Fast flowing fluids like steam are characterized by a turbulent flow profile.
Two ultrasonic transducer pairs are mounted at a defined distance on the pipe forming two channels. They send ultrasonic signals through the pipe. This measurement method exploits the fact that ultrasonic signals are modulated by the turbulence pattern of the fluid, which conducts them. The modulation of the two signal channels are continuously compared to each other. Turbulence patterns, which pass both signal channels are thus recognized, and the travel time is computed. Since the distance between the channels is known and the travel time of turbulence patterns between them is measured, the flow velocity is determined.
Since ultrasounds propagate in solids, the transducers can be mounted on the the pipe surface. The measurement is thus non-invasive, requiring no cutting or welding for the installation of the transducers.