Experimental characterization of water droplet deformation and breakup in the vicinity of a moving airfoil

An experimental study is presented on the deformation and breakup of water droplets in the vicinity of an incoming airfoil. The experimental campaign was carried out at the INTA rotating arm facility in the context of the INTA-NASA collaboration. In this experimental setup, the flow past the droplets accelerates (in the reference frame of the droplets) which is a situation similar to the one found in actual aircraft flight. This is in contrast to most of the previous more fundamental experimental studies in which the flow past the droplet has a constant velocity. During the tests, the velocity of the incoming airfoils ranged between 50 m/s and 90 m/s. Three different symmetric airfoil models were tested. They all had a spanwise length of 0.2 m. The chord lengths were 0.690 m, 0.468 m and 0.199 m respectively. The leading edge radii of curvature were 0.103 m, 0.070 m, and 0.030 m respectively. The undisturbed droplet diameters were in the range from 364 mu m to 1075 mu m. The base flow field was characterized using a Particle Image Velocimetry technique. The droplets were tracked using a high speed imaging system. The rate of deformation of the droplets was measured and the deformation and breakup processes were recorded and correlated to the parameters that defined the base flow. It was found that, within the range of tested experimental conditions, the favored breakup mechanism was of the so-called "bag and stamen" type. However, flow acceleration tended to anticipate significantly the onset of this breakup mechanism as compared to previous experimental studies in which flow velocity was constant. This is important, for instance, for researchers that develop theoretical and numerical models of droplet-gas interaction for aircraft simulation purposes. (C) 2015 Elsevier Masson SAS. All rights reserved.