BPS5234A Chap.10 Wind Between Buildings and Across Urban Arrays
Wind Between Buildings and Across Urban Arrays
From one building to many
Once a second building is added, each one changes the wind the other receives. The two-building example places a smaller building upwind of a larger one. The upstream wake shelters the lower facade of the tall building, the flow lifted over the small building strikes the tall one higher up and turns into downwash, side gaps produce lateral jets, and the two wakes merge downstream.
Research summarised in the deck adds consequences for peak suction, pollutant re-entry and wind-driven rain.
An urban array at pedestrian height
The urban district lab puts six 40 m blocks into a large domain, applies a 5 m/s inlet, solves a steady realizable k–&epsilon model and reads the result on a plane at 1.5 m.
Sample points in open ground, between blocks and in the wake give velocity ratios close to one, slightly above one and far below one, showing acceleration and stagnation within a few tens of metres.
The deck turns these into design insights about spacing, orientation, height and wind-mitigation features.
Is the passage a Venturi?
The second individual assignment includes a V-shaped passage between two buildings. Students mesh the throat finely, apply an atmospheric inlet, solve with steady RANS and compute a velocity amplification factor, then ask whether the acceleration resembles a classical Venturi.
The answer depends on how much of the wind is diverted over and around the buildings rather than forced through the gap.
What to be able to do
Expect to explain interference effects, compute and interpret velocity ratios, and connect each pattern to a design response.
What this chapter covers
- 01
Tandem buildings: sheltering, downwash and side jets
- 02
Pressure redistribution on the downstream building
- 03
Pedestrian-level velocity ratios in an urban array
- 04
Common set-up mistakes in urban wind labs
- 05
The V-shaped passage and velocity amplification
- 06
Design responses to acceleration and stagnation
Worked example · free
Classifying pedestrian wind round a tandem pair
- 2Divide each probe speed by the reference speed.
- 2Classify each value as accelerated, sheltered or close to ambient.
- 1Name one design response for the worst location.
Key terms
- Downwash
- Flow driven down the windward face of a tall building toward street level by the pressure difference over its height.
- Tandem Arrangement
- Two buildings placed one behind the other in the wind direction, so the downstream one sits in the upstream wake.
- Pedestrian Level
- The height of about 1.5 to 2 m at which outdoor wind comfort and ventilation are evaluated.
- Venturi Effect
- The acceleration of flow through a narrowing passage, strictly a consequence of continuity in a confined duct.
- Lateral Jet
- An accelerated stream of air passing through the gap at the side of a building or between two buildings.
Wind Between Buildings and Across Urban Arrays FAQ
How does a smaller upstream building change the wind on a taller one?
Its wake shelters the lower part of the tall building, lowering wind speed and pressure coefficient there, while the flow deflected over it is accelerated toward the upper facade, increasing suction and downwash. The recirculation length behind the small building depends on the height ratio and plan dimensions.
Why can pollutants re-enter a building in a tandem arrangement?
The deck notes that for certain wind directions an upstream building increases recirculation, so exhaust can be carried downward onto the windward facade or upward along the leeward facade and drawn back in through openings.
What velocity ratios did the urban array lab report?
About 0.94 in open ground, 1.06 in the canyon between blocks where the flow accelerates, and 0.1 in the wake where the flow recirculates weakly, each for a 5 m/s reference at pedestrian height.
Does wind between two buildings really follow the Venturi effect?
Not fully. A closed duct forces all the flow through the throat, but an open passage lets the wind build pressure at the mouth and go over and around the buildings, so the amplification is far smaller than the area ratio. The assignment asks students to test this with streamlines and velocity amplification.
What mistakes do students make in urban wind labs?
The deck lists the wrong inlet direction, buildings not subtracted from the domain, a mesh too coarse near walls, too few iterations and a result plane at the wrong height. Each produces plausible pictures with wrong numbers.
Assessment move
For every urban result you produce, place three probes, one in open ground, one in the tightest gap and one in the deepest wake, and report their velocity ratios. Those three numbers reveal more about a layout than any single contour plot.
