Experimental investigation of the kinematics of post-impact ice fragments

Hail is more hazardous for aircraft engines compared to rain and snow, mainly, because of its solid nature and high water content. In extreme cases it can lead to engine flame out. In order to avoid such situations, aero engines should be designed to withstand hail ingestion. For this purpose we have studied the post-impact characteristics of ice, such as particle velocity and directions of travel. To achieve this goal, a large experimental program has been undertaken, in which spherical ice specimens were projected against a rigid plate. Three specimen diameters (6.2, 12.9 and 27.5 mm) and four impact angles (20, 45, 67.2 and 90°) were considered, as well as a wide range of impact velocities (60–200 m/s). From this experimental work, we can conclude that the ice fragments formed after impact do not bounce back and that the post-impact ice trajectory angle is lower than 2°. This is in line with observations found in the literature. On the other hand, the ice fragments are mainly organised in a circular cloud, when observed in the target plane. The center of this cloud has the same velocity as the initial ice ball tangential impact velocity. Furthermore, the cloud radius expands with a rate proportional to the ice ball normal impact velocity. Finally, each fragment inside the cloud has a relative velocity which varies linearly with its distance from the cloud center. These experimental observations should be very helpful in developing models and simulations of hail ingestion by aircraft engines.