Thesis (MEng)--University of Stellenbosch, 2001. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52485 |
| Date | 12 1900 |
| Creators | Gunther, U. K. (Uwe Karsten) |
| Contributors | Rooseboom, A., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various foliations) : ill. |
| Rights | Stellenbosch University |
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