Direct numerical simulations towards ultimate turbulence: UltimateRB

Programme(s): H2020-EU.1.1. - EXCELLENT SCIENCE - European Research Council (ERC)

Topic(s): ERC-2018-STG - ERC Starting Grant

Call for proposal: ERC-2018-STG

Funding Scheme: ERC-STG - Starting Grant

Grant agreement ID: 804283

Objective: Turbulent thermal convection plays an important role in a wide range of natural and industrial settings, from astrophysical and geophysical flows to process engineering. The paradigmatic representation of thermal convection is Rayleigh-Bénard (RB) flow in which a layer of fluid is heated from below and cooled from above. A major challenge is to determine the scaling relation of the Nusselt number (Nu), i.e. the dimensionless heat transport, with the Rayleigh number (Ra), which is the dimensionless temperature difference between the two plates, expressed as Nu∼Ra^γ. Theory predicts that the scaling exponent γ increases for extremely strong driving when the boundary layers transition from laminar to turbulent. Understanding the transition to this so-called ‘ultimate’ regime is crucial since an extrapolation of results from lab-scale experiments and simulations to astro- and geophysical phenomena becomes meaningless when the transition to this ‘ultimate’ state is not understood. So far, there is no consensus among experimental efforts for obtaining the ‘ultimate’ regime. We propose using direct numerical simulations (DNS) to gain a better understanding of the transition towards the ‘ultimate’ regime. While obtaining ‘ultimate’ thermal convection in simulations has been elusive, new developments make this feasible now. The benefit of simulations is that they allow full access to the flow and temperature fields, while all boundary conditions are set exactly and independently. This allows us to test various physical effects at full dynamic similarity. To trigger the excitation of the ‘ultimate’ regime at lower Ra than in standard small aspect ratio cells, we want to study the effect of roughness, additional shear, and large domains in which a stronger flow can develop than in confined small aspect ratio cells that are traditionally considered. The addition of rotation will be studied to disentangle the complicated effect of rotation on high Ra number thermal convection.