Abstract of SPECKLE DIAGNOSTICS OF TURBULENT FLOWS,
N A Fomin,
Heat and Mass Transfer Institute 15, P Brovka, Minsk, 22072, Republic of Belarus.

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New possibilities in the diagnostics of turbulence are discussed based on recent advances in speckle photography. It is shown that double exposure speckle photography (DESP) in the frame mode allows the derivation of a two dimensional map of deflection angle experienced by light passing through a turbulent flow. The map may be used in the reconstruction of 3D density correlation functions and energy spectra in turbulence studies. Single exposure speckle -photography allows the determination of a density correlation function by using temporal averaging of the deflected light with improved spatial resolution and allows both visualisation and a quantitative measure of the anisotropy of the turbulence field. This is illustrated in the present work through presentation of a 2D map of temperature fluctuations in a turbulent flame. The speckle tomography method is based on multi-directional speckle photography. A mathematical description of light propagation in an optically inhomogeneous medium is presented. The relationship between the resulting time-averaged speckle patterns and refractive index is discussed. The temperature distribution in a multi-jet combustor was obtained using 12 (light) projection measurements. The obtained results show that speckle photography may be used as a basis for tomographic studies of 3D temperature fields in such combustion zones. The precision of temperature measurement using this technique strongly depends on the flow complexity and the number of projections measured.

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