Degradation of some mechanical current meter measurements by high-frequency mooring or wave motion

Comparison of current measurements from moored (paddle-wheel rotor) Aanderaa current meters and acoustic Doppler current profilers in a strong tidal flow on Georges Bank indicates rate under-reading by the Aanderaa meters at some vertical positions. The under-reading may arise from mooring-line vibrations induced by vortex shedding from spherical buoyancy packages, and shielding of the paddle-wheel rotors due to the meters' inability to remain aligned with the fluctuating relative water velocity. Field tests and a simple model are used to investigate this explanation. The authors briefly review the Georges Bank observations that instigated this investigation. An explanation for the degradation of the Aanderaa measurements is proposed, observational information from field tests is discussed, and the model and its implications are presented. The model results support the hypothesis that high-frequency mooring-line vibration causes the degradation of the Aanderaa measurements.< >