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Le signal complexe de la diffusion collective de la lumière et les écoulements turbulents

Abstract : Collective light scattering is an observation device for turbulent gas. A laser lights the observed zone, and the gas molecules scattered field is detected at a small angle from the forward propagation direction. The collected information is proportional to the spatial Fourier transform of the gas density, inside the observation zone, at the scattering wavevector. Heterodyne detection allows to measure the modulus and phase of the complex signal.
The Doppler effect implies that the flow velocity can be measured: The signal spectrum, under some conditions, is proportional to spatial mean velocity distribution. We show that instantaneously, under the same kind of conditions, the signal phase derivative is proportional to the spatial mean velocity. The velocity evolution is then known. The velocity frequency spectrum can be calculated. We also compute the turbulent diffusion coefficient from the phase derivative. Experimental results were obtained in a supersonic mixing layer inside windtunnel at the LEA, Poitiers.
Scattering selects a specific wavevector. Information about a fixed lengthscale of the turbulent flow is present inside the signal modulus. A special device was built to observe simultaneously two scatterings at different wavevectors. The experience is an axissymmetric air jet. Correlation between those signals shows two different timescales: The shorter one appears only on selfcorrelation. Its value depends on the observation wavelength. The longer one also appears on crosscorrelation. Its value only depends on the production lengthscale and on the mean velocity. This timescale is peculiar to large structures inside the turbulence.
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Contributor : Cyrille Honoré Connect in order to contact the contributor
Submitted on : Friday, June 13, 2008 - 1:51:18 PM
Last modification on : Sunday, June 26, 2022 - 9:39:05 AM
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  • HAL Id : tel-00287591, version 1


Cyrille Honoré. Le signal complexe de la diffusion collective de la lumière et les écoulements turbulents. Physique [physics]. Ecole Polytechnique X, 1996. Français. ⟨tel-00287591⟩



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