Measurements of flow velocity with cross correlation functions of ultrasonic signals show that the travelling time of structures deviates from the mean flow velocity. This difference usually is explained by the difference between the line integral of measurement and the area integral of the mean flow velocity. A comparison of the probability distribution of velocity components shows that the most frequent components in the fluid are in accordance with the travelling time of structures. The explanation is given by systems theory. In vortex shedding flow meters an ultrasonic wave is modulated by vortices behind a bluff body. The frequency of the vortices is proportional to the flow velocity. It depends on the size and arrangement of bluff bodies. As ultrasound is very sensitive to all kinds of modulating effects the size of bluff bodies can be drastically reduced in comparison to measurements with pressure sensors. Additionally the sensitivity can be increased, pressure losses behind the bluff body are considerably decreasing.

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