Use of tow tanks to study sensitive current meters

Abstract

The importance of accurate calibration of current sensors has increased as current sensors have become more sensitive, and as scientists study turbulence and mixing in boundary layers. Current sensors are often calibrated in tow tanks. This paper discusses and presents some measurements of some of the complexities of calibration in tow tanks. Residual convective currents cause a noise floor in calibration. Sensor wakes cause gain errors and increase the ambient turbulence if not dissipated before a successive measurement is made in the same water. Proximity to surfaces can affect the velocity gain of current sensors. Density stratification in water and the internal waves it allows add complexity to measuring slow flows. Strumming of current meter mounts greatly increases the velocity noise of current meters. Measurements of velocities in current sensor wakes are compared with far-field turbulent wake models to characterize how long one must wait between tank tows in order for wakes to dissipate. Wake velocities caused by sensors that looked like they would leave two-dimensional wakes, in fact, decayed as velocities in three-dimensional wakes. Three-dimensional wake velocities dissipate more rapidly than two-dimensional wakes velocities, reducing the time required to wait between measurements in tow tanks.