University of Illinois Urbana-Champaign

Flow physics and air interactions of magnetically driven plasma discharges

Hristov, Georgi K.

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HRISTOV-DISSERTATION-2022.pdf

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https://hdl.handle.net/2142/116167

Description

Title
Flow physics and air interactions of magnetically driven plasma discharges
Author(s)
Hristov, Georgi K.
Issue Date
2022-07-01
Director of Research (if dissertation) or Advisor (if thesis)
Ansell, Phillip J.
Doctoral Committee Chair(s)
Ansell, Phillip J.
Committee Member(s)
  • Chamorro, Leonardo P.
  • Dutton, J. Craig
  • Elliott, Gregory S.
  • Zimmerman, Joseph W.
Department of Study
Aerospace Engineering
Discipline
Aerospace Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
  • flow control
  • plasma flow control
  • vortex generator
  • vorticity generation
  • magnetically driven discharges
  • plasma
  • axisymmetric jet
  • swirling jet
  • magnetohydrodynamics
  • MHD
  • EHD
  • electrohydrodynamics
  • arc
Abstract
Motivated by the use of magnetohydrodynamic devices in fluid dynamic flow control and plasma assisted combustion research, the current work experimentally studied the physics and the aerodynamic interactions of magnetically driven low-current arc-plasma discharges. The plasma characterization was performed using nonintrusive optical techniques, high-speed imaging, emission spectroscopy, electric circuit measurements, and simulations. The plasma took the shape of a thin filament under atmospheric pressure and a diffuse glow under low pressure. Its spectrum was characteristic of glow discharges with the main emission peaks resulting from molecular and ionized nitrogen. Alternating current waveform was provided to sustain the plasma. Breakdown was observed only during the positive half-cycle of the excitation which caused a steady unidirectional Lorentz force and discharge motion. The resulting electric field was inferred empirically and was simulated through finite element methods. The results demonstrated that the electrode shape directly influenced the strength and the spatial distribution of the electric field. Several device geometries were developed for aerodynamic characterization in different flow regimes. Firstly, the interactions of the coaxial magnetically driven discharge in quiescent air were studied, to characterize the actuator-induced flow. Schlieren visualization was used to reveal the thermal effects of the plasma on the surrounding air. The induced flow velocity close to the actuator face was directly related to the influence of the moving discharge, whereas the flow farther away from the actuator face was dominated by thermal instabilities. Stereo-PIV data phase-locked to the discharge rotation showed an upward flow due to the rapid expansion of the fluid local to the plasma discharge, and the subsequent relaxation and entrainment of the flow back towards the actuator after the plasma discharge had passed. A toroidal region of vorticity with two swirling components was identified in the three-dimensional reconstruction of the flowfield. Due to the combination of these two vortical motions, the three-dimensional streamlines of the flow traced a helical path around the annular region. A model based on electromagnetic field interactions with the air and the resulting momentum transfer through intermolecular collisions was suggested to explain the plasma-induced flow. A magnetic Lorenz force contribution dominant during the positive half cycle of the excitation caused a flow in the direction of the discharge motion. Additionally, a second flow motion followed the curvature of the electric field lines, whose contribution dominated during the negative half cycle of the voltage waveform. This model further served to understand and describe the flow interactions of the different geometries in the context of externally imposed canonical flow regimes. All fundamental aerodynamic interactions were considered by studying flows perpendicular and parallel to the plane of discharge motion. First, the discharge interactions with an axisymmetric jet of air were characterized. In this case, the magnetic Lorentz force component had a dominant influence. As a result of the actuation, the jet was swirled, and its turbulence levels were increased significantly. Additionally, the boundary layer interactions of magnetically driven discharges were studied in low-speed crossflow conditions through the use of a coaxial and a v-shaped geometries. In this case, the electric field contribution was more significant than the magnetic Lorentz force component. The resulting flow had s-shaped streamwise velocity profiles as measured in the wall-normal direction characteristic for the flow downstream of a conventional vortex generator pair, due to the three-dimensional mixing induced by coherent vortex structures.
Graduation Semester
2022-08
Type of Resource
Thesis
Handle URL
https://hdl.handle.net/2142/116167
Copyright and License Information
Copyright 2022 Georgi Hristov

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