Topic
Vector measuring current meter
About: Vector measuring current meter is a research topic. Over the lifetime, 19 publications have been published within this topic receiving 143 citations.
Papers
TL;DR: In this article, an intercomparison experiment was conducted between a near-surface vector-averaging current meter (VACM) and an Edgerton, Germeshausen, and Grier, Inc., vector-measuring current meters (VMCM) at a midshelf mooring site (38°32.8′N, 123°22.6′W) located in 90 m of water off northern California between April and July 1981.
Abstract: An intercomparison experiment was conducted between a near-surface vector-averaging current meter (VACM) and an Edgerton, Germeshausen, and Grier, Inc., vector-measuring current meter (VMCM) at a midshelf mooring site (38°32.8′N, 123°22.6′W) located in 90 m of water off northern California between April and July 1981. The VACM and VMCM were deployed at 9 and 11 m, respectively, beneath a 1.5-m spherical surface buoy on a slack mooring. The VACM and VMCM 1-hr vector-averaged velocities were highly correlated with a complex correlation coefficient amplitude of 0.991, indicating the relative effectiveness of the internal vector-averaging sampling scheme of the VACM and the flow sensors of the VMCM to filter much of the wave-induced high-frequency currents and mooring motion. The vector-averaged VACM speeds Sa generally exceeded the VMCM speeds Sm, with a mean and maximum speed difference of 4 and 14 cm/s, respectively. Since the observed difference in vector-averaged speeds, ΔS = Sa − Sm, was larger on average than an independent estimate of the true shear by a factor of 4 to 5, most of the difference in the observed VACM and VMCM vector-averaged speeds has been attributed to instrumental error. The surface wave climate was measured nearby over the shelf during part of the experiment, and a significant correlation was found between estimates of the rms horizontal velocity and rms difference vertical velocity on the basis of the observed wave data and a VACM-derived rms wave speed W, defined as (R2 − Sa2)½, where R is the hourly averaged VACM rotor speed. This result allows the fractional vector-averaged speed difference ΔS/Sm to be determined experimentally from the field data as a function of Sm/W, a signal-to-noise parameter expressing the ratio of the observed VMCM vector-averaged speed to the VACM-derived rms wave speed W. Although significant scatter occurs between simultaneous 1-hr vector-averaged VACM and VMCM speeds, the average fractional speed difference ΔS/ Sm decreases from about 0.57 ± 0.10 (at 95% confidence) at Sm/W = 0.15 to about 0.20 at Sm/W = 1 and remains approximately constant at 0.16 ± 0.02 for Sm/W between about 1.5 and 2.0. Thus the fractional speed difference observed in the field is a relatively well-defined function of the relative magnitudes of the mean horizontal and wave-associated flow components. Comparison of these field measurements with published laboratory data on the performance of the VACM and VMCM in unsteady flow suggests that the field and laboratory measurements are consistent and can be used to estimate the absolute fractional speed error of each current meter as a function of u/w, where u is the true mean horizontal vector speed and w the amplitude of the rms wave-induced oscillatory velocity. The combined laboratory and Coastal Ocean Dynamics Experiment 1 field data thus indicate that in an absolute sense the VACM overresponds by about 18–20% at u/w = 0.5 to about 10–11% at u/w = 2.0, while the VMCM underresponds by about −5 to −6% for u/w between 0.5 to 2.0. These results should be independent of mooring type and thus allow estimation of absolute error in velocity measurements made with VACMs and VMCMs in the range of u/w between 0.5 and 2.0.
44 citations
TL;DR: In this article, the Aanderaa RCM11 and the Nortek Aquadopp were compared with the Doppler shift of backscattered acoustic signals, and the Falmouth Scientific ACM and the travel time of acoustic signals between pairs of transducers.
Abstract: As part of a program aimed at developing a long-duration, subsurface mooring, known as Ultramoor, several modern acoustic current meters were tested. The instruments with which the authors have the most experience are the Aanderaa RCM11 and the Nortek Aquadopp, which measure currents using the Doppler shift of backscattered acoustic signals, and the Falmouth Scientific ACM, which measures changes in travel time of acoustic signals between pairs of transducers. Some results from the Doppler-based Sontek Argonaut and the travel-time-based Nobska MAVS are also reported. This paper concentrates on the fidelity of the speed measurement but also presents some results related to the accuracy of the direction measurement. Two procedures were used to compare the instruments. In one, different instruments were placed close to one another on three different deep-ocean moorings. These tests showed that the RCM11 measures consistently lower speeds than either a vector averaging current meter or a vector measuring current meter, both more traditional instruments with mechanical velocity sensors. The Aquadopp in use at the time, but since updated to address accuracy problems in low scattering environments, was biased high. A second means of testing involved comparing the appropriate velocity component of each instrument with the rate of change of pressure when they were lowered from a ship. Results from this procedure revealed no depth dependence or measurable bias in the RCM11 data, but did show biases in both the Aquadopp and Argonaut Doppler-based instruments that resulted from low signal-to-noise ratios in the clear, low scattering conditions beneath the thermocline. Improvements in the design of the latest Aquadopp have reduced this bias to a level that is not significant.
