The drifting, rotating deep-ocean shearmeter

Abstract

The shearmeter is a new member of the family of neutrally-buoyant subsurface Lagrangian floats (Swallow floats). It is a 10-m spar buoy with anemometer-type vanes rigidly mounted at each end, designed so that opposed horizontal flow at the ends due to shear causes the entire instrument to rotate. The vanes must provide torque at very low flow speed, so design work has included tank testing of components and subassemblies. A complete prototype shearmeter has been built and was tested in Seneca Lake, NY. It incorporated 3-axis BASS current meters at each end for calibration purposes. Both lake testing and tank testing suggest that shear as low as 0.4 or 0.5 cm/s over 10-m can be measured, with no realistic upper limit. The lower limit is due to spurious rotation from expected oceanic vertical flow past the float. Tank testing of vanes indicates that they respond at the lowest speed tested, 0.12 cm/s (0.24 cm/s velocity difference). Vane design improvements are under investigation.