Subcritical Contraction for Improved Open-Channel Flow Measurement Accuracy with an Upward-Looking ADVM

Acoustic Doppler velocity meters �ADVMs� provide an alternative to more traditional flow measurement devices and proce­ dures such as flumes, weirs, and stage rating for irrigation and drainage canals. However, the requirements for correct calibration are extensive and complex. A three-dimensional computational fluid dynamics �CFD� model was used to design a subcritical rapidly varied flow contraction that provides 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. CE Database subject headings: Open channel flow; Subcritical flow; Flow measurement; Acoustic techniques; Doppler systems; Laboratory tests; Hydraulics. Author keywords: Open channel flow; Subcritical flow; Flow measurement; Acoustic techniques; Doppler systems; Laboratory tests; Hydraulics.