Yes if the gap is so small you don’t need the wind to move though it, then align the buildings to remove it. Same goes for setbacks in facades etc.
Rule of thumb meshing rules (I’m open for alternatives!)
My general rule of thumb is that a setback needs at least 2-3 cells in order to “work” meshwise and 5-10 in order to be reliable simulationwise.
For corridors etc at least 10-20 cells for it to be sort of reliable. At 3-5 cells it can “run” but it will slow the wind and your results look too good. It’s like cutting a corner off a bag of peanuts. If the hole is size of 2 peanuts, the flow will be slow. If you use same hole for rice it goes fast. So either refine it to finer cells (rice) or increase the hole.
What does this mean? If your outer cell size is 20m and your refinement level is 5 then the cells will be 20/(2^5) = 0.6m. this should be ok for setbacks of 1.5m size to “work”. If you need the precision to show the effect of the 1.5m setback, you would need a cell size of 0.3m ish.
Also remember to model edges explicitly. So two rectangles overlapping into a L shaped building should be boolean unioned. So the inner corner of the L has a visible edge.
So… This reveals the issue usually: the context mesh needs less of precision so people turn up the cell size here. However this is imported garbage so you actually need smaller cell size or cleaner geometry. Paradox.
Rule of thumb is around 0.5m cell size close to your buildings of interest. And trees at least 3-5 cells wide.
And always check your mesh manually in para view before running. Do vertical and horizontal slices so you can inspect the cells.
Also run a checkMesh command. If it fails the checkMesh there’s a big risk of the simulation “exploding” where pressure and velocities keep accelerating up to astronomical values until the simulation stops.
All the best,
Mathias Sønderskov Schaltz