Browsing by Subject "Sprays"

Now showing 1 - 3 of 3
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    Advanced simulation and modeling of turbulent sprays
    (2014-03) Liu, Wanjiao
    Sprays have wide applications in agriculture, pharmaceutical synthesis, engines, ink jet printing and so on. The successful spray applications and the control of spray param- eters require a thorough understanding towards the physical mechanisms. Numerical tools have been developed in the past few years for simulating the multiphase turbu- lent flows like sprays. Several researchers have successfully carried out direct numerical simulations (DNS) to investigate the primary breakup in such flows. DNS is accurate but requires extensive computational resources. In comparison, large eddy simulation (LES) is more practical, resolving only the large-scale flow structures and modeling the small-scale effects. The major difficulty with LES of multiphase turbulent flows is the need to model the interfacial subgrid-scale terms. Subgrid surface tension force, for ex- ample, plays an important role in the small droplet formation process. Subgrid surface tension force is, however, a highly non-linear term and can be difficult to model. In this research, we propose a new approach that combines the filtered density function (FDF) approach with the large eddy simulation. The major advantage of FDF is that the non-linear surface tension force appears in a closed form and thus needs no sub- grid modeling. The FDF transport equation is solved conveniently via a Lagrangian Monte-Carlo method. The Lagrangian approach is attractive in that it facilitates the transport of the liquid-gas interface without the diffusive or dispersive errors found in the Eulerian approaches. The surface tension source term in the momentum equation is closed using a Lagrangian volume of fluid (LVOF) approach. We utilize concepts from the smoothed particle hydrodynamics (SPH) in the LVOF approach to obtain the surface tension source term based on the Lagrangian particles. Several modifications have been made towards the original SPH formulation such that it is more suitable for the large-scale, turbulent multiphase flow simulations. Multiple particles are seeded in each Eulerian cell to achieve higher statistical accuracy, while the original SPH seeds one particle in each cell. What's more, a weighted SPH formula for the color function is adopted and is shown to be capable of handling variable particle number density. Performance assessment is via the rotation of Zalesak's disk and an oscillating elliptical droplet. Results show that the modified approach is able to handle the variable particle number density case appropriately. The simulations of multiphase turbulent flows are then performed with the proposed FDF-VOF methodology. At the same time, results from the simulations are compared with the DNS approach for validation and com- parison. Results show that the FDF-LES based approach can be a promising method, in that it models the flow with lower computational cost than DNS, yet maintaining accuracy in a model-free manor.
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    Atomization of viscous fluids using counterflow nozzle
    (2020-08) Rangarajan, Roshan
    In the present work, we study the enhanced atomization of viscous liquids by using a novel twin-fluid atomizer. A two-phase mixing region is developed within the nozzle using counterflow configuration by supplying air and liquid streams in opposite directions. Detailed qualitative and quantitative measurements for droplet size were conducted using shadowgraph technique. Near-field spray images from the nozzle exit suggest that the spray emerges out as a fine droplets with little scope for further atomization. The performance of this nozzle is compared to that of ‘flow-blurring’ nozzle. Three test liquids (Water, Propylene Glycol & Glycerol 85% soln) are used to vary the liquid viscosity in the range from 1 to 133.5 mPa.s. The counterflow nozzle produces a spray whose characteristics are relatively insensitive to fluid viscosity over the range of gas-liquid mass flow ratios between 0.25 and 1.
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    Methods for the Modeling and Simulation of Sprays and Other Interfacial Flows
    (2019-09) Wenzel, Everett
    Interfacial multiphase flows involve the motion of at least two fluids separated by surface tension. Atomizing interfacial flows, colloquially known as sprays, are among the most important fluid dynamic systems because of their ubiquity; power generation, delivery of aerosolized medicines, and productive produce farming all depend fundamentally on the detailed control of sprays. Atomization remains poorly understood because of a historical and persisting inability to accurately and affordably measure the dynamics inside and near the spray orifice outlet -- it is therefore desirable to be able to numerically simulate sprays with high fidelity. This dissertation presents computational methods that aim to improve current shortcomings in the modeling and simulation of sprays. Accurately characterizing the interfacial curvature of poorly-resolved liquid structures is addressed by deriving a series of finite particle methods for computing curvature. The methods are verified in analytical curvature tests, and validated against the oscillation frequency of ethanol droplets in air. The finite particle method, leveraging dynamic length scale modification, is demonstrated to out-perform the widely-used height function approach. Tracking the location of interfaces is also addressed, for which a coupled Eulerian-Lagrangian point mass particle scheme is introduced that preserves a well-distributed particle field, can be applied to an arbitrary number of fluids, and does not limit the simulation time step. The Eulerian-Lagrangian method is demonstrated to out-perform contemporary geometric volume of fluid methods at resolutions relevant to spray simulation in a variety of analytical phase tracking tests, and is dynamically evaluated by simulating extending three-phase elliptical regions, droplet dynamics, and Rayleigh-Taylor instabilities. The Eulerian-Lagrangian method is then extended to an approach for consistently and conservatively solving multiphase convection-diffusion problems -- this extension is verified via two analytical heat transfer problems, and robustness is demonstrated by simulating heated air blast atomization. Each of these tests conserves thermal energy and preserves boundedness of the temperature field. This dissertation concludes by outlining paths for consistently and conservatively solving the multiphase Navier-Stokes equations and the multiphase large eddy simulation equations in the coupled Eulerian-Lagrangian point mass particle framework.