Dynamic Response of Spherical Electromagnetic Current Meters

The dynamic response of spherical electromagnetic current meters has been clarified through a comprehensive laboratory measurement program. Studies of current meter response for pure steady, pure oscillatory (horizontal plane), and combined steady/oscillatory motion were carried out. Flowmeter response under pure steady flow and pure oscillatory flow in the absence of high levels of free-stream turbulence shows an r.m.s. error of only 1-5 cm/sec. Combined steady/oscillatory flows result in larger residual errors and significant differences (up to 20%) in meter sensitivity compared with either pure steady or pure oscillatory tests. Horizontal cosine response showed systematic deviations from pure cosine behavior. The analysis shows electromagnetic current meters to be accurate for certain kinematic estimates, but could be erroneous when used to estimate dynamical quantities such as Reynolds stress, bottom friction, and log-layer friction velocities. A careful error analysis must precede any use of the meters for estimating dynamical quantities.