An orifice flow meter is widely used in industry to measure the flow rate of gas or liquid in the pipe because of its simplicity. The accuracy of measurement is acceptable when the steady flow is measured. However, it is often the case in industry that the unsteady flow must be measured. It is not yet known, under unsteady flow conditions, how accurate the flow rate can be measured using the orifice flow meter and where the measurement errors come from. In the present paper to answer these questions a pulsating air flow is considered as the simplest of the various types of unsteady flow patterns and the measurement of the flow field in the pipe with orifice is made in detail. The sound pressure level is also measured by probe tube microphone. A comparison is made of power spectrum densities obtained by measurement of sound pressure level and by the velocity measurement from laser Doppler anemometer.

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