ID 412

Spatial-temporal characterization of ligaments in drop breakup using high-speed digital in-line holography

 

Abstract:

Ligaments are important structures in drop breakup whose three-dimensional (3D) morphologies are difficult to measure. Compared to the small fragments from earlier atomization, they usually contain higher ratio of the initial volume and will finally disintegrate into relatively larger fragments. High-speed digital in-line holography is applied to characterize the ligaments and fragments together in the breakup of a water drop exposed to the compressed gas stream. In our newly developed method, the 2D skeleton and edge of the ligament is first extracted in a depth-of-field extended image from reconstructed hologram at different depth distances. Then the local thicknesses and 3D positions of segmented sections are determined by applying the automatic locating algorithm. The entire ligament is retrieved and visualized in 3D after stitching the local sections together. As a development and complement of our recently proposed method that only deals with toroidal rim, this method can be used to study complicated multi-branch ligaments. Thus it is capable of characterizing the multi-mode break. Holograms are recorded at 16 kHz and the spatial-time evolution of ligaments in 1.94 milliseconds are quantified in a typical multi-mode breakup case. 3D position and morphology of the ligaments and secondary droplets are visualized and compared with the time. The stamen surrounded by secondary droplets and a rim node possibly caused by gravity are connected by the bag residues to form a multi-branch ligament. Results show that the ligaments are elongated and crack into relatively large droplets. The volume fraction (normalized by the initial drop volume) of ligaments decreases gradually. Volume fluctuation caused by measurement uncertainty is relatively large and is supposed to be addressed in the future work. The volume fraction of the fragments decreases gradually most of the time but sometimes increases rapidly. Both the 3D visualization and quantification show good accordance to observations.