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Detlef Lohse – University of Twente

Wednesday, July 24th, 10:45AM EDT 

Transition to the Ultimate Regime in Rayleigh-Bénard Turbulence

In this talk I will reconcile the various experimental observations for large Rayleigh number Rayleigh–Bénard (RB) turbulence. In this so-called ultimate turbulence regime, the Nusselt number (the dimensionless heat flux) has a steeper dependence on the Rayleigh number (the dimensionless temperature difference between top and bottom plate) than Nu~Ra^1/3. Here the analogy between RB flow (Fig. 1a-c) and parallel flow along a flat plate (Fig. 1d-f) is illuminating. In turbulent RB convection, the core part of the flow is always turbulent (a, b), while the kinetic boundary layers (BLs) can vary from scaling-wise laminar Prandtl–Blasius type boundary layer (a, bluish) to fully turbulent Prandtl– von Karman type boundary layer (b, reddish). These two different cases correspond to two distinct dependences of Nu vs. Ra, illustrated in (c) with, respectively, the blue and red lines. Analogously to RB flow, parallel flow along a flat plate (d-f) undergoes a transition between laminar (d) and turbulent (e) boundary layers that have different dependences of the skin friction coefficient Cf  on the Reynolds number Re, which are sketched in (f) with the blue and red lines, respectively. There is a similar analogy between RB flow and pipe and channel flows and Taylor-Couette flow.