A Determination of the Terminal Velocity and Drag of Small Water Drops by Means of a Wind Tunnel

K. V. Beard Dept. of Meteorology, University of California, Los Angeles

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H. R. Pruppacher Dept. of Meteorology, University of California, Los Angeles

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Abstract

Measurements of the drag on small water drops falling in water-saturated air at terminal velocity were carried out in a wind tunnel for Reynolds numbers R between 0.2 and 200. The fractional deviation (D/Ds) − 1 of the actual drag D from the Stokes drag Ds was determined as a function of R and empirical formulae for (D/Ds) − 1 were derived for the three ranges 0.2≤R≤2,2≤R≤21 and 21≤R≤200. From these relations drag coefficients were computed and the terminal velocity of water drops of radii between 10 and 475 μ calculated for drops falling in water-saturated air and at pressure levels of 400, 500, 700 and 1013 mb, where the temperature was assumed to be −16, −8, 14 and 20C, respectively.

It is shown, for 0.2≤R≤200, that the values derived for the drag on water drops are in good agreement with those for the drag on solid spheres experimentally determined by Pruppacher and Steinberger, and with those for the drag on solid spheres theoretically computed by Rimon. It is pointed out that there is a strong scatter among the values for the terminal velocity of water drops given in literature. Out data agreed quite closely with those of Gunn and Kinzer.

Abstract

Measurements of the drag on small water drops falling in water-saturated air at terminal velocity were carried out in a wind tunnel for Reynolds numbers R between 0.2 and 200. The fractional deviation (D/Ds) − 1 of the actual drag D from the Stokes drag Ds was determined as a function of R and empirical formulae for (D/Ds) − 1 were derived for the three ranges 0.2≤R≤2,2≤R≤21 and 21≤R≤200. From these relations drag coefficients were computed and the terminal velocity of water drops of radii between 10 and 475 μ calculated for drops falling in water-saturated air and at pressure levels of 400, 500, 700 and 1013 mb, where the temperature was assumed to be −16, −8, 14 and 20C, respectively.

It is shown, for 0.2≤R≤200, that the values derived for the drag on water drops are in good agreement with those for the drag on solid spheres experimentally determined by Pruppacher and Steinberger, and with those for the drag on solid spheres theoretically computed by Rimon. It is pointed out that there is a strong scatter among the values for the terminal velocity of water drops given in literature. Out data agreed quite closely with those of Gunn and Kinzer.

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