Path-averaged turbulent dissipation measurements using high-frequency acoustical scintillation analysis

Abstract

The path-averaged turbulent kinetic energy dissipation (per unit mass) is estimated in a shallow tidal channel using acoustical scintillation analysis. The tidal current ensures that fully developed turbulence prevails. In order to make measurements of the turbulence parameters, a high-frequency (67-kHz) acoustic propagation experiment was conducted. Our acoustic data is compared to the weak scattering theory of Tatarski assuming a Kolmogorov turbulence model and compared with available oceanographic data. Analysis of log-amplitude, phase, and phase-difference spectra shows close agreement with the theory. Comparison of the acoustic data with direct measurement of temperature and salinity fluctuations using in situ sensors allows evaluation of the contribution of turbulent velocity fluctuations to the scintillation signal. The results show that turbulent velocity fluctuations are the dominant (∼95%) component of the observed acoustic scintillation, leading to estimates of the path-averaged turbulent energy dissipation, which rises and falls with the tidal current (ε∼10−7–10−5 m2s−3). A confirmation of the validity of the weak scattering theory together with the Kolmogorov turbulence model in this environment is given within the context of the scattering strength–refractive index variance parameters (Γ,X).