Turbulent transition in Rayleigh-Bénard convection with rough and smooth boudaries.

Density gradients occur naturally in the oceans, the atmosphere or the interior of stars. When they are stable, they support internal gravity waves. When they are unstable, they are at the origin of coherent structures called plumes. The development and the interaction of the plumes give birth to large scale turbulent motions. These phenomena are not well understood but are mandatory in the modelling of mixing and energy transport in geophysical flows.

Natural flows are indeed difficult to characterize and physicists often use some model

systems, such as the Rayleigh-B\'enard flows which can mimic a lot of situations and are easy

to manage in laboratory.

We present measurements of the global heat-transfer and the velocity field in Rayleigh-B\'enard convection over two decades of Rayleigh and up to 2.10¹². The velocity field is inferred from sequences of shadowgraph pattern using a Correlation Image Velocimetry (CIV) algorithm.

The Rayleigh-Bénard flow is largely inhomogeneous and the velocity change from one place to another one, which makes the definition of the Reynolds number difficult.

Despite this, the scaling of the Reynolds number with the Rayleigh number remains robust among the studies, and only the amplitude can differ.

In addition, the joint heat-transfer and velocity measurements allow to compute the scaling of the kinetic dissipation rate which features a transition from a laminar Re^5/2 scaling to a turbulent Re³ scaling. The addition of roughness permit to evidence a roughness triggered regime with enhanced heat transfer efficiency, link to the destabilization of the boundary layer, sooner than with smooth boundaries.