Large Eddy Simulation

Large Eddy Simulation (LES) is often viewed as the most efficient computational technique to study turbulent flows and helped researchers to have a better understanding of the complex phenomena. In the framework of COMTESSA, the LES model PALM (The Parallelized Large-Eddy Simulation Model) is used along with the eddy covariance measurement to simulate the atmospheric boundary layer flows.

As a first attempt, a set of high-resolution LES numerical experiments has been performed to investigate the characteristics of the scalar field dispersing from a localized small source. We performed simulations reproducing the dispersion from a continuous tracer release in a neutrally stratified boundary layer generated in a wind tunnel. An example is shown in  Figure 1. It shows a snapshot of instantaneous scalar concentration in the xy plane at source elevation.


Figure 1: A simulated plume dispersing in a wind tunnel.

With respect to dispersion modelling, a literature study showed that no previous study could clearly demonstrate the ability of LES to model higher-order moments of the concentration fluctuations generated from a small source in the highly turbulent conditions encountered in the ABL. We could, for the first time, show that the LES approach can simulate higher-order concentration moments (up to the fourth) but only when run at very high resolution.

LES model simulations of the field experiments were done in an unprecedented nesting set-up, where first 3 nesting levels of a mesoscale model were used to scale down meteorological reanalysis data. Four more nesting levels of the LES itself brought the model resolution around our experimental site to ~1 m. A publication, in which the model simulations are compared with eddy covariance measurements at our tower, is currently being prepared. Another publication will compare the model results with the tomographic tracer reconstruction.

Figure 2 WRF simulation of the meteorology at Rena military facility (red mark) showing wind direction (arrows) and wind speed (contour lines in meters per second) are reported for 16-07-2018 at 10 a.m.


Figure 3: LES simulation. The coarser LES grid level is shown together with the innermost most resolved grid level. Colored by the northern component of the turbulent wind velocity (y-direction in the figure). The most resolved grid level covers a large part of  the military facility. The horizontal colored plane is at the elevation of the military facility. The  topography rising above the level of the military facility is also shown (shaded solid color).