Improvement of Acoustic Doppler Velocimetry in steady and unsteady turbulent open-channel flows by means of seeding with hydrogen bubbles

Velocimetry is used in various fields of research. In hydraulics, Ultrasonic Velocimetry based on the Doppler Shift effect can accurately resolve the quasi-instantaneous flow field [Takeda Y. Velocity profile measurement by Ultrasonic Doppler method. Experimental Thermal and Fluid Science 1995;10:444–53; Takeda Y. Ultrasonic Doppler method for flow measurement. In: International symposium on advanced fluid information. 2002. p. 69–76] if the acoustic scattering level is sufficiently high. But Ultrasonic velocity instruments are known to perform poorly in clear water with low acoustic scattering level, such as often found in laboratory applications.Artificial seeding of the flow can be used to increase the acoustic scattering level. Blanckaert and Lemmin [Blanckaert K, Lemmin U. Means of noise reduction in acoustic turbulence measurements. Journal of Hydraulic Research IAHR 2006;44:3–17] have proposed a technique to seed the flow with micro hydrogen bubbles generated by means of electrolysis. This technique is optimized under steady flow conditions and subsequently applied to measure the flow characteristics around the front of a surge wave.