Dynamic response of electromagnetic current meters

Abstract

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.