A Novel Integrated Framework to Study Buoyant Turbulent Plumes Released into Atmosphere

Date
2020
Authors
Bhimireddy, Sudheer Reddy
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Abstract

Turbulent Buoyant plumes are common in the environment and can occur by accidental, natural, or man-made releases, for example, wildfire emissions, hot gases from volcanic eruptions, overflows from avalanches, and ocean mixing. These plumes are maintained by pure buoyancy forcing, created by either temperature difference in case of wildfires, or buoyancy difference in case of ocean mixing. This thesis includes understanding the community Weather Research and Forecast (WRF) model at different scales to simulate atmospheric conditions, performance assessment of the model at Large-Eddy Simulation (LES) scales by comparing against field data and finally implementing a new buoyant flow transport in the model to release gas mixtures at different densities.

A well-resolved LES has been conducted for the first time to simulate realistic geophysical plumes, which include thermal plumes arising due to surface heating and buoyant plumes representing the toxic emissions from large wildfires. For thermal plumes studied, fundamental flow dynamics investigated near to the source revealed characteristic feature of spiraling columnar neck region that feeds into the active and unsteady head region which entrains ambient air through the instabilities occurring at the interface with the ambience.

For Ammonia plumes released into unstratified ambience with no winds, unstable temperature boundary layer with no winds, and convective atmospheric boundary layer with different geostrophic and surface heating forcing, the plume rise followed a 3/4 power law with time. Plumes with strong source buoyancy flux penetrated the boundary layer top and the amount of plume penetrated scales with the mean plume rise reached. Buoyant plumes are observed to spread radially after they rise to the boundary layer top. This radial spread distance from source scales linear with time, resembling the propagation of a density current front in the slumping phase. For equal source buoyancy fluxes, an increase in geostrophic forcing resulted in uniform mixing of plume within the boundary layer. Overall mixing of the plume inside boundary layer scales with convective time scale based on the convective velocity and boundary layer depth.

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Keywords
Large-Eddy simulation, Transport and Dispersion, Turbulent Buoyant plumes, Weather Research and Forecast model
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Department
Mechanical Engineering