Publication Date:
2022-05-26
Description:
The dynamic response of electromagnetic current meters (manufactured
by Marsh-McBirney, Inc.) has been clarified through a comprehensive laboratory
measurement program combined with a thorough literature review. Elucidation
of the behavior of these flowmeters under a variety of dynamic conditions has
been neglected in the past. Since flow past a spherical body has considerable
hydrodynamic complexity for different dynamic conditions, a careful laboratory
study was carried out for pure steady, pure oscillatory (horizontal plane), and
combined steady/oscillatory conditions at two test facilities. Test results
indicate that flowmeter behavior under pure steady flow is excellent in the
absence of high levels of free-stream turbulence, with an r.m.s. error of 1-5
cm/sec. These errors could· be reduced with a higher-order polynomial regression
fit. Pure oscillatory response was also excellent, with r.m.s. errors of
1-2 cm/sec, and sensitivity which is correlated with the oscillatory Reynolds
number, (Re)o, and the Keulegan-Carpenter number, (A/d). Combined steady/oscillatory flows degraded current meter performance with larger residual
errors (1-6 em/sec) and significant differences in sensitivity (up to
20°/o). Horizontal cosine response showed systematic deviations from pure
cosine behavior, with a notable inter-cardinal undersensitivity and cosine
"shoulder" at lower Reynolds numbers. Error analysis shows these current
sensors are adequate for many kinematic measurements, but may lead to excessive
errors when using velocity to calculate dynamical quantities (such as bottom
friction, Reynolds Stress, or log-layer friction velocities). A careful error
analysis must precede any use of these meters for estimating dynamical quantities.
These studies pointed out a potential difficulty in using these meters
in areas of large ambient turbulence levels (20°/o turbulent intensities),
which are characteristic of many near-bottom shallow water environments.
Further study is needed to clarify this behavior.
Description:
Prepared for the U.S. Army Corps of Engineers, Coastal
Engineering Research Center, Waterways Experiment Station,
Vicksburg, MS, under contract DACW/2-82-C-0014; work was
initiated with funding from the NOAA National Office of Sea
Grant under grant number NA80-AA-D-00077; the Coastal
Research Center of the Woods Hole Qceanographic
Institution provided support.
Keywords:
Ocean currents
;
Water current meters
;
Oceanographic instruments
Repository Name:
Woods Hole Open Access Server
Type:
Technical Report
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