The influence of vegetative windbreaks on fugitive dust (PM10) mitigation and transport flux: the role of turbulence in enhancing particulate deposition

Quantifying deposition efficiency for fugitive dust on vegetation is essential for developing more accurate computational models. This work focuses on the role that turbulent motions play in deposition enhancement. This research combines field and wind tunnel experiments to study particle deposition onto vegetation resulting from small-scale interactions of turbulent flows. These experiments will help to optimize the design of vegetative windbreaks as a mitigation tool for fugitive dust removal from the atmosphere. The long-term goal is to use these data for model development and parameterization within the Quick Urban and Industrial Complex (QUIC) dispersion modeling system. Experimental testing in a full scale wind tunnel seeks to quantify deposition efficiencies by varying the relevant Stokes number parameters (i.e., wind speed, deposition area of substrate, and particle size). Experimental results indicated that grid induced turbulence enhances deposition in all six directions (x-upstream and downstream, y-right and left, z-up and down). Deposition was enhanced on the upstream impaction surface (-x direction) by a factor of two compared to the no-grid "laminar" case. Deposition on all other directional surfaces increased by about an order of magnitude. This work investigates the effect of isotropic turbulence on the enhancement of particle deposition to surfaces for inertial impaction dominated processes. Turbulence and particle deposition were quantified using hot-wire anemometry and fluorimetry measurement techniques, respectively. The contribution of turbulence on deposition is shown to scale with a dimensionless parameter formed from the combination of the classical Stokes number (Stk) and the Taylor-microscale Reynolds number (Rλ). This scaling helps to understand the role that the intermediate eddies (λ) and turbulent fluctuations (u') have on deposition fraction (DFλ). A modified Stokes number (Stk*=Stk·Rλ 0.3) parameterization for an empirical equation (DFλ = 100-100/(440.5·(Stk*)3.88+1)) was devised to utilize this new scaling and incorporate physically significant turbulent deposition parameters (i.e., λ and u') into the solution. Experimental results indicate that past impaction parameterizations substantially underestimate deposition in the presence of turbulence.