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Loom Street canopy.
The question
A canopy over a café's tables had lifted off its posts twice in one winter. Both times the wind was westerly, blowing over the five-storey mill next door. The landlord wanted to know whether to move the canopy, strengthen it, or shelter it.
32
16
−41%
3
What the smoke showed
With the wind from the west, air coming over the mill's roof doesn't carry on over the courtyard. It rolls down the mill's back wall and curls back into the yard, so the canopy sees air coming at it from below. The smoke made the loop plain within the first run: you can see it in the picture above.
Scale matters less than you'd think here. Flow separating from sharp roof edges looks much the same at 1:50 as at full size, which is why building studies in tunnels work at all.
Three options
| Option | What changed | Peak uplift |
|---|---|---|
| Today | Canopy 1.2 m from the mill wall | 100% |
| A | Canopy moved 2.5 m further from the wall | 78% |
| B | Solid 2 m wall on the west side of the yard | 84%, and a new gust at the door |
| C | Slatted screen on the west side, half open | 59% |
The solid wall made things worse near the café door: air went over it and dropped straight down behind. The slatted screen let some air through, which slowed the loop instead of moving it. The screen went up in March.