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Développement d'une instrumentation ultrasonore pour la mesure des vitesses des liquides au-delà de la limite de Nyquist par une approche spectrale

Abstract : Doppler velocimetry by pulsed ultrasounds makes it possible to obtain the velocity profile of a fluid flow. The mean velocity estimation is carried out, for each measurement volume, from a so called Doppler signal, obtained from the retrodiffused demodulated sampled signal, whose frequency is proportional to the particles velocitiy. It is shown, in the case of a plane circular transducer, for a homogeneous uniform flow in the measurement volume, that the power spectral density (PSD) of this signal can be modelled by a gaussian function. An algorithm of white noise suppression, based on parametric identification of the PSD, is proposed and validated. It consists in identifying, in real-time, the PSD components of the Doppler signal by using the Levenberg-Marquardt method with a general gaussian model. The identified white noise is withdrawn from the density in order to allow a calculation of moment unbiased by this one.

The Doppler signal being sampled by nature, it is subjected to the Shannon theorem which imposes a maximum measurable velocity called Nyquist velocity. This one is inversely proportional to the pulse repetition frequency (PRF). The exploration depth is also related to the PRF. This imposes the principal limit of this method, namely the inverse relation binding the exploration depth and maximum measurable velocity. In some applications, this limit is penalizing. A method allowing velocity measurements beyond the Nyquist limit is proposed. It is based on the use of several PRF. Different folding up for each PRF allows information contribution necessary to the resolution of the velocity ambiguity. An original algorithm of spectral rebuilding is proposed. It makes it possible to recombine the folded up PSD obtained for each PRF in order to reproduce the PSD of the Doppler signal.
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https://tel.archives-ouvertes.fr/tel-00011601
Contributor : Stéphane Fischer <>
Submitted on : Tuesday, February 14, 2006 - 10:51:45 AM
Last modification on : Thursday, April 23, 2020 - 2:26:28 PM
Long-term archiving on: : Saturday, April 3, 2010 - 10:23:07 PM

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  • HAL Id : tel-00011601, version 1

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Stéphane Fischer. Développement d'une instrumentation ultrasonore pour la mesure des vitesses des liquides au-delà de la limite de Nyquist par une approche spectrale. Traitement du signal et de l'image [eess.SP]. Université Louis Pasteur - Strasbourg I, 2004. Français. ⟨tel-00011601⟩

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