Herman CLERCX – Rotating Rayleigh-Benard convection : probing the transition to the rotation-dominated regime

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Date(s) - 6 juillet 2018

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Séminaire commun I2M (équipe ALEA-SI) / IRPHE (Seminar I2M / IRPHE)-Most geo- and astrophysical flows are driven by strong thermal forcing and affected by high rotation. In these systems, direct measurements of the physical quantities are not possible due to their large scales, remoteness and complexity. A model containing the main physical constituents is rather beneficial. This approach is given by the problem of rotating Rayleigh-Benard convection (RRBC) : a rotating fluid layer heated from below and cooled from above. For large-scale systems, the governing parameters of RRBC take extreme values, leading to a regime of geostrophic turbulence.-Background rotation causes different flow structures and heat transfer efficiencies in Rayleigh-Benard convection. Three main regimes are known : rotation-unaffected (regime I), rotation-affected (regime II) and rotation-dominated (regime III). Regimes I and II are easily accessible with experiments and numerical simulations (see right figure [1]), thus they have been extensively studied, see for a recent review [2]. On the other hand, access to regime III is more troublesome. Thus, regime III and the transition to this regime are less explored. Approaching the geostrophic regime of rotating convection, where the flow is highly turbulent and at the same time dominated by the Coriolis force, typically requires dedicated setups with either extreme dimensions or troublesome working fluids (e.g., cryogenic helium). In this study, we explore the possibilities of entering the geostrophic regime of rotating convection with classical experimental tools : a table-top conventional convection cell with a height of 0.2 m and water as the working fluid [3], see figure below. In order to examine our experimental measurements, we compare the spatial vorticity autocorrelations with the statistics from simulations of geostrophic convection reported earlier [4]. Our findings show that we have indeed access to the geostrophic convection regime and can observe the signatures of the typical flow features reported in the aforementioned simulations. As a next step we explored the role of coherent flow structures on the transition to regime III in RRBC. There are two main hypotheses proposed for the driving mechanisms of the transition to regime III one of them directly related to flow coherency. These hypotheses are usually examined through different parameters such as viscous and thermal boundary layers thicknesses and heat transfer efficiency [5,6]. In this work, we study regime III and these hypotheses from a new perspective : Lagrangian velocity and acceleration fluctuations and autocorrelations of tracers from experiments. We have found that the transition to regime III coincides with three phenomena ; the vertical motions are suppressed, the vortical plumes penetrate further into the bulk and the vortical plumes interact less with their surroundings. These findings allow us to evaluate the available hypotheses and to understand more about regime III [7].These regime transitions will be discussed in this talk.-[1] R.J.A.M. Stevens, H.J.H. Clercx and D. Lohse, PRE 86, 056311 (2012).[2] R.J.A.M. Stevens, H.J.H. Clercx and D. Lohse, EJMB/F 40, 41-49 (2013).[3] D. Nieves, A.M. Rubio and K. Julien, PoF 26, 086602 (2014).[4] H. Rajaei, R.P.J. Kunnen and H.J.H. Clercx, PoF 29, 045105 (2017).[5] E.M. King, S. Stellmach, J. Noir, U. Hansen and J.M. Aurnou, Nature 457, 301-304 (2009).[6] K. Julien, E. Knobloch, A.M. Rubio and G.M. Vasil, PRL 109, 254503 (2012).[7] H. Rajaei, K.M.J. Alards, R.P.J. Kunnen and H.J.H. Clercx, submitted to JFM (2018).– http://www.tue.nl/en/research/researchers/herman-clercx/ Herman CLERCX -Accès IRPHE[