Numerical Modelling of the Fluid Structure Interaction using ALE and SPH: The Hydrodynamic Ram Phenomenon

Vulnerability against high-velocity impact loads is a critical issue for the design of aerospace structures due to the fact that aircrafts can be subjected to different types of loads during their service life which may cause a catastrophic failure. Bird strikes, hailstones, runway debris or even the ice released from the edge of a propeller blade may impact different parts of the fuselage. Wings represent the largest exposed area to impact threads of all the vulnerable components of an aircraft, therefore impacts onto a fuel tank inside the wings are considered of special relevance in aircraft vulnerability. Hydrodynamic Ram (HRAM) is a phenomenon that occurs when a high-energy object penetrates a fluid-filled container. The projectile transfers its momentum and kinetic energy through the fluid to the surrounding structure increasing the risk of excessive structural damage leading to a catastrophic failure. HRAM consists of four principal stages: shock, drag, cavitation and exit. Each stage contributes to structural damage in a different way and extent. The study of the HRAM phenomenon is not only important for the aircraft industry. High velocity impacts on fluid filled containers are of great interest for different industrial fields such as safety of industrial facilities or road haulage. In those cases, an impact in the vessel may produce the failure of the tank and serious consequences on the environment or even toxic and flammability effects. This work shows the numerical modelling developed to reproduce the effects of the HRAM phenomenon in different fluid filled square tubes (aluminum and CFRP) when impacted by steel spherical projectiles at different velocities and filling levels. The simulations are performed with the finite element code LS-Dyna using two different techniques for the fluid phase: the ALE and SPH formulations. Experimental tests providing the pressure in different points of the fluid, deformation of the walls and cavity evolution are compared with the numerical results in order to assess the validity and accuracy of both ALE and SPH techniques in reproducing such a complex phenomenon. In addition, the numerical results revealed that ALE is the most appropriate technique to simulate HRAM attending to its computational cost. The numerical model validated has contributed to a better understanding of the phenomenon as well as to study some possibilities to attenuate the effects of the HRAM on the affected structures. With the aim of reducing both the pressure wave generated by the impact and the cavity growing, thin sandwich structures with two metallic skins and a core of air have been located in different positions inside the fluid filled tube. The results show improvements regarding the vulnerability of the fluid filled impacted tubes.