Irrigation Training
and Research Center

Subcritical Contraction Design for Improved Open Channel Flow Measurement Accuracy with an Upward-Looking Acoutsic Doppler Velocity Meter

Summary

Acoustic Doppler velocity meters (ADVM) provide an alternative to more traditional flow measurement devices and procedures such as flumes, weirs, and stage-rating for irrigation and drainage canals. However, the requirements for correct calibration are extensive and complex. A 3D computational fluid dynamics (CFD) model was used to establish design criteria of a subcritical rapidly varied flow (RVF) contraction that provides a constant cross section in addition to a consistent, linear relationship between the upward-looking ADVM sample velocity and the cross-sectional average velocity, in order to improve ADVM accuracy without the need for in situ calibration. CFD simulations validated the subcritical contraction in a rectangular and trapezoidal cross section by showing errors within +1.8% and -2.2%. Physical testing of the subcritical contraction coupled with an upward-looking ADVM in a large rectangular flume provided laboratory validation with measurement errors within +/-4% without calibration.

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