Analysis of the effects of droplets collisions on thestability of a pure water jet by means of 3D numericalsimulations

Water jet cutting has always been a promising technology because of its extreme simplicity and flexibility, even if it often suffers a lack of control on its process parameters, especially if compared to technologies such as laser cutting or electrodischarge machining. In order to gain competitiveness and satisfy quality requirements, water jet cutting is in need of a more systematic insight aiming to achieve awareness on the physics and the main causes of the disturbances which systematically affect the jet: these disturbances can be neglected in industrial applications, but can play a relevant role in case of high-precision water jet machining. As a matter of fact, it is possible to notice naked-eye that sometimes the jet pulsates losing its coherence for short whiles, spreading wider and then coming back to a stable condition: this is a random phenomenon which happens with no regular frequency and whose causes are basically still unknown. Previous numerical simulations identified a possible cause to this problem with the collision of some water droplets with the main jet, even if the former simulations were 2D axis-symmetrical models based on strong physical assumptions and numerical simplifications in order to reduce the overall computational effort required and speed up convergence. The aim of the present work is to set up improved 3D numerical models in order to solve the previously mentioned limits and improve the reliability and the precision of the analysis.