Droplet Transport in Flow Turbulence

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Dilute poly-disperse mixtures of small liquid droplets in gas phase flow is a common feature of both environmental and industrial processes. For example: clouds, humidification and cooling, combustion, surface coating, fire suppression, drug delivery, and pesticide application. In many practical cases the droplets are small enough that their motion is strongly influenced by the gas phase turbulence. The effect of turbulence on the droplet motion depends on the droplet inertia and size relative to the length and velocity scales of the turbulence. In general terms, turbulence causes a diffusion of droplets across the flow and in such a manner as to generate intermittent regions of droplet concentration at the mixing and dissipation scales. Prediction methods for droplet concentration and its transport based on Computational Fluid Dynamics are valuable to the analysis of the above mentioned processes. The applicant is a member of a research team at UNB focussed on development a Large Eddy Simulation (LES) tool for tracking pesticide droplets in the lower layer of the atmosphere from release to deposition on the ground cover. LES simulation of the atmospheric turbulence over local topography covering 1 sq km has been accomplished using advanced GPU based computing developed at UNB. The present challenge is developing an efficient and accurate model of droplet motion through the atmospheric turbulence. One of the articles provided as a significant contribution describes a RANS simulation of an aircraft wake with Lagrangian droplet tracking. The next research objective is a LES simulation which includes the aircraft wake and atmospheric turbulence together with an Eulerian model of droplet transport. The present grant proposal describes an experimental research program which is necessary to support the larger objectives described above. It provides measurements of droplet concentration and transport in turbulent flow for the purposes of testing and developing phenomenological transport models used in Eulerian LES based computational simulations. The model problem that will be studied is a horizontal axisymmetric spray plume in isotropic grid turbulence with model scales chosen to approximate the scales of a Large Eddy Simulation of the near ground layer of the atmosphere. The droplet sizes will be chosen to extend the range of Stokes number and settling velocity currently considered in the literature to include those of relevance to pesticide transport. The experiments will be coordinated with model testing in LES simulations of atmospheric turbulence.