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The effects of spatial averaging on airfoil probe measurements of oceanic velocity microstructure

University of British Columbia

The effects of spatial averaging on the measurement of velocity power spectra by an airfoil probe have been studied. The probe is a miniature airfoil of revolution with a piezo-ceramic sensor that outputs a voltage proportional to the time-varying cross-stream velocity component. The spatial transfer function of the probe was measured in grid turbulence set up in a water tunnel by comparing the power spectra obtained with the airfoil probe to those measured using high spatial resolution laser Doppler anemometry (LDA). The LDA system was designed and constructed specifically for this purpose. The LDA was shown to be capable of measuring low intensity turbulence, giving results consistent with similarity theory and previous constant temperature anemometry measurements. The transfer function exhibited unexpected behavior at low wavenumbers, namely, an initial positive slope and peak away from the origin. A three-dimensional model is proposed and used to explain the experimental transfer function and to derive a large-scale transfer function suitable for correcting shear probe measurements of oceanic rates of dissipation of turbulent kinetic energy. Although in general the transfer function depends on the form of the energy spectrum, it is shown that for oceanic spectra the transfer function is independent of scaling in the wavenumber domain and hence independent of the rate of dissipation. For the two probe designs studied, the large-scale transfer functions have a 1/2-power response at a scale of about 1.5cm and approximately 50% of the total dissipation is recovered at the highest dissipation value considered, 10⁻¹ cm²/s³, (which is larger than the dissipation rates encountered in most of the ocean). The dissipation measurements can be corrected to an accuracy of 5%.

Text

2010-06-13

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http://hdl.handle.net/2429/25656

Oceanography

University of British Columbia

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