A curr ent meter that is capable of measuring the high-frequency fluctuations of the three-dimensional velocity vector has been developed. The meter works on a doppler-shift principle. At high velocities, the meter has been shown to have an accuracy of better than 3%

Crosby, R. M.

Pritchard, D. W.

Wiseman, Wm. J., Jr.

English

Yale University

      P.O. BOX 208118 | NEW HAVEN CT 06520-8118 USA | PEABODY.YALE. EDU   JOURNAL OF MARINE RESEARCH The Journal of Marine Research, one of the oldest journals in American marine science, published important peer-reviewed original research on a broad array of topics in physical, biological, and chemical oceanography vital to the academic oceanographic community in the long and rich tradition of the Sears Foundation for Marine Research at Yale University.  An archive of all issues from 1937 to 2021 (Volume 1–79) are available through EliScholar,  a digital platform for scholarly publishing provided by Yale University Library at  https://elischolar.library.yale.edu/.  Requests for permission to clear rights for use of this content should be directed to the authors, their estates, or other representatives. The Journal of Marine Research has no contact information beyond the affiliations listed in the published articles. We ask that you provide attribution to the Journal of Marine Research.  Yale University provides access to these materials for educational and research purposes only. Copyright or other proprietary rights to content contained in this document may be held by individuals or entities other than, or in addition to, Yale University. You are solely responsible for determining the ownership of the copyright, and for obtaining permission for your intended use. Yale University makes no warranty that your distribution, reproduction, or other use of these materials will not infringe the rights of third parties.  This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. https://creativecommons.org/licenses/by-nc-sa/4.0/   A Three-dimensional Current Meter far Estuarine Applications 1 Wm. J. Wiseman, Jr.,2 R. M. Crosby,3 and D. W. Pritchard Chesapeake Bay Institute The :Johns Hopkins University Baltimore, Afaryland 2I2I8 A BSTRACT A current meter that is capable of measuring the high-frequency fluctuations of the three-dimensional velocity vector has been developed. The meter works on a doppler-shift principle. At high velocities, the meter has been shown to have an accuracy of better than 3 °/o · Introduction. As part of a program to study the turbulent regime of oceanic motions, a current meter4 that is capable of sensing the high-frequency veloc-ities found in estuaries was developed at the Chesapeake Bay Institute. Besides a rapid response time, the design criteria for the meter were: (i) output data should be true vector averages of the three-dimensional velocity, and (ii) the velocity should be remotely sensed so that the disturbance to the fi eld of fl ow, at the point of interest, should be minimal. The idea developed was that of an acoustic doppler-shift current meter. The Meter.5 The operation of the meter is conceptually straightforward. Sound is emitted from a transmitter and scattered by parti culate matter that is assumed to be moving with the ambient fluid 's velocity. The sound, scattered in three independent directions, is recorded, and the doppler-shifts in frequency of the scattered sound relative to the transmitted sound are measured, care being taken to distinguish between an up-doppler-shift and a down-doppler-shift . These frequency shifts give suffici ent information to determine the veloc-1. Contribution No. 167 from Chesapeake Bay Institu te, The Johns Hopkins Universit y. Accepted for publication and submitted to press 10 November 1971. 2 . Coastal Studies Inst itute, Louisiana State University, Baton Rouge, La. 70803. 3. T exas Instruments, Inc., Dallas, Texas 7 5222. 4. A prototype has been described by Kronengold and Vlasak (1965). 5. Schematics for the current meter can be obtained from the Chesapeake Bay Institute. 