Natural ventilation

Natural ventilation exchanges indoor and outdoor fluid through openings using pressure differences created by buoyancy or wind.

Displacement ventilation supplies relatively cool air at low level and removes warmer air at high level. In the ideal two-layer model, buoyant plumes feed a warm upper layer above a sharp density interface while the supply occupies the lower layer.

At steady state, the displacement-ventilation interface lies where the total plume volume flux equals the imposed ventilation flux. For one triangular-profile wall line plume with kinematic buoyancy flux $B_0$, entrainment coefficient $\alpha$, and ventilation flux $A$ per unit span,

A tall opening between fluids of different densities can support simultaneous inflow below a neutral pressure level and outflow above it. For a symmetric opening with a Boussinesq hydrostatic pressure difference and equal discharge coefficients, the neutral level lies at mid-height.

The neutral pressure level is the height at which indoor and outdoor hydrostatic pressures agree. Flow through an opening reverses direction across this level.

A stratified-room plume-exchange model balances the upward volume transported by a warm plume against downward transport by cold wall plumes. A global buoyancy balance relates the source flux to the buoyancy carried out through the vents.