Reproducing Field Measurements of Deposition Velocity: Large Eddy Simulation of Dry Deposition over Rough Surfaces

Abstract:

The deposition velocity (v_d) of dust on rough surfaces are much larger than on flat surface. The largest discrepancies between estimations and field measurements of deposition velocity also occur on rough surfaces, these discrepancies often reach 2 to 3 orders of magnitude. For decades, these discrepancies are repeatedly reported, yet not understood.

Previous deposition models use the flow over a single cylinder to approach the flow over more complex surfaces, and use a constant surface drag for all surfaces. These models cannot distinguish the drag for different roughness densities and different wind speeds, particularly on rough surfaces where the influence of roughness elements and shear flow on drag is significant.

This study proposes a deposition model, which reproduces the measurements of deposition velocity over rough surfaces, based on a study of turbulent shear flow over rough surfaces, a proposed physical drag theory and a series of large eddy simulation data. The numerical simulations of particle-laden flow are conducted on 11 rough surfaces with different roughness density (λ) at 6 wind speeds with particle diameters from 0.1 to 10 μm. These data are studied to associate the geometry of surfaces, the structure of shear flow and the drag on surfaces with deposition velocity. The wind profile and shear stress on surfaces are measured to validate the simulation. The simulated deposition velocities agree well with these field measurements for the first time.

The attached figure shows the comparison between deposition velocity from field experiments (markers) and the estimation from this study (curves), for 2 surfaces (λ = 0.033 and λ = 0.5) under 6 wind speeds (1 - 25 ms^-1) and different particle sizes (d_p). The measurements are well reproduced by the proposed deposition model. Deposition velocity increases as wind speed increases, thus the curves are not distinguished for each wind speed on each surface.