Regime transitions in turbulent horizontal convection at low Prandtl numbers.

Transitions between flow regimes in natural horizontal convection are analyzed using Direct Numerical Simulations starting from laminar and up to turbulent dynamics. This particular flow is driven by a gradient of buoyancy along a horizontal boundary and it has attracted increasing attention over the last ten years. Horizontal convection is thought to play an important role in driving large-scale overturning circulations as found in the oceans and the atmosphere. Recently, a new picture of the different types of theoretical regimes has been proposed by Shishkina et al. (2016) but this theory has yet to be validated. The scaling laws describing the transport of momentum and heat inside the domain are verified against numerical experiments at low Prandtl numbers. Our numerical experiments show clear evidence of the regimes predicted by Shishkina et al. (2016) and include other regimes previously observed by Gayen et al. (2014). In particular, we report the first observations of two new limiting turbulent regimes in natural horizontal convection at low Prandtl numbers.