Numerical simulation of the accumulation process in modulated liquid jets of defined volume using the volume-of-fluid methodology

Axial modulation of liquid jets leads to the well-known jet 'bunching'effect. The associated accumulation of liquid in certain parts of the jet is advantageous, as it can increase the mechanical effect of the jet when it hits a surface. This advantage is known from empirical studies on cleaning with pulsating jets as well as from biological studies on the hunting method of archerfish. So far, however, there have been no systematic studies on modulating jets to enhance their impact on a solid surface. In this paper the VOF methodology is used to simulate jets with an axial velocity modulation. The modulation aims to accumulate the ejected liquid in a single droplet at the tip of the jet. This accumulation process has a decisive influence on the jet impact, as it changes the distribution of pressure and wall shear stresses in the impact area, which are difficult to determine experimentally. The validity of the numerical model is ensured by benchmark experiments in which both the drop shape and the impact force are used as comparison criteria. For this purpose, the velocity at the nozzle outlet was measured with high temporal resolution in the experiments, as it defines the inflow boundary conditions for the simulations. The results lead to a better understanding of the basic accumulation process. Furthermore, the improvement of the impact behaviour by jet modulation can be estimated.