Location: Hydraulic Engineering ResearchTitle: Instrumentation performance for air entrained flows Author
Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/15/2018
Publication Date: N/A
Technical Abstract: Stepped chutes are commonly used for embankment overtopping protection, energy dissipation, and increased spillway capacity for aging dams. The stepped features of chutes can induce air entrained flows if the chute is of sufficient length. Specialized instrumentation is required to properly measure velocities and air concentrations in flows descending stepped chutes as well as in the energy dissipation basin associated with them. Scientists at the USDA-Agricultural Research Service Hydraulic Engineering Research Unit have been conducting research on stepped chutes for over a decade. Velocities and air concentrations were recorded, and observations were noted related to multiple instrumentations (i.e. dual-tipped fiber optic probe, back-flushing Pitot tube, ADV, and propeller meter) used in testing. The fiber optic probe is the only instrument used in the studies that provides air concentration measurements. The ADV and Pitot tube worked well in measuring velocities in non-air entrained environments. As air entrainment began to develop, the ADV velocity measurement system reported multiple errors and some of the results became unrealistic. The back-flushing Pitot tube provided reasonable velocity measurements in the middle and lower portion of the depth profile for air entrained flows but was unable to provide accurate velocity measurements near the surface. The fiber optic probe is not applicable for velocities measurements in non-air entrained flows since it relies on the detection of air bubbles to calculate velocity. The fiber optic probe performed very well in highly air entrained flows for measuring velocity and air concentration but has some limitations for flows with limited air entrainment. Also, if the flow has erratic behavior and the velocity vectors are not parallel to the probe tips then the velocity results may be incorrect. It was difficult to acquire reliable velocity measurements in the stilling basin area with the fiber optic probe and the ADV due to the erratic behavior of the flow and the high air entrainment. A propeller meter provided reasonable velocity measurements for the downstream section of the hydraulic jump and the exit of the stilling basin. Each instrument works best in the environment for which they were developed. The results are intended to provide scientists information on the best methodology for collecting velocities in air entrained flows.