BPS5234A Chap.11 Street Canyons and Urban Morphology
Street Canyons and Urban Morphology
Geometry decides ventilation
The street canyon lab holds the street width at 20 m and raises the buildings from 10 to 20 to 40 m, giving height-to-width ratios of 0.5, 1.0 and 2.0. With the wind perpendicular to the street, the shallow canyon behaves as isolated roughness, the square canyon holds a single vortex in wake interference, and the deep canyon shows skimming flow, with a shear layer at roof level over a trapped vortex and possibly a weaker secondary vortex beneath.
Ventilation and pollutant removal worsen in that order.
Setting up the canyon
The canyon is run in two dimensions with a steady RNG k–&epsilon model, enhanced wall treatment, a 3 m/s inlet and a T-shaped domain after a published study. Refinement at roof edges, ground and walls with ten inflation layers and a target y+ of 1 to 30 decides whether the shear layer and vortex are captured.
The deliverables are plots of speed, streamlines and vorticity together with vertical velocity profiles.
Four urban forms
The morphology lab compares open, medium-density, dense and mixed-height block layouts with an atmospheric inlet at 3 m/s, rough ground and a realizable k–&epsilon model.
Open layouts ventilate well; medium layouts show recirculation and channelling; dense layouts stagnate; mixed-height layouts mix strongly through downwash. The course follow-up limits the exercise to nine buildings, four layouts and one wind direction.
Reading amplification
The velocity amplification factor divides pedestrian-level speed by the reference speed.
Dense layouts show both very low and very high values in the same district.
What this chapter covers
- 01
Canyon aspect ratio and the three flow regimes
- 02
Shear layer, main vortex and secondary vortex
- 03
Canyon set-up decisions and vortex diagnostics
- 04
Open, medium, dense and mixed-height layouts
- 05
The velocity amplification factor at pedestrian height
- 06
Design levers for high and low amplification
Worked example · free
Placing a real street on the regime scale
- 2Compute the height-to-width ratio of each street.
- 2Place each against the lab cases and their regimes.
- 1Rank them for pollutant removal with a reason.
Key terms
- Street Canyon
- A street flanked by continuous buildings on both sides, characterised by its height-to-width ratio.
- Skimming Flow
- The regime in a deep canyon where the wind passes over a roof-level shear layer and drives a trapped vortex below.
- Wake Interference Flow
- The intermediate regime in which the wake of one building reaches the next and a single vortex fills the canyon.
- Building Coverage Ratio
- The share of site area covered by building footprints, one of the descriptors of an urban morphology.
- Q-Criterion
- A field quantity used to identify vortex cores more reliably than velocity magnitude alone.
Street Canyons and Urban Morphology FAQ
Why does a deeper canyon ventilate worse?
As the canyon deepens the wind stops descending into it and skims over a shear layer at roof level. The trapped vortex beneath exchanges air with the flow above only weakly, so pollutant released at street level is retained rather than removed.
Why use vorticity or the Q-criterion for canyon results?
Velocity contours can make a weak vortex invisible, especially a secondary vortex low in a deep canyon. Vorticity and the Q-criterion locate vortex cores directly, which is what the lab asks for when it requires the vortex centre to be extracted.
Why does a mixed-height layout often ventilate better than a uniform dense one?
Uneven heights generate extra turbulence and drive air vertically, through downwash and colliding shear layers, so air is brought down into the street even when building coverage is similar. The model answers favour it for tropical cities on that ground, since it also keeps the streets shaded.
What does a velocity amplification factor above 2.0 indicate?
That the pedestrian-level wind is more than twice the reference speed, typically at corners and gaps in dense layouts. The model answers point to four levers for shrinking such areas: a more porous layout, varied heights, wider passages, and podiums that let air pass.
Why scale the morphology domain on the tallest building?
Domain extents set in multiples of the tallest height let the inflow develop and the wakes recover without blockage or artificial effects. Shortening the upstream length from 5H to 2H would distort the inlet turbulence and velocity profiles and so the pedestrian-level speeds.
Assessment move
Draw the three canyon regimes and the four morphologies as thumbnail sketches with arrows, then write one sentence under each about ventilation. Rehearse explaining the change from one sketch to the next, because the tutorial questions ask why the regime changes, not only what it is.
