Document Type


Date of Original Version



An examination is made of the use of broad bandwidth high-frequency acoustic scattering to infer remotely the spatial structure of the temperature field of a thermally driven buoyant plume. Application of the far-field Born approximation results in a linear relationship between the transfer function of the scattering process, G, the ratio of received to transmitted pressure, to the spatial Fourier transform of the temperature field, φ (K), where K = ks – ki is the Bragg wave-number vector. A series of experiments are devised to test this hypothesis. These experiments involve a geometry of scattering in which pairs of sources and receivers are placed on opposite sides and equidistant from the scattering volume, a buoyant plume generated by a small circular heating element at the base of a water tank. It is shown that the far-field approximation assuming incident plane waves breaks down when the scales of temperature variability of the plume are of order the Fresnel radius. These results are discussed for both an unstable and turbulent plume. Conditions for the recovery of the Bragg scattering condition are established. © 1996 Acoustical Society of America.