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La spirale turbulente : motif de grande longueur d'onde dans les écoulements cisaillés turbulents

Abstract : An experimental study of the spiral turbulence, the helix shaped laminar-turbulent coexistence regime of the Taylor-Couette flow (the shear flow between two independently rotating coaxial cylinders), is reported. For this purpose Couette apparatus with unequalled dimensions have been built. It is shown that spiral turbulence is a large wavelength pattern compared to the gap size between cylinders which can't be observed when the azimuthal aspect ratio is lower than 50. It is revealed for the first time that this regime has an equivalent in plane Couette flow, the shear flow between plates moving at the same speed but in opposite directions, provided that it has large enough aspect ratios. This result inclines us to consider spiral turbulence as the charcteristic expression of the Taylor-Couette flow of a striped laminar-turbulent coexistence pattern common to shear flows. Studying its emergence from the fully turbulent flow, one may notice that the pattern appears progressively, following the same scenario in both flows. The quantitative study conducted in the Taylor-Couette flow shows that it appears as the result of an instability of the homogenous turbulent flow, the laminar-turbulent regime being obtained far from threshold through local relaxation to the laminar state in the minima of the modulations when these are strong enough. The pattern appears via a supercritical bifurcation and we show that a coherent amplitude-equation description can be given in terms of two coupled Ginzburg-Landau equations with real coefficients provided that noise is added. We demonstrate this by estimating, from experimental data, all the coefficients of the amplitude equations. This leads to consider a global picture of the transition in the framework of a noisy potential dynamics.
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Submitted on : Thursday, March 6, 2008 - 1:22:14 PM
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Arnaud Prigent. La spirale turbulente : motif de grande longueur d'onde dans les écoulements cisaillés turbulents. Physique [physics]. Université Paris Sud - Paris XI, 2001. Français. ⟨tel-00261190⟩

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