Aeroelasticity and Structural Dynamics
Resistance of tanks
to the hydraulic ram generated on impact and penetration by a projectile at high speed
The objective of the work is to enable the industrial company to limit the vulnerability of aircraft tanks to impacts of projectiles that may be ballistic. In practice, under certain conditions, a projectile may cause catastrophic damage in a tank: the "hydraulic ram".
After having passed through the wall to enter the tank, the projectile finds in its path a space more or less full of liquid and internal walls. It is slowed, or even stopped, by the fluid which opposes it with a drag force that is a function of the kinematics, geometry and characteristics of the projectile and the fluid. In this process, the kinetic energy of the ammunition is transferred to the liquid near the path. Either the tank is partially filled with air near the path and the fluid, displaced by the projectile, naturally finds its place there, or it is not and the structure must then adapt to the hydrodynamic loading. This is the hydraulic ram phenomenon. Concretely, the more the projectile is suddenly stopped and near a wall, the more likely the structure is to distort locally, up to possible rupture of the tank by cascading cracks.
The CRD unit's experimental contribution in this activity concerns the implementation of shots campaigns (in pool or on different types of tanks representing aircraft wings). A specific test rig was designed and developed for such experiments on structures (figure 1). The objectives of the dynamic characterization studies were to provide the input data necessary for the numerical simulations: dynamic properties of materials and assemblies and, in this instance, initial angle and speed of the projectile, level of the dynamic pressure in the tank, speed on exit from the tank, deformation fields of the structures, etc. The multiplicity of the dynamic measurements enabled the CRD Unit to evaluate the pertinence of new models for simulating the complex and strongly non-linear phenomena brought into play ("multi-physics" problems, with particularly in this case fluid/structure interactions).
Fig.1 - Equipment around the firing test rig
The particularly short duration of the hydrodynamic phenomena studied (a few ms) naturally increases the difficulty of experimental observation.
Fig.2 - Pressure field in the fluid environment (SPH +/- LAG numerical simulation)
Video views (Dassault aviation tank)
Figure 2, representing the propagation of a pressure field in a FE fluid model under the effect of a theoretical projectile (first image), and a few images of a real test (below), gives an idea of the state of the art in the field of modeling and the complexity of the real phenomena that are still to be understood.
