See how wind moves between buildings at street level
Pick an area of Hong Kong and a wind direction. Your browser runs a small wind simulation on a flat slice about 2 m above the ground, through the government's building footprints. Draw a new block and watch the street wind change.
揀一個地區同風向，你部電腦會即時計一個離地大約 2 米嘅風場切面。加一座新樓，睇下條街嘅風點變。
A qualitative 2D sketch for teaching and early design feel. Not an Air Ventilation Assessment (AVA) under HPLB/ETWB TC No. 1/06: no wind velocity ratio (VR) figures, no pass or fail. 只係教學同早期設計參考嘅二維草圖，唔係 AVA，冇 VR 數字，冇合格不合格。
Paused to rest your computer after 90 seconds with no input. Press Run to carry on. 冇人用 90 秒，自動暫停咗。
How to read it 點樣睇
- Wind corridors 風道: long yellow or orange bands along streets that line up with the wind.
- Calm zones 靜風區: dark blue patches behind blocks (the wake) and inside courtyards.
- Corner acceleration 轉角加速: orange or red spots where the wind squeezes round a corner.
- White streaks follow the air. Eddies keep moving after the pattern settles; real wind is unsteady too.
- Compare, don't measure: change one thing (a block, a gap, a direction) and look at what changes.
What height does here 高度有咩用
The simulation is one flat slice about 2 m above the ground. A block either stands in that slice or it doesn't, so height decides only one thing: a block at least 2.5 m tall from its own base blocks the slice. A 10-storey and a 60-storey tower look the same to this sketch.
In real streets tall towers also drag faster air down to the pavement (downwash), and wind comes over roofs as well as round the sides. A 2D slice shows neither. 二維切面睇唔到高樓將高空風帶落地面，亦睇唔到風由屋頂吹過。
What this is not 呢個唔係乜
- Not an AVA and not the TC No. 1/06 method, which needs 3D, wind directions weighted by how often they blow, VR at test points, and usually a wind tunnel for a Detailed Study.
- No wind speeds, no VR, no comfort or safety rating, no pass or fail.
- Nothing about wind loads on buildings (that is the Code of Practice on Wind Effects in Hong Kong 2019, for structural engineers).
- 2D only: no terrain, trees, footbridges or covered walkways; open ground floors are not known; cells are about 2 m; the incoming wind is uniform and made up.
How it works 原理
The 600 m square is split into a grid of cells (300 x 300 at Normal). Each cell holds nine numbers: how much air is moving in each of nine directions. Every step the air hops to the next cell and the nine numbers relax towards a balanced mix; air that hits a wall bounces back. This is the lattice Boltzmann method (D2Q9, with a Smagorinsky turbulence model). Thousands of steps later, the street pattern appears. It runs on your own computer: nothing is uploaded.