The application of Doppler velocity meters in the measurement of open channel discharges

[摘要] ENGLISH ABSTRACT:This report deals with the use of Doppler meters to measure flow velocitiesand hence discharges in streams. The Doppler meter measures the shift infrequency of an acoustic wave, which it emits and then becomes reflected bya moving particle. The reading is converted into a velocity by dividing theshifted frequency by a calibration constant. The particles that reflect thesignal need to follow the flow sufficiently closely so that their velocity maybe assumed equal to the flow velocity.A previous study on the use of the Doppler meter at a Crump weir (Du Toitand Venter, 1999) indicated that velocities measured with a Doppler metershowed a distinct relationship with recorded water levels. However, the widescatter of the observed frequencies in this study, necessitated further tests onthe use of the Doppler meter at measuring structures as well as calibrationtests on the instrument in the hydraulic laboratory of the University ofStellenbosch.The mam objective of this investigation was to establish the relationshipbetween measured Doppler velocities at a Crump weir and the approachvelocities in the stream. The instrument was to be tested in both modular andnon-modular flow ranges. In addition, the instrument had to be calibrated inthe hydraulic laboratory under varying flow conditions, such as very lowflow velocities and different sediment concentrations. The placement of theprobe at different depths of the flow was also investigated to comment onthe accuracy of the Doppler readings at these depths. The results of thesetests should serve as guidelines for any additional tests required for use ofthis instrument in open channel discharge measurements.The Doppler meter used for this study was supplied and manufactured inStellenbosch by Flotron, and is being marketed as DFM-P-067. It wascalibrated in the laboratory in a channel with limited width and hence non-two-dimensional flow conditions. Conclusions were drawn on the calibrationconstant that was established. The calibration of the instrument requires thedivision of the cross-sectional flow area into a number of sub-divisions overwhich the flow was integrated. The calibration constant of 1460 establishedin this study differs by approximately 6 percent from the theoretical constantvalue of 1375.The sensitivity of the Doppler meter to different sediment concentrationswas also investigated. For the instrument to read a shifted frequency, it isessential that suspended particles that follow the water movementsufficiently closely are present in the stream. It was observed that readingsof the instrument in sediment-free water differed only by 3.6% from thereadings taken in water containing sediments. The instrument was thus notvery sensitive to different sediment concentrations. It was also found thatthe angle at which the probe was placed in the water had no effect on theaccuracy of the observed Doppler velocity. It was furthermore found that theDoppler meter worked reliably at all depths, including levels very close tothe channel floor and levels just below the water surface. One drawback ofthe apparatus was the minimum velocity that it can measure accurately. Thisminimum velocity of 0.046 mis does not compare well with that for othercommercially available Doppler meters. The Argonaut-Acoustic Dopplermeter for example can measure velocities as low as O.OOOlm/s, meaning thatthe DFM-P-067 measures a minimum velocity 460 times swifter than theminimum velocity of the Argonaut-Acoustic Doppler meter.After the Doppler meter had been calibrated, it was tested at a Crump weirin the laboratory to determine the relationship between the Dopplervelocities, measured at the weir's crest, and the velocities in the approachchannel. These tests were performed for both modular and non-modular flowconditions. The report concludes that, within the flow range in which the instrument wastested, there is a linear relationship between the two velocities mentioned. Itis likely that the results obtained in the modular flow range can be used toextrapolate for high flows, especially for submergence ratios less than 0.93.The wide scatter of results obtained in the previous study was due to thereadings not being averaged. The Doppler meter does not measure a pointvelocity but an average velocity within the acoustic field that it emits. Thisacoustic field is very small and depends on the geometry of the probe.Finally it is recommended that the linear relationship in the non-modularflow range be investigated further in a larger model, where the submergenceratio can be better controlled. The Doppler meter should in future also becalibrated in a wide channel in which two-dimensional flow conditions areapproached and these results should be compared to the results obtained inthis study. Every instrument is expected to have its own calibration constant,and depending on its application, it can either be calibrated at a weir or inthe laboratory. The calibration of the instrument at a Crump weir shouldallow for a wider range of flows, and also very low flow velocities.At the end of this report guidelines were drawn up that are based on theresults and conclusions obtained in this investigation. They may serve as anaid for measurements that could be carried out with this instrument in openchannels.

[发布机构] Stellenbosch University