Study of anabatic flows using large-eddy simulations in a simplified geometry

In the present work, the turbulent anabatic flow generated over a uniformly heated slope in neutral stratification is originally studied through a large-eddy simulation (LES) technique. The present study is, to the best of the author's knowledge, the first case of a LES applied to anabatic flows in neutral stratification. The simulation approach is succesfully validated against three data sets: experimental, DNS and theoretical. One of the primary objectives of the study is to characterise the instantaneous turbulent structures triggered by the vertical buoyancy force responsible for the increase the turbulent mixing in the boundary layer. Such structures are hardly detected in both field and laboratory experiments and cannot be reproduced by steady-state numerical simulations. A new expression of the characteristic length scale of the thermal boundary layer is proposed and applied to derive alternative scaling parameters. Three principal regions are detected in the near-surface temperature profiles: a conduction region that contains most of the temperature decrease, a convective region dominated by flow convection and an equilibrium region that is almost not influenced by the heated slope. The newly proposed length scale resulted to be linked to the evolution of instantaneous turbulent structures identified as Rayleigh-Taylor instabilities which are analyzed and described. Their characteristic frequency is determined through a spectral analysis and their geometric dimensions are studied and linked to the extension of the vertical mixing zone inside the convection region. Three simulations are performed at different Rayleigh numbers to understand if there is a critical value above which the anabatic flow results Rayleigh-independent. the sensitivity analysis is carried out concluding that the analyzed flows are not Rayleigh-independent.