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

Gunther, U. K. (Uwe Karsten) (2001-12)

Thesis (MEng)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: This report deals with the use of Doppler meters to measure flow velocities and hence discharges in streams. The Doppler meter measures the shift in frequency of an acoustic wave, which it emits and then becomes reflected by a moving particle. The reading is converted into a velocity by dividing the shifted frequency by a calibration constant. The particles that reflect the signal need to follow the flow sufficiently closely so that their velocity may be assumed equal to the flow velocity. A previous study on the use of the Doppler meter at a Crump weir (Du Toit and Venter, 1999) indicated that velocities measured with a Doppler meter showed a distinct relationship with recorded water levels. However, the wide scatter of the observed frequencies in this study, necessitated further tests on the use of the Doppler meter at measuring structures as well as calibration tests on the instrument in the hydraulic laboratory of the University of Stellenbosch. The mam objective of this investigation was to establish the relationship between measured Doppler velocities at a Crump weir and the approach velocities in the stream. The instrument was to be tested in both modular and non-modular flow ranges. In addition, the instrument had to be calibrated in the hydraulic laboratory under varying flow conditions, such as very low flow velocities and different sediment concentrations. The placement of the probe at different depths of the flow was also investigated to comment on the accuracy of the Doppler readings at these depths. The results of these tests should serve as guidelines for any additional tests required for use of this instrument in open channel discharge measurements. The Doppler meter used for this study was supplied and manufactured in Stellenbosch by Flotron, and is being marketed as DFM-P-067. It was calibrated in the laboratory in a channel with limited width and hence non-two-dimensional flow conditions. Conclusions were drawn on the calibration constant that was established. The calibration of the instrument requires the division of the cross-sectional flow area into a number of sub-divisions over which the flow was integrated. The calibration constant of 1460 established in this study differs by approximately 6 percent from the theoretical constant value of 1375. The sensitivity of the Doppler meter to different sediment concentrations was also investigated. For the instrument to read a shifted frequency, it is essential that suspended particles that follow the water movement sufficiently closely are present in the stream. It was observed that readings of the instrument in "sediment-free" water differed only by 3.6% from the readings taken in water containing sediments. The instrument was thus not very sensitive to different sediment concentrations. It was also found that the angle at which the probe was placed in the water had no effect on the accuracy of the observed Doppler velocity. It was furthermore found that the Doppler meter worked reliably at all depths, including levels very close to the channel floor and levels just below the water surface. One drawback of the apparatus was the minimum velocity that it can measure accurately. This minimum velocity of 0.046 mis does not compare well with that for other commercially available Doppler meters. The Argonaut-Acoustic Doppler meter for example can measure velocities as low as O.OOOlm/s, meaning that the DFM-P-067 measures a minimum velocity 460 times swifter than the minimum velocity of the Argonaut-Acoustic Doppler meter. After the Doppler meter had been calibrated, it was tested at a Crump weir in the laboratory to determine the relationship between the Doppler velocities, measured at the weir's crest, and the velocities in the approach channel. These tests were performed for both modular and non-modular flow conditions. The report concludes that, within the flow range in which the instrument was tested, there is a linear relationship between the two velocities mentioned. It is likely that the results obtained in the modular flow range can be used to extrapolate for high flows, especially for submergence ratios less than 0.93. The wide scatter of results obtained in the previous study was due to the readings not being averaged. The Doppler meter does not measure a point velocity but an average velocity within the acoustic field that it emits. This acoustic field is very small and depends on the geometry of the probe. Finally it is recommended that the linear relationship in the non-modular flow range be investigated further in a larger model, where the submergence ratio can be better controlled. The Doppler meter should in future also be calibrated in a wide channel in which two-dimensional flow conditions are approached and these results should be compared to the results obtained in this 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 in the laboratory. The calibration of the instrument at a Crump weir should allow 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 the results and conclusions obtained in this investigation. They may serve as an aid for measurements that could be carried out with this instrument in open channels.

