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Channel Cooling by Turbulent Convective Mixing

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Memorandum rept.

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Results from a series of experiments are described which show that hot, reduced-density channels in the atmosphere usually cool by a process of turbulent convective mixing. Five different types of channels were created a by the interaction of a pulsed CO2 laser with aerosols in the atmosphere, b by electric discharges in the atmosphere, c by laser-guided, electric discharges in the atmosphere, and d and e by the absorption of CO2 laser radiation in nitrogen doped with sulfur hexafluoride. For channels in which the energy deposition was almost cylindrically symmetric and axially uniform, e, the rate of cooling, after reaching pressure equilibrium, was within an order of magnitude of thermal conduction. But for channels in which the energy deposition was asymmetric andor non-uniform, the rate of cooling was typically one thousand times faster than thermal conduction for channels whose radius at pressure equilibrium was approx. 1 cm. These channels were seen to be turbulent and to cool by mixing cold, surrounding air into the hot channel. Such turbulence has been explained by Picone and Boris in terms of a residual vorticity that is caused by the non-cylindrical energy deposition. A simple empirical formula is deduced relating the rate of cooling growth of channel envelope to the radius of the channel at pressure equilibrium and to the ambient sound speed, which indicates that the effect of velocityturbulence saturates for variations in the energy deposition of greater than about 2 to 1.

Subject Categories:

  • Atmospheric Physics
  • Plasma Physics and Magnetohydrodynamics

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