Vertical profiles of salinity and temperature in the ocean reveal fluctuations with length scales as small as a few mil 1imeters. These fluctuations, called microstructure by oceanographers, are the small scale results of turbulent mixing processes. It is hoped that through the study of microstructure we can evaluate the relative importance of different sources of the turbulent energy (e.g., tides, large scale internal waves, etc.) and the mechanisms for generation of the turbulence (e.g., wave overturn, shear instability, etc.).
There is a considerable body of evidence to suggest that shear instability is an important mechanism in the generation of ocean microstructure. Measurement of the velocity shear in the ocean is difficult for it is necessary to profile the fluctuations in the horizontal velocity vector with a vertical resolution of a few centimeters.
After consideration of alternatives, it became apparent that a sensor based on the airfoil probe concept of Ribner and Siddon offered the greatest potential for making such measurements. The technique employs a tiny axi-symmetric side force transducer integrated into the nose portion of a cylindrical probe. The probe is mounted at the lower end of an instrumented pressure housing which free falls vertically through the ocean structure. Fluctuations in the two horizontal velocity components are instantaneously resolved into time varying voltages by a special two-channel piezoelectric transducer. This paper describes problems encountered in preparing probes suitable for oceanic work. The major problems that had to be solved were increasing the sensitivity, assuring orthogonality of the channels, performing calibrations and reducing mechanical vibration of the instrument body.
Since early 1972, numerous trials have been made in British Columbia Fjords which show the technique to be very successful. Characteristic traces of velocity structure show remarkable correlation with the corresponding records of temperature and salinity variation. Energy spectra of velocity shear confirm that there is complete spatial resolution of the small scale structure, ensuring that turbulent energy dissipation can be estimated from the data.
In summary, the present instrument provides profiles to study the importance of shear instability in the generation of oceanic turbulence and microstructure as well as information on local rates of energy dissipation. The airfoil probe has proven completely successful for measuring cross stream velocity components in view of the simple rugged nature of the sensing element, linear response, invariance with respect to fluid temperature, and ease of calibration.
Osborn, Thomas and Siddon, Thomas, "Oceanic Shear Measurements using the Airfoil Probe" (1973). Symposia on Turbulence in Liquids. 101.
3rd Biennial Symposium on Turbulence in Liquids (1973: Sep., Rolla, MO)
Chemical and Biochemical Engineering
Article - Conference proceedings
Special Methods of Measurement
© 1973 University of Missouri--Rolla, All rights reserved.