High-frequency acoustic scattering from thermally generated turbulence

Abstract

The results of a laboratory experiment are presented for the case of high-frequency broadband multistatic acoustic scatter from a thermally generated buoyant plume. The dominant scattering mechanism is changes in the relative compressibility, which is related to the temperature difference from ambient. Far-field weak scattering theory is used to describe the scattering process. This results in a simple relationship between the complex acoustic scatter and the one-component, three-dimensional Fourier transform of the temperature field measured at the scattering producing Fourier component K, defined as the Bragg wave number. This wave number is the magnitude of the difference wave vector between the scattered and incident wave vectors. Theory predicts for measurements made in the same scattering direction that spectral estimates for overlapping Bragg wave numbers be identical. Utilizing data collected from simultaneous multiple bistatic configurations with parallel Bragg wave vectors yields a more fully resolved spectral estimate of the scattering producing temperature field. A time series of the spectral estimate of the turbulent temperature field is shown for a three-channel bistatic common Bragg direction measurement. The results indicate agreement between the spectral estimates in the regions of overlapping Bragg wave numbers, validating the theoretical description. The time series also shows strong, yet highly variable nulls in the wave-number spectrum of the turbulence. [Work supported by ONR.]