Timothy F. Duda

Bigelow 202, AOPE Department
Woods Hole Oceanographic Institution
Woods Hole MA 02543
 
 
 
 
 
 

The shearmeter is a spar buoy which drifts horizontally in the ocean at the depth where its own density matches that of the water. While drifting, horizontal flow past the ends will induce a net torque on vanes (photo) similar to those of a cup anemometer, rigidly mounted to the pipe. The torque, measured to be of order 1 ounce-inch, causes the instrument to rotate, since the shear-stress on the rotating pipe is calculated to be two or more orders of magintude less. Any rotation caused by vertical flow past the unit or by vorticity in the water would constitute an error. All the units built have been 10 m in length, measuring shear at that scale.

These instruments promise to allow flow measurements near 0.1 cm/s, one-tenth or one-twentieth of other methods (except for impulse-type acoustic travel-time current meters, (i.e. BASS), which have comparable sensitivity, or possibly digital video particle imaging). The shear of the deep ocean is very small (probability density function), but is potentially unstable because of the weak stratification, so its role in the generation of turbulence and in mixing does not scale down with the speed.

Prototype One (P1) for the lake is anything but simple, however. At the ends are mounted BASS 3-axis acoustic current meter cages, developed by Dr. Williams of my department at WHOI. Also included is an acoustic command release and communicator, from Benthos Inc., slightly altered by Tom (upstairs in the Oceanographic Systems group) to drive a waterproof solenoid. (PHOTOS )

The BASS, an order-$100 k scientific instrument, has since been returned to the proprietor to be used for other purposes. We are grateful to Dr. Williams for lending the instrument for the sensitive business of testing freely-floating, self-buoyant instruments (i.e. loaning at risk of loss).

The mechanical design and fabrication of P1 was handled by Webb Research Corporation. Mechanical design, fabrication, and electronics procurement for two ocean-going prototypes, without BASS, was also handled by Webb. The electronics are based on an enhancement of the RAFOS-float system of a particular RAFOS vendor. The floats are complete, and one was lanunched in the South Atlantic (photo above).

 
Here are the two variables plotted against each other, each integrated over periods of one revolution.

 
The Seneca tests were augmented by tank testing. The vanes were towed, suspended by monofilament line which exerted ummeasurable torque. Currents in the tank were monitored to be less than 10% of the tow speeds. The rotation was measured with a hand-held adjustable triangle. The tank is managed by the Advanced Engineering Laboratory in the WHOI AOPE Dept. The green dots show the data points, rotation rate vs. current speed. The response of the vanes at low speed appears to be divisible into two simple linear regimes, monotonic overall with speed. Meaurements of paired 13-cm vane cups (our chosen design) in a flow channel show torque of 1e-3 N at 0.4 m radius at 1 cm/s, about 1000 times the estimated skin drag on a 10-m long, 3 inch OD float rotating at 1/4 rev per hour. These measurements were made by suspending the pair in such a manner that torque would elevate them as they twisted, allowing the very small force to be calculated.

Sinusoidal vertical towing of the vane assembly at 4.3, 3.0 and 6.7 cycle per hour rates over 2.44 meters shows rotation rates quite small, of order one revolution in 5, 9 and 3 hours, respectively. Further testing is now underway (6/97), using the facilities of McLane Research Laboratories. The fastest measured 0.33 rev/hour rate corresponds to a noise shear of 4 mm/s over 10 m (4e-4 inv sec). These tests were done with our original design, not our improved design,

 
The figure show the vertical tow results. The upper panel shows the vertical position (red) and the angular position (green). The lower panel shows the vertical velocity (line) and the angular velocity (o's). The angular velocity is very low for the middle time period of 3 cph towing and maximum vertical velocity of 0.64 cm/s.

Further information can be found in this publication:

T. F. Duda and D.C. Webb (1997): The Drifting, Rotating Deep-Ocean
Shearmeter. Proceedings of Oceans '97 Conference, MTS/IEEE, volume 2,
pp 796-801.