153 154 Journal of Marine R esearch [30, I ity vector associated with the water at the intersection of the transmitter's and receivers' beam patterns. Information is fed from the meter to the signal pro-cessing equipment through underwater cables. This type of doppler-shift Rowmeter is not fundamentally new. Kronengold and Vlasak (1965) and Vlasak (1968) have described such devices, using acoustic sources and analog procesing equipment, for measuring one velocity component. Yeh and Cummins (1964) and Foreman et al. (1965) used an optical source in a similar one-dimensional meter. Fridman et al. ( 1968) have done the same for a three-dimensional meter. Only the last device senses the full three-dimensional velocity vector, but optical systems of this type have stability problems when used in the field. The acoustic meters, besides sampling a single component of the velocity, have the disadvantage of analogue processing that yields only a mean speed and possibly a spectrum. The signal processing associated with our system yields a time series. The transmitter of our system consists of a crystal-controlled oscillator which, after appropriate amplification and impedance matching, drives a disc (one-quarter-inch diameter, lead-zirconate-titanate, piezo-electric) at a fre-quency of approximately 9.965 MHz. The transmitter and all receivers are battery powered. The receivers, located symmetrically about the transmitter and making angles of 45° to it (Fig. 1 ), detect the scattered sound waves with piezo-electric discs that are similar to that used in the transmitter. The received signal is amplified by a low-Q tuned amplifier and mixed with a sinusoidal signal of approximately 9.985 MHz; then it is low-pass filtered, amplified, and finally fed to the processing equipment. The intersection pattern of the transmitter and receiver beams is tripodal in shape. When measured it was found to be somewhat less than 2 cm along a leg of the tripod and less than I cm across a leg. Immediately upon reception of the signal above water, an analog tape is written. One channel of the tape contains a timing pulse of 1-m sec duration once every 50 m sec. The outputs of the three receivers, now centered about a frequency of 20 kHz, are fed through band-pas filt ers into Schmitt triggers. The Schmitt triggers act as wave-shaping devices. The outputs of these trigger circuits are recorded on three other channels of the tape. The analog tape is passed through a frequency to digital (F-to-D) converter. This system counts the number of cycles of signal on each of the three chan-nels associated with the receivers during each 49-m sec sampling interval and records the counts on magnetic tape. It was hoped that these counts would provide reliable measures of the doppler-shifts of interest. When the meter was first tested, however, a phe-nomenon that was designated "drop-out" was observed. Drop-out occurs when the received signal level falls below the no-signal noise level of the receiver. This results in an inability to track the doppler frequency and thus the velocity. There are many possible causes of this phenomenon. Some are eliminated in 1//iseman, Jr. et (//.: A Current l/1/etcr 155 --Figure 1 . Photograph of the current meter sitting on it s battery charger. 156 'Journal of Marine Research [30, I the system's design and others are dealt with indirectly during the signal pro-cess111g. A clock records, for each receiver and for each sampling interval, the length of time during which drop-out occurs. The estimated doppler frequency for each receiver, for each sampling interval, can then be found by using the formula Ci; /i; = (0.049 - di;)' where: / i; is the computed estimate of the output frequency from receiver i during the jth interval; Ci; is the number of cycles of signal recorded from receiver i during the j th interval when drop-out did not occur; and di; is the length of time, in seconds, when drop-out occurred at receiver i during the j th interval. The fr equencies are converted to velociti es on an IBM 7094 through use of the lin ear doppler equati on (Middleton r 967): c(t,s,p) x (Joi - /i;) Ui j = _2_c_o---'s- 2--'2'--'. 5-0~x~9-. 9- 6~5-x-1 o-,6' where: c, the speed of sound in water, is determined from the temperature, salinit y, and pressure recorded at the depth of the meter by using the equati on of Wil son ( 1 960); /oi is the frequency recorded from the ith receiver if the water is not moving; and Uij is the speed,6 during the j th interval, along the bisector of the angle formed by the ith receiver and the transmitter. Cali bration. The meter was calib rated in the CBI circulating test flume. The flume speed is controllable to within ± 0.05 cm/s or ± 0.5%, whichever is greater, except at very low velociti es or at velocities greater than 50 cm/s (Cannon 1969). With the flume at rest and with a single large stati onary scatterer, the meter indicated a mean speed of o. r o cm/s. The least-count error associated with any one receiver is o. 16 cm/s. The discrepancy between the indicated