Daniel J. Howes

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 procedures 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.

Center Pivot Sprinkler Distribution Uniformity Impacts on the Spatial Variability of Evapotranspiration

AbstractUnderstanding variable evapotranspiration (ET) throughout a field can help maximize yield on a per-acre basis, as well as assist with proper irrigation scheduling. The results from this study indicate that irrigation system distribution uniformity (DU) has a significant effect on the uniformity of ET during water-stressed periods. The study site involved intensely managed forage (alfalfa and winter grain hay) irrigated by center pivots being supplied with reclaimed water near Palmdale, California. During spring and early summer 2007 the center pivots were operating under deficit irrigation. In 2010, after the installation of reservoirs, water was applied to meet full evapotranspiration (ETc) demands. Using remote sensing of actual evapotranspiration, the variability in ETc for the same pivots with the same crop was quantified. During the non-water-stressed period, ET uniformity was significantly better than during the water-stressed period (2007). The difference in uniformity was found to be attri...

Velocity Contour Weighting Method. II: Evaluation in Trapezoidal Channels and Roughness Sensitivity

The Velocity Contour Weighting Method (VCWM) was developed in Part I to accurately estimate the cross-sectional average velocity of a prismatic channel flow using acoustic Doppler velocity meter (ADVM) measurements of centerline velocity. Here, the VCWM is validated by its successful application to 25 different concrete-lined trapezoidal channels used for irrigation water delivery. At each site, the cross-sectional distribution of velocity is measured by an acoustic Doppler velocimeter (ADV), which is moved horizontally and vertically through a sampling grid. Multiple tests at some sites led to a total of 51 sets of cross-sectional measurements. ADVM measurements are simulated by interpolating ADV measurements along a set of vertically aligned centerline coordinates typical of ADVM deployments. Subsequent application of the VCWM gives an estimate of the cross-sectional average velocity. Secondly, the velocity-area method is applied to the ADV data to directly measure the cross-sectional average velocity f...

Velocity Contour Weighting Method. I: Algorithm Development and Laboratory Testing

An algorithm is developed for real-time estimation of the cross-sectional average velocity of a channel flow by using an upward-looking pulsed wave acoustic Doppler velocity meters (ADVM). The Velocity Contour Weighting Method (VCWM) is applicable to gradually varied flows in prismatic channels and requires little to no calibration. VCWM estimates the average velocity as a weighted average of ADVM bin velocities. Weights are based on the velocity distribution sampled by the ADVM. Collectively, the VCWM is able to adapt to a wide range of channel geometry and roughness features. Expressions for the velocity weights are developed by first applying a validated 3D computation fluid dynamics (CFD) channel flow model to a wide range of flow scenarios including differing channel geometries, discharge rates, depths, and boundary roughness. CFD simulation data are then reduced empirically with the aid of dimensional analysis to obtain the velocity weight equation. Special attention is given to the first weight accounting for near-wall velocity where the ADVM does not measure. Application of the method to a large rectangular flume shows that the VCWM predicts the average velocity with an uncertainty less than ±5% and that this uncertainty can be reduced by minimizing the buffer distance between the channel bottom and the first velocity measurement. In a companion paper, the performance of the VCWM is examined in irrigation canals with trapezoidal cross sections.