Mooring Vibration as a Source of Current Meter Error and Its Correction

Abstract

Abstract Data from instrumented moorings are used to show that rate underreading of Aanderaa paddle-wheel rotor current meters is a result of mooring vibration induced by vortex shedding from spherical in-line buoyancy. Direct evidence of this mooring motion is provided by accelerometer records that reveal high-frequency (0.2–0.4 Hz) displacements normal to the absolute flow direction that are as much as 0.5 m in amplitude. Current meter measurements are up to 40% lower than concurrent rates measured by a ship-mounted acoustic Doppler current profiler (ADCP). The result supports the hypothesis of Loder and Hamilton that rate underreading occurs when cross-flow motion of the current meters causes a rotor shielding effect arising from meter misalignment with the fluctuating relative water velocity. Measurements from an instrumented mooring in which spherical in-line buoyancy is replaced with streamlined flotation are also presented. Here the variance of the measured cross-flow acceleration is typically redu...