HR: 0800h
AN: OS21C-1245    [Abstracts]
TI: Measuring Dissipation Rates of Turbulence Kinetic Energy with A Lagrangian Float
AU: Lien, R
EM: lien@apl.washington.edu
AF: Applied Physics Laboratory University of Washington, 1013 NE 40th St, Seattle, WA 98105 United States
AU: * D'Asaro, E A
EM: dasaro@apl.washington.edu
AF: Applied Physics Laboratory University of Washington, 1013 NE 40th St, Seattle, WA 98105 United States
AB: This study tests the ability of a neutrally buoyant float to estimate the dissipation rate of turbulence kinetic energy $\varepsilon$ from its vertical acceleration spectrum using an inertial subrange method. A float was equipped with a SonTek Acoustic Doppler Velocimeter (ADV), which measured the vector velocity a meter below the float's center, and a pressure sensor, which measured the float's depth. Measurements were taken in flows with $\varepsilon$ varying from $ 2 \times 10^{-8}$ to $10^{-3}$ W kg$^{-1}$. The Lagrangian acceleration spectrum is white within the inertial subrange with a level proportional to $\varepsilon$. The finite size of the Lagrangian float introduces a highly reproducible spectral attenuation at high frequencies. Estimates of $\varepsilon_{float}$ were obtained by fitting the observed spectra to a model spectrum which included the attenuation effect. The ADV velocity measurements were converted to a wavenumber spectrum using a variant of Taylor's hypothesis. The spectrum exhibited the expected -5/3 slope within an inertial subrange. A value of $\varepsilon_{ADV}$ was computed from the level of this spectrum. These two independent estimates $\varepsilon_{ADV}$ and $\varepsilon_{float}$ were highly correlated, with a correlation coefficient of 0.92, over the 5 decades of $\varepsilon$ measured. This analysis confirms that $\varepsilon$ can be reliably estimated from Lagrangian float acceleration spectra in turbulent flows. For the meter-sized floats used here, the finite size of the float and the noise level of the pressure measurements sets a lower limit of $\varepsilon_{float}> 10^{-8} \rm \,W kg^{-1}$ under most oceanic conditions.
DE: 4294 Instruments and techniques
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting