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Droplets in a continuously accelerated flow field, under certain conditions, deform as oblate spheroids and breakup. This type of flow field differs from the traditional shock tube experiments where droplets are suddenly exposed to a constant high velocity airstream. Bag, bag and stamen and shear breakup were encountered in previous works for water droplets under these conditions. The aim of this investigation is the visualization of the breakup modes of ethyl alcohol droplets in a continuously accelerated flow field. In order to generate this continuously accelerating flow field allowing for the illumination, the rotating arm facility at INTA is used. In this facility, droplets were allowed to fall in the path of an incoming airfoil of a chord of 1050 mm mounted at the end of a rotating arm and moving at velocities up to 60m/s. A high-speed camera recording images at 40 000 fps and shadowgraph illumination technique was first employed. Direct illumination technique and a high-resolution camera will also be used in this investigation. Under certain conditions, these droplets deform and breakup before impinging on the airfoil. The influence of the droplet diameter (between 0.5 mm and 1.5 mm) and the airfoil velocity (30-60 m/s) were investigated. In particular, a new type of breakup has been encountered for ethyl alcohol droplets of 1mm of diameter. Figure 1 shows the evolution of the droplet breakup as the air velocity is increasing. The air moves from the right to the left. After the deformations as an oblate spheroid, a bulge appears in the front part as in bag and stamen and it resemble the shape of a hat. However, then, instead of forming the bag, all the rear part of the droplet seems to collapse inside the droplet and a stamen is formed in the rear part of the droplets that grows a high quantity. At the same time, the droplet front part layer seems to shear back. The present work provides not only experimental data on the features during the breakup process, but additionally it provides insight in the underlying physics of the problem.
