Values of Diffusion Coefficients Deduced from the Closing Times of Helicopter-Produced Clearings in Fog

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  • a Air Force Geophysics Laboratory, Bedford, MA 01731
  • | b 29 Argilla Road, Andover, MA 01810
  • | c Combustion Engineering Co., Windsor, CT 06708
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Abstract

Values of diffusion coefficients determined from the observed closing times of nine conical-shaped clearings in fog produced by hovering helicopters at Lewisburg, WV, in September 1969 are presented. The values were established following the method of Elliott (1970) assuming that the geometric and diffusive properties of the clearings and surroundings could be approximated by theoretical equations of the type governing the diffusion of heat and water substance in a bounded, circular cylinder of infinite length, with appropriate specification of the condensation conditions. The coefficient values for the nine experiments were found to range from 0.7 × 105 to 1.9 × 105 cm2 s−1.

The physical, thermal and kinematic characteristics of the Lewisburg fog were investigated that might explain the variation of the coefficient values across this range. These efforts revealed that the values varied primarily, and inversely, with 1) the depth of the fog and 2) the Richardson number within the fog layer. Secondary correlations were also indicated which are noted.

The coefficient values determined by the described method are appreciably larger than those reported previously in the literature for radiation fog of the Lewisburg type. Possible reasons for this include the uncertainties of specifying the initial and “closed-in” conditions of the theory, the residual, turbulence effects of the helicopters, and the fact that the method emphasizes the horizontal components of diffusion, whereas previous methods emphasized the vertical component.

Theory predictions are illustrated which relate the initial temperature-humidity conditions of artificially produced clearings to the closing-in times of the clearings.

Abstract

Values of diffusion coefficients determined from the observed closing times of nine conical-shaped clearings in fog produced by hovering helicopters at Lewisburg, WV, in September 1969 are presented. The values were established following the method of Elliott (1970) assuming that the geometric and diffusive properties of the clearings and surroundings could be approximated by theoretical equations of the type governing the diffusion of heat and water substance in a bounded, circular cylinder of infinite length, with appropriate specification of the condensation conditions. The coefficient values for the nine experiments were found to range from 0.7 × 105 to 1.9 × 105 cm2 s−1.

The physical, thermal and kinematic characteristics of the Lewisburg fog were investigated that might explain the variation of the coefficient values across this range. These efforts revealed that the values varied primarily, and inversely, with 1) the depth of the fog and 2) the Richardson number within the fog layer. Secondary correlations were also indicated which are noted.

The coefficient values determined by the described method are appreciably larger than those reported previously in the literature for radiation fog of the Lewisburg type. Possible reasons for this include the uncertainties of specifying the initial and “closed-in” conditions of the theory, the residual, turbulence effects of the helicopters, and the fact that the method emphasizes the horizontal components of diffusion, whereas previous methods emphasized the vertical component.

Theory predictions are illustrated which relate the initial temperature-humidity conditions of artificially produced clearings to the closing-in times of the clearings.

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