A high resolution constant beam width pulse-to-pulse coherent 3-D acoustic Doppler profiling current meter for use in laboratory and environmental studies

An ultrasonic constant beam width transducer system (1 MHz) is described which is capable of generating an extended focal zone by electronically focusing the beam over the desired water depth range. Compared to a plane disc transducer, the higher beam directivity and the reduction in side lobe level leads to an increase of the signal-to-noise ration by up to 15 dB and allows to significantly reduce undesirable spectral broadening effects. The potential of the system in directly measuring instantaneous sediment flux profiles by making use of the backscattering intensity in combination with the simultaneous Doppler phase information is demonstrated. Using the focalized transducer in a pulse-to-pulse coherent 3D acoustic Doppler profiling current meter, simultaneous 3D velocity component profile measurements over the whole water depth are carried out in uniform, open-channel flow. They verify existing laws for the distribution of mean velocity, turbulence intensities and Reynolds stresses and reveal the presence of coherent structures extending over the whole water depth. Experimental results obtained in an open-channel, sediment-laden flow document the event structure of sediment transport by the correlation between sediment concentration peaks and coherent flow structures. The instrument system is well suited for boundary layer studies and has been applied in the laboratory, rivers and lakes