Vendredi 6 septembre à 11h00, salle des séminaires IRPHE
Abstract: Turbulent flows develop in different situations: atmospheric winds, ocean currents, or industry. Although there is a large diversity of turbulent flows, they all share some similarities and require a comprehensive understanding. In particular, the large-scale regime of turbulent flow is of main interest since the large turbulent eddies control the transport of momentum and dispersion of pollutants. To better understand the large-scale dynamics of turbulent flows, we present a new technique for generating turbulent flow with a scale separation between the forcing scale and the container size. This technique consists of using magnetic stirrers immersed in a fluid and energized by a vertical oscillating magnetic field to inject energy at scales smaller than the container size.
We will first demonstrate that the generated flow satisfies the assumptions of the Kolmogorov theory, implying that the kinetic energy injected into the fluid is transferred to small scales by a self-similar turbulent cascade mechanism. We will then report measurements of the large-scale regime showing that the energy modes are in equipartition and that the averaged energy flux is zero. These observations suggest a statistical equilibrium regime at large scales using this technique and, thus, the use of statistical mechanics concepts to describe turbulent flows. We will finally measure the temporal decay of such a turbulent flow made of turbulent blobs initially in statistical equilibrium. The conservation of linear/angular momentum during the decay of the turbulent flow can be measured by computing the Saffman or Batchelor invariants, which both depend on the integral length scale and the turbulent velocity of the flow. Our experimental measurements robustly support the Saffman model and present the first experimental evidence of the connection between the Saffman invariant and the large-scale energy spectrum.