Large eddy simulation of a sheared convective atmospheric boundary layer: analysis and diagnostic

Details

Supervisors

Chatelain, Philippe ; Duponcheel, Matthieu ; Winckelmans, Grégoire

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil mécanicien, à finalité spécialisée

Abstract

The Atmospheric Boundary Layer (or ABL) is the region of the atmosphere in direct proximity to the Earth’s surface: this is where wind energy is harnessed by wind turbines, but this is also where severe weather events occur. In the age of climate change, a thorough study of the air flow in the ABL is crucial. This is best achieved via atmospheric simula- tions which have become a powerful tool thanks to dramatic improvements in computing performance. The initial aim of this master thesis was to study the interactions between physical flows in the atmosphere and wind turbine fields, but it became apparent that this objective was unattainable due to various problems encountered during the simulations. The focus was therefore shifted on diagnosing and quantifying the problems encountered by an in-house code, developed by the TFL group at UCLouvain, to simulate an atmospheric boundary layer in which both wind and ground radiation are simulated. An analysis focusing on the physical properties of the simulated flow was also carried out. This work has clearly highlighted and quantified the problems encountered by the code during simulations, as well as the various key factors in the development of these problems. The main issues arising from the simulations were the appearance of parasitic temperature variations, and consequently of significant vertical movements due to convection in the free atmosphere. To best capture and quantify these problems, metrics based on the amplitudes of local temperature variations were developed. Using these metrics on different profiles and cross-sections of the various flow variables, a systematic study of the impact of several simulation parameters on the code’s performance was carried out. Some variables showed a strong impact on performance, such as the wind horizontal (or “streamwise”) velocity, the atmospheric stability, or the value of the Courant–Friedrichs–Lewy (CFL) coefficient. Also, vertical structures developing close to the Earth’s surface (i.e., where wind turbines are installed) suggest that there are problems with the current implementation of the temperature wall model. At last, as far as the physics of the results returned by the code is concerned, simulations are in line with what is expected and observed in the literature, apart from a few parasitic structures close to the surface. A streamwise velocity profile very similar to the one described in meteorology textbooks was simulated, showing the homogenization of velocity in the mixed layer due to turbulent mixing. The greater importance of buoyancy compared with shear on turbulence generation was also observed.