Interpretation of Velocity Profiles Measured by Freely Sinking Probes

For most freely sinking instruments that measure horizontal velocity, it is necessary to understand the response of the vehicle to the flow to properly interpret the measurements. Three physical effects that influence the measurements are identified and modeled. First, the vehicle tends to follow the low vertical wave number variability in the velocity profile. The horizontal velocity of the vehicle is shown to be the low pass of the true current profile. The filter has a single pole at a wave number that does not depend on the sink rate. Second, when there is a horizontal velocity sensor on the vehicle, the sensor may be displaced from the vehicle´s center of mass. In this case there is a phase difference between the measured profile and the vehicle motion which may be accounted for. The third effect arises when the drag on the vehicle cannot be adequately modeled as a point force. The effect of the distributed drag force is only significant for high wave number variations. These results are applied in correcting measurements from a freely sinking velocity profiler. The corrected measurements are in good agreement with simultaneous velocity measurements from an instrument, whose performance in the band of comparison does not depend on the vehicle motion.