Noninvasive acoustical property measurement of fluids
Methods for noninvasive determination of acoustical properties of flowing in pipes having a large ratio (>10) of pipe diameter to wall thickness, and in highly attenuating fluids are described. When vibrations are excited on the outer surface of the wall of a pipe, the resulting vibrations propagate directly through the wall in a normal direction and through the pipe wall as guided waves, appearing on the opposite side of the pipe. This dual path propagation through pipes, where guided waves take the circumferential path in the wall of the pipe and may interfere with the time of-flight measurement obtained from the direct path through the fluid, is at least in part resolved by subtracting the signal from the guided wave from the combined signal, thereby permitting improved observation of the direct path propagation through the fluid.
1. A method for noninvasively measuring acoustical properties of a fluid, said method comprising the steps of:
applying a shaped pulse signal to a first ultrasonic transducer in vibrational communication with an outside surface of a pipe having a wall and through which said fluid is flowing, the shaped pulse signal being a Gaussian modulated sine pulse, whereby vibrations are generated in said fluid and in the pipe wall;
detecting the generated vibrations on a second ultrasonic transducer disposed on the outside surface of the pipe diametrically opposite to the first ultrasonic transducer with fluid flowing through the pipe, whereby a first time-dependent electrical signal is obtained;
introducing a chosen gas into the fluid to a gas volume fraction such that no vibrations pass through the fluid;
applying the shaped pulse signal to the first ultrasonic transducer, whereby vibrations are generated solely in the pipe wall;
detecting the generated vibrations on the second ultrasonic transducer with no vibrations passing through the fluid, wherein a second time-dependent electrical signal is obtained;
subtracting the second electrical signal from the first electrical signal whereby a difference electrical signal is generated;
determining a time-of-flight of the vibrations between the first transducer and the second transducer using the difference electrical signal; and
determining acoustical properties of the fluid using the determined time-of-flight of the vibrations.
2. The method of claim 1 , wherein the time-of-flight of the generated vibrations between the first transducer and the second transducer is determined from the difference electrical signal using at least one of: pulse-overlap frequency mixing, signal deconvolution, or signal cross-correlation.
3. The method of claim 1 , wherein a direction of propagation of the vibrations is perpendicular a direction of flow of the fluid through the pipe.
4. The method of claim 1 , further comprising the step of storing the second signal as a reference signal.
5. The method of claim 1 , wherein the shaped pulse signal comprises frequencies chosen to maximize the ratio of the first signal to the second signal.