= Source-volume blocking of displacement ventilation
{title2=$Q_c=(A_e^2BH)^{1/3}$}
A source volume $Q$ makes roof outflow exceed floor inflow by $Q$. At zero floor inflow, an entraining plume cannot support a steady cold lower layer; the warm layer reaches the floor. With <reduced gravity> $g'=B/Q$, the entire hydrostatic head drives roof discharge $Q=A_e\sqrt{g'H}$. This gives the stated threshold. Equal splitting of <pressure> head is inappropriate at blocking. The same result in physical-area notation is $Q_c=(2C_d^2A_{\rm geom}^2BH)^{1/3}$.
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