Droplet drag in an accelerating and decelerating flow

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Abstract
An experimental study of the motion of small water droplets in both accelerating and decelerating conditions is presented. Droplets with diameters in the range 115-187μm were exposed to propagating N-waves having strengths smaller than 0.03. Droplet-displacement data were obtained by single-frame stroboscopic photography, at an equivalent framing rate of 4000 pictures per second. The data were fitted by means of best-fit polynomials in time, which were used to obtain drag coefficients in accelerating and decelerating flow conditions. In addition to providing drag data for impulsive-type motions, these data show that the unsteady drag follows two entirely distinct trends. In one, applicable to decelerating relative flows, the unsteady drag is always larger than the steady drag at the same Reynolds number. In the other, applicable to accelerating relative flows, the unsteady drag is always smaller than the corresponding steady value. These trends have not been previously known. They give some support to a mechanism recently proposed (see Temkin & Kim 1980) to explain departures of the drag coefficient for a sphere from its steady value; namely, the changes in size of the recirculating region behind the sphere, relative to its steady counterpart at the same Reynolds number.

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