Acoustic vorticity meter for benthic boundary layer flow measurements

An acoustic travel-time flow-meter that had been configured to measure vorticity on scales of 15 and 45 centimeters has now been configured to measure benthic boundary layer vorticity over a scale of 1.5 meters. Closed square paths in each sensor define three orthogonal axes of vorticity. Originally constructed to measure shear in the presence of waves, the 15 cm and 45 cm sensors have an expected accuracy for shear of 10/sup -2/ s/sup -1/ and 3/spl times/10/sup -3/ s/sup -1/ respectively, The 1.5 m sensor appears to have a noise floor of 2/spl times/10/sup -5/ s/sup -1/. Strong boundary layer shear generates turbulence and turbulent vorticity. Measurements of vorticity are being made with these sensors at several scales in strong tidal flows from 1.2 m to 3.2 m above the bottom. Data are being stored internally and in a pop-up data logger for recovery at the end of the deployment and part way through the deployment, Absolute shear can be determined with the 45 cm sensor through careful zero flow calibration before the deployment. The 1.5 m sensor is part of the deployment tripod and too large to be calibrated in controlled conditions. Consequently, fluctuations in vorticity can be determined with the 1.5 m sensor.<
>