BPS5234A Chap.3 Particle Transport and Wind-Driven Rain
Particle Transport and Wind-Driven Rain
When air carries something that is not air
Many building questions involve a dispersed phase: dust and particulate matter indoors, respiratory droplets in an infection-risk study, mist from a humidifier, aerosols in a street, and raindrops driven onto a facade. The course treats these with an Eulerian–Lagrangian approach.
The air is solved on the grid as before, and particles or parcels of particles are tracked through it by integrating Newton’s second law, with drag, gravity, buoyancy and, for droplets, evaporation.
Choosing the coupling and the wall rule
For the dilute loadings typical of indoor air quality, the particles respond to the air but do not change it, which is one-way coupling.
Dense sprays and heavy loading need two-way coupling. At every wall the modeller decides whether a particle is trapped, reflected or allowed to escape, and that decision defines what a deposition result means.
Rain on buildings
Wind-driven rain is a multiphase problem in which drops of many sizes are carried by a turbulent wind and pulled down by gravity. Three frameworks exist.
A grid-based Eulerian model treats rain as a volume fraction per drop size and suits city-scale exposure and guideline studies. Lagrangian tracking follows individual drops and suits facade details and rain entering openings. A volume-of-fluid model captures a sharp water surface and suits runoff, not airborne rain.
The results are summarised with a catch ratio, the facade rain flux divided by the horizontal rainfall intensity.
What to be able to do
Expect to justify a framework for a stated objective, list the inputs a rain simulation needs, compute and interpret catch ratios, and explain coupling and wall-interaction choices.
What this chapter covers
- 01
The Eulerian-Lagrangian approach for dilute flows
- 02
Drag, gravity and buoyancy on a particle
- 03
One-way and two-way coupling and wall interaction
- 04
Eulerian, Lagrangian and volume-of-fluid rain models
- 05
Rain injection inputs and drop-size distributions
- 06
The catch ratio and vulnerable facade zones
Worked example · free
Which drag law applies to a fine droplet?
- 1Write the particle Reynolds number in terms of slip velocity and diameter.
- 2Substitute and evaluate it.
- 2Select the drag regime and compute the coefficient.
Key terms
- Lagrangian Tracking
- Following individual particles or parcels through a computed airflow by integrating their equations of motion.
- One-Way Coupling
- A particle model in which the air moves the particles but the particles exert no effect back on the air.
- Drag Coefficient
- The non-dimensional factor that scales the drag force on a particle, set by its Reynolds number.
- Discrete Random Walk
- A stochastic method that adds turbulent fluctuations to particle paths so drops do not simply follow mean streamlines.
- Volume of Fluid
- A method that tracks a sharp interface between immiscible fluids, suited to runoff rather than airborne rain.
Particle Transport and Wind-Driven Rain FAQ
Why are particles tracked individually instead of being solved on the grid like air?
Because the dispersed phase is dilute in most building problems and each particle responds to drag, gravity and evaporation according to its own size and history. Tracking them as individuals keeps that information, while the air remains a continuum solved on the mesh.
What is the difference between one-way and two-way coupling?
In one-way coupling the air pushes the particles and nothing pushes back, which suits indoor aerosol and droplet studies with low loading. Two-way coupling also transfers momentum from particles to air and is needed for dense sprays or heavy particle loading.
Which rain model should a compliance study use?
The course matches frameworks to objectives: grid-based models for city-scale exposure, guideline studies and comparing catch ratios; drop tracking for facade detailing, rain ingress and research; and a hybrid when both accuracy and efficiency are needed. It also notes that drop tracking is widely adopted and compliant with the Green Mark code for wind-driven rain.
What inputs does a rain simulation need?
A rainfall intensity in millimetres per hour, a drop-size distribution such as Marshall-Palmer, and an initial drop velocity, usually the terminal velocity for each size, together with the wind field around the building.
Why report a catch ratio instead of a rain flux?
Because the ratio divides out the storm intensity, so results can be compared across rainfall rates and wind speeds, vulnerable zones identified, and the values linked to hygrothermal and durability analysis.
Assessment move
Make a three-column card for each rain framework listing what it represents, the question it answers, and the reason it fails elsewhere. Then practise the reverse: given a facade question, name the framework in one sentence with its reason.
