Large Eddy Simulations of Volume Restriction Effects on Canopy-Induced Increased-Uplift Regions

ABSTRACT Previous modeling and empirical work have shown the development of important areas of increased uplift past forward-facing steps, and recirculation zones past backward-facing steps. Forests edges represent a special kind of step - a semi-porous one. Current models of the effects of forest edges on the flow represent the forest with a prescribed drag term and does not account for the effects of the solid volume in the forest that restrict the airflow. The RAMS-based Forest Large Eddy Simulation (RAFLES) resolves flows inside and above forested canopies. RAFLES is spatially explicit, and uses the finite volume method to solve a descretized set of Navier-Stokes equations. It accounts for vegetation drag effects on the flow and on the flux exchange between the canopy and the canopy air, proportional to the local leaf density. For a better representation of the vegetation structure in the numerical grid within the canopy sub-domain, the model uses a modified version of the cut cell coordinate system. The hard volume of vegetation elements, in forests, or buildings, in urban environments, within each numerical grid cell is represented via a sub-grid-scale process that shrinks the open apertures between grid cells and reduces the open cell volume. We used RAFLES to simulate the effects of a canopy of varying foliage and stem densities on flow over virtual qube-shaped barriers under neutrally buoyant conditions. We explicitly tested the effects of the numerical representation of volume restriction, independent of the effects of the leaf drag by comparing drag-only simulations, where we prescribed no volume or aperture restriction to the flow, restriction-only simulations, where we prescribed no drag, and control simulations, where both drag and volume plus aperture restriction were included. Our simulations show that representation of the effects of the volume and aperture restriction due to obstacles to flow is important (figure 1) and leads to differences in the strengths and locations of increased-updraft and recirculation zones. Particularly, introduction of volume restriction shifted the location of the uplift zone and anchored it to the downwind edge of the barrier. In addition. We propose that volume restriction due to the presence of the stems and branches is an important improvement to LES that should be considered. Figure 1: Horizontal and vertical wind components (arrows) illustrated across a vertical cross section through the simulation domain. The colors illustrate the variation of the vertical wind component. The canopy location is marked by a thick continuous line. A. Drag-only simulation (Drag = 0.04π, Volume blockage = 0%), B. Control simulation (Drag = 0.04π, Volume blockage = 40%).