AFRIKAANSE OPSOMMING: Hierdie verslag handeloor die gebruik van die Doppler-meter om vloeisnelhede en derhalwe die vloeitempos in riviere te meet. Die Doppler meter word gebruik om die verandering in die frekwensie van 'n akoustiese golf wat deur bewegende deeltjies in die water gereflekteer word te meet. Die lesing word dan omgeskakel in 'n snelheid deur die gewysigde frekwensie deur 'n kalibrasie konstante te deel. Die bewegende deeltjies wat die sein reflekteer, volg die vloei genoegsaam sodat aanvaar kan word dat hulle snelhede gelyk aan die vloeisnelheid is. 'n Vorige studie in die gebruik van die Doppler meter by 'n Crump meetwal het baie belowende resultate getoon deurdat daar gevind is dat die gemete Doppler snelheid 'n duidelike verwantskap toon met veranderings in gemete water vlakke. As gevolg van die wye band in die waargenome frekwensies in die studie is aanbeveel dat verdere toetse op die gebruik van die Doppler meter by meetstasies gedoen moet word. Die instrument moet ook in die laboratorium gekalibreer word. Die hoofdoel van hierdie ondersoek was om die verwantskap tussen die gemete Doppler snelhede by 'n Crump meetwal en die aankomssnelhede in die stroom te bepaal. Dit moes gedoen word in beide die modulêre en niemodulêre vloeibestekke. Behalwe vir die kalibrasie van die instrument in die laboratorium moes die betroubaarheid daarvan onder verskillende vloei toestande ook getoets word, soos byvoorbeeld by lae vloei snelhede en by verskillende sediment konsentrasies. Die instrument is ook op verskillende vlakke binne die vloei getoets om te bepaal of daar op hierdie vlakke betroubare lesings verwag kon word. Resultate verkry, kan dan dien as riglyne vir enige verdere toetse wat nog op die instrument in oop kanale uitgevoer moet word. Die Doppler meter wat vir die ondersoek gebruik is, word in Stellenbosch vervaardig deur Flotron en word onder die naam DFM-P-067 bemark. Dit is in die laboratorium in 'n kanaal met 'n beperkte breedte getoets en IS daarom in nie-twee dimensionele vloei gekalibreer. Gevolgtrekkings IS gebaseer op die kalibrasie konstante verkry uit die toetse. Die kalibrasie van die instrument vereis dat die deursnee area van die vloei in verskeie segmente onderverdeel moes word. Die kalibrasie konstante van 1460 bepaal in hierdie studie verskilongeveer 6% van die teoretiese waarde van 1375 vir die konstante. Die Doppler meter se sensitiwiteit vir verskillende sediment konsentrasies is ook ondersoek. Dit is noodsaaklik dat daar gesuspendeerde deeltjies teenwoordig in die water is en dat die deeltjies saam met die water beweeg om te verseker dat die instrument die gewysigde frekwensie kan registreer. Daar is egter gevind dat die lesings van die instrument in sediment-vrye water slegs met 3,6% verskil van lesings wat in water met sediment geneem is. Dit lei tot die gevolgtrekking dat die instrument nie baie sensitief vir veranderlike sediment konsentrasies in die water is nie. Daar is ook gevind dat die hoek waarteen die sender in die water geplaas word nie die akkuraatheid van die Doppler snelhede beinvloed nie. Verder is gevind dat die Doppler meter bevredigende resultate lewer, ongeag op watter diepte lesings geneem word. Tydens toetse is waarnemings baie nabyaan die kanaal bodem asook nabyaan die water se oppervlak gedoen. 'n Tekortkoming van hierdie instrument is die minimum snelheid wat dit akkuraat kan meet. Daar is gevind dat die Doppler meter se muurnum snelheid lesing van 0.046 mis nie goed vergelyk met dié van ander meters wat kommersieël beskikbaar is nie. Die Argonaut-Acoustic Doppler meter kan byvoorbeeld vloeisnelhede so laag as 0.0001 mis meet wat beteken dat die DFM-P-067 se minimum betroubare vloeisnelheid 460 keer vinniger is as die Argonaut-Acoustic Doppler meter se minimum betroubare vloeisnelheid. Nadat die Doppler meter gekalibreer is, is dit by 'n Crump meetwal in die laboratorium getoets om die verhouding tussen die Doppler snelhede gemeet by die oorloopkruin en die snelhede wat in die aanloopkanaal gemeet is, te bepaal. Hierdie toetse is uitgevoer op beide modulêre en nie-modulêre vloei toestande. Daar is gevind dat daar binne die vloeibestek waarin die toetse plaasgevind het 'n liniêere verband tussen die twee bogenoemde snelhede bestaan. Dit is hoogs waarskynlik dat die resultate wat in die modulêre vloeibestek gevind is gebruik kan word om vir hoë vloeie te ekstrapoleer, veral vir grade van versuiping laer as 0.93. Die vorige studie se uiteenlopende resultate kan toegeskryf word aan lesings waarvan die gemiddelde lesing vir 'n spesifieke vloeitoestand nie bepaal is nie. Die Doppler meter meet nie 'n bepaalde punt-snelheid nie, maar 'n gemiddelde snelheid binne die akoestiese veld wat dit uitstraal. Hierdie akoestiese veld is baie klein en afhanklik van die geometrie van die sender. Ten slotte word aanbeveel dat die lineêre verband in die nie-modulêre vloeibestek in 'n groter model, waar die graad van versuiping makliker beheerbaar is, verder ondersoek moet word. Die Doppler meter moet ook in 'n breë kanaal waarin twee dimensionale vloei voorkom, gekalibreer word. Resultate so verkry moet vergelyk word met die wat in hierdie studie behaal is. Elke instrument behoort sy eie kalibrasie konstante te hê en afhangende van waar dit gebruik word, kan dit of by 'n meetwal of in die laboratorium gekalibreer word. Die kalibrasie van die instrument by 'n Crump meetwal behoort 'n wyer reeks vloeie toe te laat met ook baie lae snelhede. Die verslag word afgesluit met riglyne gebaseer op die resultate en gevolgtrekkings wat uit die ondersoek voortgespruit het. Hierdie riglyne en gevolgtrekkings kan dan dien as 'n hulpmiddel vir metings wat met hierdie instrument in oop kanale uitgevoer word.

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