Formation of undulatory cavities during hydroelastic water entry

When a rigid body enters the water with sufficient speed, it forms a trailing subsurface air cavity that typically features smooth walls until it eventually collapses. When the impactor is flexible, however, the interplay between the hydrodynamic forcing and elastic deformation can produce nested undulations in the cavity walls. These so-called "matryoshka cavities" have been observed previously in experiments with free-surface impacts of droplet streams and elastic spheres. In our experiments, we explore a much simpler hydroelastic problem in which a flexible 2 degree-of-freedom system enters a quiescent water bath with normal incidence. An axisymmetric nose is connected to the impactor body via a compliant mechanism acting as a linear spring. For some impact parameters, we observe a similar phenomenon in which the excited structural vibrations of the impactor lead to pronounced undulations in the cavity walls that persist for several nose oscillations. We present oscillatory cavities at a range of impact speeds and impactor stiffness values and explore their implications on the coupling between the hydrodynamics and impactor elastic response: interestingly, regardless of the stiffness, oscillatory cavities are only observed when the nose mass is sufficiently small compared to the characteristic added mass of the surrounding fluid. Additionally, we measure the force of impact and make a related observation that the cases with the smallest nose mass experience the lowest impact forces.