Experiments on the Dynamics of Flexible Cylindrical Shells Impacting on a Water Surface

This work presents a comprehensive set of experimental results on the water entry of compliant cylindrical shells. Free fall experiments are conducted on a flexible thin cylinder varying the drop height. The problem studied here is not representative of a free cylinder as this is hold by a sledge, which acts as a concentrated mass. The impact dynamics is analyzed from accelerometers, linear position sensors, and through the analysis of high speed images. Further, an experimental methodology based on the modal decomposition method is developed and utilized to reconstruct the overall structural deformation and the distributed strain field on the base of local strain measurements. Fiber Bragg gratings are utilized for this purpose. Results show that the flexibility of the structure plays an important role on the impact dynamics, which is found to completely differ from the impact of rigid structures. The overall deformation of the shell follows the first mode shape of vibration of a free ring, while the stresses are influenced by the superposition of the higher mode shapes that are excited during the impact.