The Spatial Response of Acoustic Travel-Time Current Meters in the Measurement of Turbulence

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This technology development project deals with acoustic travel- time current meters (ACMs) that are widely used in oceanography and the atmospheric sciences to measure boundary layer turbulence. These sensors infer velocity by measuring the difference in time required for acoustic pulses to travel both ways between opposing transducers separated by a distance L. The response of such a device to turbulence having spatial scales equal to or smaller than L should be attenuated, and a simple model of the ACM response function based on this idea has been known for almost 30 years. However, recent field and laboratory measurements of the spatial response of ultrasonic ACMs to turbulence in water show that measured velocities are less attenuated than this simple model predicts. Research will be conducted to test whether anomalous behavior of ACMs is due either to flow distortion or to spatial variations in the amplitude and phase of the acoustic field, and to distinguish between these alternatives. It is expected that an improved understanding of the spatial response of ACMs will considerably enhance their usefulness as turbulence sensors in the ocean, and should lead to improved sensor design and performance.