Drifting Deep-Ocean Shearmeter
Timothy F. Duda
Bigelow 202, AOPE Department
Woods Hole Oceanographic Institution
Woods Hole MA 02543
Shearmeter concept drawing.
This page covers the original development project. Information on more recent projects can be found on these web pages:
1. Ocean test mission, 1998
2. Brazil Basin Shearmeters, 2000
3. Guiana Basin Shearmeters, 2001
4. DIMES Shearmeters, 2009
A prototype shearmeter (PHOTO
) has been assembled and tested in a Lake. A second ocean-going unit
was launched on a 100-day mission in the Atlantic on March 26, 1998 (link
above). The shearmeter is intended measure flow velocity difference in
the vertical coordinate at scales of about 10 meters in a simple and reliable
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
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
Tests of P1 in Seneca Lake, NY in June, 1996 showed good
correlation between shear and rotation rate. Plotted here are shear
data computed from the onboard BASS (red), and the rate of rotation (green).
The rotation rates (rev/hr) have been divided by 1333, an estimated calibration
coefficient. In addition to the visual agreement between shear and
rotation, the statistics agree well. The BASS recorded mean and standard
deviation shear of 0.0192 and 0.0070 inverse sec, while the estimates from
rotation are 0.0199 and 0.0072, respectively.
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
Shearmeter. Proceedings of Oceans '97 Conference,
MTS/IEEE, volume 2,
This material is based on work supported by the National
Science Foundation under Grant No. 9416014 from the Ocean Technology
and Inderdisciplinary Coordination section of the Ocean
Sciences Division. Any opinions, findings and conclusions or recomendations
expressed in this material are those of the author and do not necessarily
reflect the views of the National Science Foundation (NSF).
Coastal and Ocean Fluid Dyn. Lab Webpage
Ocean Acoustics Lab Webpage
(My formal affiliation.)