speed and the true speed is probably due to our inabilit y to exactly determine the differ-ence fr equencies between the local oscill ators in the receivers and that in the transmitter. These difference fr equencies,/oi , had been recorded at two-degree Celsius intervals and were used in the lin ear doppler equati on when the veloc-iti es were computed. All of the other tests were performed with the solid scatterer replaced by polyethylene spheres at a concentration of 5 ppm. In this configuration and 6. These speeds are space-time averages. The weighting funct ion in the time domain is zero when drop-out occurs and is unity otherwise. In space, the weighting function is the product of the trans-mitter and receiver beam patterns. 1972] Wiseman, 'Jr. et al: A Current Meter 157 with the flume at rest (a small residual velocity of approximately 0.5 cm/s al-ways remained), the meter indicated a mean speed of 1.97 cm/s, even after bad points had been corrected by a linear interpolation scheme. 7 At speeds higher than 10 cm/s, however, the meter was always better than 3% accurate. W e believe that the change in accuracy of the meter between high and low mean-flow rates is due to a change in the character of the drop-out. At low speeds, the drop-out occurs as long periods of loss of signal preceded by, and followed by, periods of short duration when a single cycle of doppler information is missing from the record. At high speeds, the drop-out occurs in many periods of moderate duration. Our processing scheme can cope effectively with only the latter form of drop-out. The meter has been used with moderate success in the fi eld when the mean velocity of the flu id was approximately 20 cm/s. The results of this fi eld work will be described in another paper. Conclusions. A current meter has been developed and tested that is capable of recording the high-frequency turbulent fluctuations of fluid velocity that occur in an estuary. The meter has the advantages over similar meters of being able to sample without disturbing the fluid at the point of sampling, of taking a vector average, and, most important, of simultaneously measuring all of the three components of the fluid velocity. Under adequate sampling conditions, the meter can be made to operate in a satisfactory manner, although it is hoped that future improvements will extend the range of "adequate" sampling condi-tions. Acknowledgments. We would like to acknowledge the assistance of the entire engineering staff of the Chesapeake Bay Institute. This work was sup-ported by Office of Naval Research Contract NoNR 4 010(1 r). 7. Processing of the data begins by computing the sample distr ibution of the corrected frequencies, fit· T he minimum distance from the ':'od_al frequency_ to th~ fir st zero in the distributio~ is determined. Any sample point lying more than this distance on either side of the modal fr equency 1s replaced by a linear interpolation between the two nearest "good" frequencies that bracket it. If a "bad" point occurs at either end of the time series, the modal frequency is used in the li near interpolation. REFERENCES CANNON, GLENN 1969. Observations of motion at intermediate and large scales in a coastal plain estuary. Chesapeake Bay Institute, Techn. Rep., 52; 144 PP· FOREMAN, JR., J. W ., E.W. GEORGE, and R . D. L EWIS 1965. M easurement of localized flo w velocities in gases with a Laser Doppler Flowmeter. Appl. Phys. L ett., 7 (4): 77-78. FRIDMAN, J. D., R. M . H UFAKER, R. F. K INNARD 1968. Laser Doppler System measures three-dimensional vector velocity and turbulence. Laser Focus, Nov.: 34-38. Journal of Marine Research [30, I KRONENGOLD, MORTON, and WELDON VLASAK r965. A Doppler Current Meter. Mar. Sci. Instrum. ISA, 3 : 237-250. MIDDLETON, DAVID r967. A statistical theory of reverberation and similar first-order scattered fields. Part I: Waveforms and the general process. IEEE Trans. Inf. Theory, IT-r3 (3): 372-392. VLASAK, WELDON r968. Further development in the Doppler method of water-velocity measurement. IEEE Trans. Geosci. Electron., GE-6 (4): r97-204. WILSON, W. D. r960. Equation for the speed of sound in water. J. acoust. Soc. Amer., 32 (ro): r35. YEH, Y., and H. L. CUMMINS r964. Localized fluid flow measurements with an He-Ne Laser Spectrometer. Appl. Phys. Lett., 4(ro): r76-r78. 

A three-dimensional current meter for estuarine applications

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A three-dimensional current meter for estuarine applications

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