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National Advisory Committee for Aeronautics, Report - Impingement of Cloud Droplets on a Cylinder and Procedure for Measuring Liquid Water Content and Droplet Sized in Supercooled Clouds by Rotating Multicylinder

Evaluation of the rotating multicytinder method for the
measurement of droplet-size distribution, volume-median droplet
size, and liquid-water content in clouds showed that small un-
certainties in the basic data eliminate the distinction between
difi'erent cloud droplet-size distributions and are a source of
large errors in the determination of the droplet size. Calcula-
tions of the trajectories of cloud droplets in incompressible and
compressible flow fields around a cylinder were performed on a
mechanical analog constructed for the study of the trajectories of
droplets around aerodynamic bodies. Many data points were
carefully calculated in order to determine precisely the rate of
droplet impingement on the surface of aright circular cylinder.
From the computed droplet trajectories, the following impinge-
ment characteristics of the cylinder surface were obtained and
are presented in terms of dimensionless parameters: (1) total
rate of water impingement, (2) extent of droplet impingement
zone, and (3) local distribution of impinging water on cylinder
surface.

The rotating multicylinder method for inrflight determination
of liquid-water content, droplet size, and droplet-size distribution
in icing clouds is described. The theory of operation, the
apparatus required, the technique of obtaining data in flight,
and detailed methods of calculating the results, including
necessary charts and tables, are presented. An evaluation of
the multicylinder method includes the eflect on final results of
droplets that do not freeze completely on the cylinders after
striking them, as well as probable errors infinal results caused
by the inherent insensitivity of the multicylinder method.

As part of a comprehensive research program directed
toward an appraisal of the problem of ice prevention on
aircraft, the NAOA has undertaken an investigation of the
impingement of water droplets on aerodynamic bodies.
Previous investigators have calculated the water-droplet
trajectories for right circular cylinders (refs. 1 to 5), for
bodies of revolution (refs. 6 to 8), for elbows (ref. 9), and for
airfoils (refs. 10 to 15).