Drag-based Cell-perturbation Method to Accelerate Turbulence Development in the Context of Meso-scale Model to Micro-scale Model Coupling

Impacts of dynamic mesoscale atmospheric processes on micro-scale atmospheric dynamics often play critical roles in predictions of micro-scale conditions and events relevant to many scientific applications.  Many scientific questions in the fields of wind energy harvesting, fire behavior prediction, modeling of water/CO2 fluxes from vegetation for global climate models, and predictions of particulate matter/fugitive gas transport and dispersion, cannot be studied without considering meso-scale impacts on micro-scales physical processes. 

Meso-to-micro scale model coupling has been widely used for numerical studies for these questions.  However, turbulence characteristics from mesoscale models are orders of magnitude larger than those represented by micro-scale models in both in time and length scales.  This disparity in resolved scales and discontinuous turbulence spectra could drive unphysical atmospheric behavior near the transition between meso- and micro-scale resolution when these models are coupled.  Without specific efforts to instigate this turbulence that would naturally exist between the scales of the meso- and micro-scale grids, it can require large simulated distances to develop a realistic turbulence spectra.  In order to bridge or fill the turbulence spectra ‘gaps’ in a more computationally efficient manner, several techniques have been used, such as synthetic methods, point-perturbation, and cell-perturbation method on potential temperature.

We explore a new drag-based cell-perturbation method of accelerating the development of turbulence in micro-scale Large-Eddy Simulation (LES) models.  Inspired by the recent work by Domingo et al., which perturbed potential temperature of cells near inflow boundaries, this new technique generates turbulence by placing virtual drag elements dynamically within the LES simulations domain, near the inlet boundary.  This technique resembles turbulent generating grids used in wind tunnel experiments.  The effectiveness and efficiency of the drag-based perturbation method are compared to simulations with stand-alone periodic boundary conditions and with the temperature-based cell-perturbation method for different atmospheric boundary layer conditions.