Calibration of a Marine Boundary Layer Flow Measuring System

While considerable attention has been paid to calibrating individual velocity sensors and evaluating their wavenumber response, little work has been carried out to examine the flow characteristics around an array of sensors, including their mounting. Upstream blocking effects of measurement systems in the field have been demonstrated by Mollo-Christensen (1979) and laboratory tests of sensor platforms have been carried out by Teunissen (1978). Theoretical models for the flow field modification have been examined by Britter (1979) based on rapid distortion theory. In order to study blockage effects, sensor spacing and angle of attack sensitivity, a tripod instrumented with up to ten triplets of small ducted impellors was deployed in a simple controlled boundary layer. The steady uniform flow was confined in a 5.5m deep trapezoidal channel, 30 m wide at the surface and 17m wide at the bottom. Different spacings of the current meters in the vertical were used to study blockage effects and the consequences of varying angle of attack of the flow relative to the sensor platform were investigated by deliberately mis-aligning the frame. Time series analysis was used to indicate the length of record required to yield stable estimates of the mean and fluctuating quantities and the cross-channel variability of the velocity field was mapped. Subsequently other tripods were placed in the channel to allow comparison among the measurement systems.