Laboratory Measurements of Small Raindrop Distortion. Part I: Axis Ratios and Fall Behavior

Abstract

The resonant interactions between eddy shedding and drop oscillations postulated by Gunn for millimeter diameter raindrops were investigated in a series of laboratory measurements of axis ratio and fall behavior for water drops of d = 0.70?1.54 mm. Drops were produced at terminal velocity using a orifice-jet drop generator and allowed to fall several meters so that the initial oscillations (produced during jet break up) decayed to a negligible amplitude before the drop shape was recorded using stroboscopic photographs. The measured axis ratios had equilibrium values for the smallest sizes (d = 0.70?0.96 mm), but scattered somewhat above equilibrium at intermediate sizes (d = 1.04?1.29 mm). An order of magnitude larger scatter in axis ratio, extending above and below equilibrium, was found for the largest sizes (d = 1.40?1.54 mm) with the average axis ratio being larger than equilibrium. All but the smallest drops displayed a sideways drift varying from 1% of the terminal velocity for d = 0.82?1.04 mm, increasing sharply to 6% for d = 1.11 mm and gradually lowering to 2% at d = 1.54 mm. The observed axis-ratio scatter and fall behavior were generally consistent with simple oscillation and drift responses to various size-dependent wake configurations as determined from observations of freely falling bodies in liquid tank experiments. The small axis-ratio scatter above equilibrium and maximum in drift corresponds to the onset of an asymmetric pulsating wake found by Magarvey and Bishop, whereas the large axis-ratio scatter above and below equilibrium is an apparent axisymmetric oscillation. Since the measured variance in axis ratio is largest for the largest size investigated, resonant oscillations extend far beyond the 1 mm size postulated by Gunn. Although such a broad coupling between eddy shedding and drop oscillations is not well understood, its existence would explain the shift in axis ratio found for small raindrop sizes using an aircraft optical-array probe by Chandrasekar, Cooper and Bringi, and postulated from distrometer-ZDR observations by Goddard and Cherry.