovements. In this paper, the model iscompared to the twin high-rise building data setof Ohba et al. (1993, hereafter OSL93).Although the model qualitatively predicts theflow field fairly well for simple canyon flow, itover predicts the strength of vortex circulationand fails to reproduce the upstream rotor.

2. MODEL DESCRIPTION

An initial wind field is prescribed (uo,vo,wo)based on an incident flow, uin, and the variousflow effects associated with building geometriesand the ground. These effects are accounted forby incorporating empirical parameterizations ofvarious flow regimes associated with flowaround buildings. The final velocity field (u,v,w)is obtained by forcing the initial velocity field tobe mass consistent. For simple geometries, theresulting complex 3D velocity field resembles atime-averaged experimental result.

For a street canyon with two buildings of equaldimensions, the upwind eddy of the first building,down-stream cavity and recovery zones behindthe second building were specified using thesame parameterizations as those used in thesingle building case (e.g., see Pardyjak and

Brown 2001). Similar to Kaplan and Dinar(1996), the canyon flow was parameterized bytwo f low regimes: skimming (when

0.15W 1.25HS +< f o r 2/H <W and 1.55HS < for W/H ≥ 2) and isolated flow. In

the skimming regime a simple vortex is imposedbetween the buildings and the horizontal velocitycomponent perpendicular to the canyon axis isspecified as:

( ) ( )

−=

S

dS

S

d

HU

zyxU

5.05.0

),,(

( )

−−

−−=S

dS

S

d

HU

zyxw

5.01

5.01

2

1),,(,

where S is the spacing between the buildings, dis the distance downwind from the backside ofthe upwind building, w is the vertical velocitycomponent and U(H) is the wind velocity at thetop of the upwind building.

3. RESULTS

FAST RESPONSE MODELING OF A TWO BUILDING URBAN STREET CANYON

Eric R. Pardyjak1 and Michael J. Brown2

1University of Utah, 2Los Alamos National Laboratory

J1.4

Fig. 1: Wind vectors comparison between Ohba et al.(1993) data and QWIC-URB along the centerline.

* Corresponding author address: Eric R.Pardyjak, University of Utah, Department ofMechanical Engineering, Salt Lake City, UT 84112, pardyjak@eng.utah.edu

Velocity vectors from the two-building high-risedata set of OSL93 and QWIC-URB calculationsare shown in Fig. 1. The building dimensions forthe case shown are L=W=H/3 and the spacingbetween buildings S=H/2. The canyon cavity inQWIC-URB is qualitatively similar to the OSL93cavity, however the OSL93 cavity is weaker andnot centered within the canyon (shifted slightlyupwind). The reattachment length for thedownwind building cavity given by the OSL93data is ~0.4H and for QWIC-URB ~0.5H.

The QWIC-URB model shows no front eddyon the upwind building, indicating that for thishigh aspect ratio flow, the mass conservationprocedure may overwhelm the parameterizationplaced upwind of the first building and the flowjust goes around the building. The experimentaldata indicate that in fact, a small recirculationregion does exist. Also, as was the case for thesingle building case, QWIC-URB generates asmall jet at the top upwind corner of the firstbuilding, but does not produce the rooftoprecirculation region.

Figure 2 and 3 are plots from a QWIC-URBcalculation of two wider buildings with L=H=W/2and the spacing between buildings S=1.5H. Asshown in Fig. 2, while not explicitly specified, themodel generates two wall normal vortices nearthe edges of the canyon that may impacttransport from the canyon. Fig. 3 shows particletraces for the same case supporting this idea.

4. SUMMARY

QWIC-URB was compared to the twin high-rise building data of Ohba et al. (1993). Theresults qualitatively predict the flow field well forsimple canyon flow, however, significantquantitative differences were found. For larger

W/H, the mass conservation method producesphysically significant wall normal canyonvortices that may be important in the transport ofpollutants out of the canyon.

5. REFERENCES

Kaplan, H. and N. Dinar (1996) A LagrangianDispersion Model for Calculating ConcentrationDistribution within a built-up domain, Atmos.Env., 30, 4197-4207.

Ohba, M., Snyder, W.H. and Lawson, R.E.(1993) Study on prediction of gas concentrationsaround twin high-rise buildings using wind tunneltechniques, Data report.

Pardyjak, E.R. and Brown, M.J. Evaluation of aFast-Response Urban Wind Model–Comparisonto Single-Building Wind-Tunnel Data, (Eds. D.Boyer and R. Rankin), Proceedings of the 2001International Symposium on EnvironmentalHydraulics 2001,Tempe, Arizona.

Röckle, R. (1990) Bestimmung derStomungsverhaltnisse im Bereich komplexerBebauungsstrukturen. Ph.D. thesis,Vom Fachbereich Mechanik, der TechnischenHochschule Darmstadt, Germany.

Röckle, R., C.-J. Richter, Th. Salomon, F.Dröscher, and J. Kost (1998) Ausbreitung vonEmissionen in komplexer Bebauung - Vergleichzwischen numerischen Modellen undWindkanalmessungen. Projekt eropäischesForschungszentrum für Maßnahmen derLuftreinhaltung, PEF 295002.

Fig. 3: Particle traces showing the upstream andcanyon vortices (freestream wind is from right to leftnormal to the obstacles).

Fig. 2: Plan view of streamlines showing wall-normalvortices generated by QWIC-URB. (freestream wind isfrom left to right).