Inhomogeneity and Lagrangian unsteadiness in turbulent thermal convection
Résumé
We present an experimental study of non-homogeneous turbulence using a Rayleigh-Bénard convection cell. The fluid motion is forced by a temperature difference between two horizontal plates. Using Lagrangian tracking on a large volume we can capture part of the Large Scale Circulation. The velocity statistics are strongly affected by the inhomogeneous mean flow but we recover the typical Homogeneous Isotropic Turbulence statistics by removing the local average. We discuss and explain a Lagrangian unsteadiness which persists because of the Large Scale Circulation oscillations. Our Lagrangian approach is a new way to study specificities of the convective roll motions in turbulent thermal convection. We propose a model based on the convolution between the Large Scale Circulation oscillations and the turbulent fluctuations to explain the shape of the velocity PDFs. However, the acceleration statistics are not affected by the mean flow.
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