Droplet pair interactions in a shock-wave flow field

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Abstract
Binary interactions between water droplets of nearly equal size in the flow field behind a weak shock wave were studied experimentally. The droplets had diameters of about 270 mm, and the Reynolds numbers, based on this diameter and on the relative velocity between the droplets and the free stream, ranged from about 130 to about 600. In this paper we report only data for Re < 150, corresponding to non-deforming droplets. The droplets in a given pair were aligned so that each pair fell on a plane parallel to the direction of the incoming flow. In this manner, the second droplet in the pair was ‘behind’ the first, at horizontal distances ranging from 1.5 to 11 diameters, and at vertical distances from the dividing streamline ranging from −3 to 6 diameters. We have quantified the interaction in terms of drag force changes on the droplets, and show that the first, or upstream, droplet is not affected by the second, but that the second experiences significant reductions for vertical distances of about one droplet diameter or less. At the smallest horizontal distances, the maximum decrease observed was about 50%, relative to its isolated value. We also show that the drag changes clearly demarcate a wake behind the first droplet. Further, on the basis of these changes, we define a region of influence attached to the first droplet, where the free-stream velocity is significantly reduced. For the droplets used in this study, this region is a slender paraboloid of revolution, having a length of about 15 diameters and a radius of about one diameter.