Vector-averaging Current Meter Research Articles

Comparison of BASS and VACM Current Measurements during STRESS

Abstract Current measurements from a vector-averaging current meter (VACM) on a subsurface mooring and a benthic acoustic stress sensor (BASS) on a bottom tripod are compared to assess their relative accuracy. The instruments were deployed off northern California at a midshelf site (water depth approximately 90 m) as part of the STRESS (Sediment Transport Events on Shelves and Slopes) field program. The subsurface mooring and bottom tripod were within a few hundred meters of each other, with the BASS 5.0 m and the VACM 6.7 m above the bottom, during two tripod deployments of 49 and 32 days in the winter of 1988/89. Speed differences between the VACM and BASS current observations have a mean of 0.2 cm s−1 and a standard deviation of 1.2 cm s−1. If the mean speed profile is logarithmic, the expected mean speed difference due to the vertical separation is about 0.4 cm s−1. The average speed difference between the VACM and BASS increases as near-bottom wave orbital velocities get large relative to hourly aver...

Response Characteristics of the VACM Compass and Vane Follower

Abstract Several simple laboratory experiments have been conducted to study the dynamic behavior of the vector-averaging current meter (VACM) compass and vane follower. They demonstrate that the behavior of the compass and vane follower can be modeled as a damped linear harmonic oscillator for small-amplitude forcing. The combined eddy-current and bearing-friction torque nearly critically damps the free oscillation of the compass and vane follower. This frictional torque is proportional to the angular-velocity difference between the instrument magnet assembly and housing. Dynamic experiments on five compasses indicate a mean (undamped) resonant period of 3–5 s at 41°N. Similar experiments on two vane followers indicate a resonant period of 2–3 s. For the VACM dynamic compass experiments, frictional torque allowed an angular oscillation of the compass housing to drive an oscillation of the compass magnet, and at resonant forcing, the compass magnet oscillates exactly in phase with its housing. For the VACM...

Comparison of Shipboard Acoustic Doppler Current Profiler and Moored Current Measurements in the Equatorial Pacific

Depth-averaged current shears computed from shipboard acoustic Doppler current profiler (ADCP) and moored Savonius rotor and vane vector-averaging current meter (VACM) measurements are compared at 35, 62.5, 100 and 140 m depths within 7 km of each other near 0°, 140°W during a 12-day interval in November 1984. The agreement between the VACM and ADCP shears was excellent. The average root-mean-square difference of hourly shear values was small, approximately 0.21 × 10−2 s−1, and the average correlation coefficient was 0.90. Spectral estimates were equivalent to within 95% significance level and the VACM and ADCP shears were 95% statistically coherent with zero phase difference for frequencies below 0.2 cycles per hour.

Comparison of Upper Ocean VACM and VMCM Observations in the Equatorial Pacific

Abstract Measurements from a Savonius rotor and vane vector-averaging current meter (VACM) and dual orthogonal propeller vector-measuring current meter (VMCM) placed 1 m apart beneath a surface-following buoy moored on the Pacific equator for 6-month intervals are described. Experiments at 13, 98, 120 and 160 m depths were completed in light winds (≈5 m s−1), small surface wave heights (1–2 m), strong current speeds (monthly averaged speeds ≈65 cm s−1) and large current shears (0.0 1 s−1). Excellent coherence was found between each VACM-VMCM doublet's data. As expected, the near-surface VACM recorded slightly larger speeds (≈10% for monthly mean 15-min vector-averaged values) than the VMCM. The only kinetic energy density mismatch at the 95% confidence level was between the 13-14 m dyad and occurred above 1.3 cph. The most striking feature of them intercomparison tests was that the VACM and VMCM observations were virtually identical below the new-surface layer. Below 98 m the average rms amplitude differe...

A comparison of the vector‐averaging current meter and new Edgerton, Germeshausen, and Grier, Inc., vector‐measuring current meter on a surface mooring in Coastal Ocean Dynamics Experiment 1

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.

Overspeeding of a Savonious rotor

The distance constant of a vector averaging current meter (VACM) rotor is determined as 50±5 cm in acceleration and 25±3 cm in deceleration. Calculations of the overspeeding errors in a rapidly reversing flow are shown to be comparable to errors arising from the lagging of the direction measurement.

On simultaneous measurements with rotor, wing and acoustic current meters, moored in shallow water

In two closely spaced moorings in the Kiel Bight, four different current meterstwo rotor current meters (Aanderaa and Vector averaging), an acoustic current meter (designed by Gytre), and a pendulum current meter (designed by Niskin) were moored for 22 days. The Vector averaging current meter (VACM) was used as reference instrument on one mooring with the floatation at 7 m depth. The floatation of the second mooring was at 5 m depth in the first 17 days of the experiment, but in 2.7 m depth in the last 5 days to make their mooring more effected by surface waves. The Niskin wing current meter was most effected by wave-induced mooring motion. The Gytre instrument showed the smallest surface-wave effects. The vector variances of this instrument in 7.4 m depth on the surface-wave effected mooring and those of the VACM in 10 m depth on the reference mooring were about equal.

Comparison of different methods of current measurements

Comparisons of different Lagrangian and Eulerian near surface current measurements in shallow water during the experiment Baltic '75 show a strong influence of winds and surface waves on the measured data. The AMF vector averaging current meter (VACM) recordings and the drifts of dye patches show a reasonable agreement in speed and direction in all observed weather conditions. the Aanderaa current meter reproduces the directions well but increasingly overestimates the speed with increasing wave height. Drifting floats proved to be only usable in calm weather.

Correction of vector-averaging current meter records from the MODE-1 central mooring for the effects of low-frequency mooring line motion

The current record from the topmost vector-averaging current meter on the central mooring of the Mid-Ocean Dynamics Experiment (MODE) is corrected for the effects of mooring motion, and power spectra of the uncorrected and corrected signals are compared. The correction is < 1 cm s −1 for this mooring line (vertical excursions < 12 m). Other mooring lines observed during the POLYMODE experiment underwent vertical excursions of hundreds of meters and would have given much different results. This exercise enabled a second evaluation of our three-dimensional mathematical model of mooring dynamics. Creep in the synthetic portion of the mooring line is also identified.

Near-surface current measurements

Near-surface current measurements made in the JASIN '72 (Joint Air-Sea Interaction) experiment have been intercompared. There is close agreement between the measurements of a 10-m VACM (Vector averaging current meter) under a toroid moored in 2500 m of water and of 12-m drifters tracked from a ship: differences were only 2 cm s −1 in currents ranging from 10 to 30 cm s −1. Comparisons of the 10-m VACM and a 12-m Aanderaa meter revealed that the Aanderaa vane failed to follow the frequent reversals associated with the surface gravity wave field and hence the instrument biased currents high (by factors of between 1 and 10). A model demonstrated this failing: it is suggested that VACM measurements also bias high (by 10%) but no sure evidence could be found.