Non intrusive measurements of the acoustic pressure and velocity fluctuations of fluids flowing in pipes

A nonintrusive flowmeter based on nonlinear acoustical interactions, has been developed. This tool measures simultaneously the pressure and velocity fluctuations of fluids flowing in pipes. A transmitter produces a CW ultrasonic beam which crosses the tube wall, propagates through the opposite side, and is collected by a receiving transducer. Both transducers are just put against the tube wall so that it is not necessary to drill holes in order to install the probes. The ultrasonic propagation across the tube is modelled using an exact matrix numerical method which allows one to specify the geometry of the device. The high frequency beam and the low frequency flow fluctuation interact nonlinearly so that the ultrasonic signal is phase modulated. The amplitude of the modulation (about a hundredth radian) is a linear combination of the pressure and velocity fluctuations. Specific electronics, based on a PLL technique, has been developed for demodulating the signal. We present measurements obtained after generating flow fluctuations in a cavity excited over a broad spectrum