Large-Eddy Simulation of the Stratified Atmospheric Boundary Layer with a Stochastic Wall Model

Abstract:

In the atmospheric boundary layer (ABL), many processes of interest which happen in the first several meters above the surface are directly impacted by turbulence and are investigated using modeling tools such as the Large-Eddy Simulation (LES). In LES of the entire ABL, the correct representation of near-wall turbulence remains a challenge due to high-computational cost of its direct simulation. In this study, a stochastic wall model is tested as an alternative for flow refinement close to the wall and for the improvement of near-wall turbulence in the LES. The model corresponds to a vertical line within each wall-adjacent LES grid, in which a one-dimensional turbulence (ODT) model is coupled to the LES. In the ODT, the vertical diffusion equation is solved and the effect of three-dimensional turbulence is obtained from stochastic eddies, which correspond to a mapping function that mixes the variables and follows a joint probability distribution of eddy size and location. The LES-ODT approach used in this study improves the one-dimensional energy spectra for all three velocity components close to the wall. When a maize field is simulated by the ODT, important canopy-flow statistics (such as skewed velocity) are obtained, providing evidence of the ODT response to the LES driving flow. The effect of atmospheric stability on the LES-ODT coupling is also tested, and positive and negative aspects are discussed. This approach provides an alternative methodology for use in studies focused on near-wall turbulence phenomena, such as sediment transport, canopy flows and urban dispersion, especially under unstable conditions, when the ABL height (and consequently the LES domain) is large.