Ventilation generated by a fluctuating pressure differential.

Ventilation produced by fluctuating pressure differences across a building appears to have received little attention . Such fluctuations are produced by gustiness of the wind or turbulence in the flow around a building. An experimantal study has been performed on a laboratory model to investigate unsteady flows through apertures simulating those in the fabric of a building . Independent variables investigated were the mean pressure difference ( p- ) across the aperture and the amplitude ( p0 ) and frequency ( f ) of a superimposed fluctuating pressure difference to give P(t) = p- + p0 sin 2pi f t . The relative amplitude (p0 ) / (p-i ) range covered was 0.2 to 0.97 and the frequency range was 0.02 to 0.6 Hz. A number of aperture geometries has been investigated . Measurements were made of instantaneous pressure difference across the aperture , air flow rate in to the plenum chamber supplying the aperture and velocity of air issuing from the aperture . The relationship between pressure difference and flow was characterised by a discharge coefficient , (Cd) . Cd values based upon the instantaneous measurements were plotted throughout a flow cycle for each operating condition . This produced a Cd loop, the size of which depended upon p0/p- and the Strouhal number. Time-averaged flows were characterised by two Cd values, based upon root mean pressure difference and the mean pressure difference , respectively. Measured values of Cd were compared with the results of a theoretical analysis which predicted successfully the generalfeatures observed.