Modélisation de la transition vers la turbulence d'écoulements en tuyau de fluides rhéofluidifiants par calcul numérique d'ondes non linéaires

Abstract : The transition to turbulence in pipe flows of shear-thinning fluids is studied theoretically. The method used is the computation of `exact coherent structures' that are tridimensional nonlinear waves. For this purpose a pseudo-spectral Petrov-Galerkin code is developped, which also allows to follow solution branches in the parameter space with continuation methods. This code is validated by recovering already published results in the Newtonian case, and by a consistency test in the non-Newtonian case. A spectral exponential convergence is obtained in all cases. This code is used to seek (guided by recent experimental results) new solutions of fundamental azimuthal wavenumber equal to 1,without success at the time being. On the contrary solutions with a fundamental azimuthal wavenumber equal to 2 and 3 are obtained by continuation from the Newtonian case. The shear-thinning effects induce, in terms of critical Reynolds numbers, a delay for the onset of these waves, as compared with the Newtonian case. This delay is characterized. An analogy is made with various experimental results that show a delay in the transition to turbulence, more precisely, in the onset of `puffs', in non-Newtonian fluids
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Nicolas Roland. Modélisation de la transition vers la turbulence d'écoulements en tuyau de fluides rhéofluidifiants par calcul numérique d'ondes non linéaires. Autre. Institut National Polytechnique de Lorraine, 2010. Français. ⟨NNT : 2010INPL037N⟩. ⟨tel-01748830⟩

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