A constant-beam-width transducer for 3D acoustic Doppler profile measurements in open-channel flows

An ultrasonic constant-beam-width transducer system which is capable of generating an extended focal zone by electronically focusing the beam over the desired water-depth range is described. Beam directivity measurements show that the higher beam directivity and the reduction in side-lobe level lead to an increase of the signal-to-noise ratio by up to 15 dB compared with that of a plane disc transducer. It also allows one to attain a significant reduction of undesirable spectral broadening effects which in the case of plane disc transducers lower the final resolution and interfere with the correct interpretation of data. Using the focalized transducer, simultaneous 3D velocity component profile measurements over the whole water depth are carried out in uniform, open-channel flow and reveal the presence of coherent structures. In the transversal direction, a stationary secondary current composed of two stationary vortices is observed. Compared with a plane disc transducer system, the focalized system increases the resolution by up to 50%.