31 citations
TL;DR: In this paper, the authors compared the performance of the ACM and the other two instruments in terms of coherence and phase with significant departures occurring only with respect to the ADCP at frequencies greater than 0.01 cph.
Abstract: Horizontal current measurements were obtained simultaneously with a vector measuring current meter (VMCM), an acoustic Doppler current profiler (ADCP), and the recently developed Falmouth Scientific, Inc., acoustic current meter (ACM) during a 110-day period from the Bermuda Testbed Mooring (BTM). The BTM site is located approximately 80 km southeast of Bermuda at 318449N, 648109W in waters of about 4550-m depth. The ACM is a relatively small, lightweight instrument, which can be deployed relatively close to the ocean surface or bottom as well as at intermediate depths. Prior to the present study, in situ data and knowledge concerning the ACM performance were quite limited. Zonal, meridional, and horizontal current speed data were obtained and time series were computed for each instrument. Several analyses were done to evaluate and intercompare instrument performance. The ACM results compared favorably with those of the VMCM and ADCP for 36-h filtered data. The ADCP typically recorded the largest horizontal current speed and the ACM typically recorded the smallest values. The strongest correlation was seen between the VMCM and ADCP data; however, the lowest correlation for all intercomparisons (by component and speed) was r 5 0.95 with most values of r 5 0.98 or greater. Directional error/bias is a primary factor in limiting current component measurement accuracy. In particular, the ACM direction was up to 208‐308 offset from those of the other two instruments. After rotating or ‘‘correcting’’ the ACM components with respect to the VMCM direction, ACM mean current component differences with respect to the other instruments’ values were less than about 2 cm s 21. Based upon the spectral analysis, both the ACM and VMCM have better performance characteristics than the ADCP in the higher-frequency portion of the spectral domain. The authors’ analysis indicates that the three different systems provide generally similar results in terms of coherence and phase with significant departures occurring only with respect to the ADCP at frequencies greater than 0.01 cph. The present study indicates that the ACM provides another viable means of obtaining accurate current measurements.
17 citations
1 Jan 1983
TL;DR: In this paper, two experiments were conducted in the summer of 1982 with "off-the-shelf" AMETEK-Straza DCP4400 acoustic Doppler current profiling systems.
Abstract: Two experiments were conducted in the summer of 1982 with "off-the-shelf" AMETEK-Straza DCP4400 acoustic Doppler current profiling systems. The first experiment compared a bottom-mounted upward-looking 300 kHz system against moored current measuring systems. Conductivity-temperature-depth (CTD) profiles, water samples, and meteorological data were collected to augment the current measurements and aid in the interpretation of the acoustic Doppler measurements. The second experiment involved real-time comparisons between the downward-looking 115 kHz acoustic profiling system installed on the NOAA Ship RESEARCHER and current measurements made with an EG&G - Vector Measuring Current Meter (VMCM) lowered from the ship while on station off the coast of Florida. This paper presents the results from both experiments along with a discussion on the performance of the AMETEK-Straza DCP4400 systems.
11 citations
1 Aug 1986
TL;DR: In this article, an EG&G Vector Measuring current meter (VMCM) and an Aanderaa Recording Current Meter (RCM4) were moored below a surface-following buoy.
Abstract: An EG&G Vector Measuring Current Meter (VMCM) and an Aanderaa Recording Current Meter (RCM4) were moored below a surface-following buoy. The RCM4 instrument at 4 m depth recorded currents similar to those measured by the VMCM at 2 m, in spite of rotor pumping and direction aliasing. The speeds measured by the RCM4 did show the effect of pumping by an absence of low speeds ( −1 ) during most of the time series. However, the mean velocities and standard deviations of the flow field for the two instruments were similar. The major differences were for current fluctuations of frequency >0.125 cph (0.00208 Hz) where the RCM4 overestimated the energy by up to an order of magnitude. A mooring design using a surface follower buoy in strong currents (50 cm s −1 ) can be used with RCM4 instruments to measure accurately low frequency current fluctuations.
8 citations
Related Topics (5)
Performance Metrics
| Year | Papers |
|---|---|
| 2008 | 1 |
| 2007 | 1 |
| 2003 | 1 |
| 2000 | 1 |
| 1999 | 1 |
| 1996 | 1 |