The manipulation of jets has since long been subject to research, due to the wide range of industrial applications in which they are used. A vast number of numerical and experimental studies concerning the physics of the breakup process of continuous jets have been published. Improvements in mixing and ambient gas entrainment have been reported experimentally when using intermittent injection, although the responsible mechanisms have not yet been completely revealed. This work presents a systematic analysis of the mechanisms of jet breakup and mixing with the surrounding fluid and its relation to vorticity generation and transport. Comparisons aremade between the redistribution of vorticity and the engulfment of ambient fluid into the core region for different injection strategies. 

It has been observed that intermittent injection leads to improved spray characteristics in terms of mixing and gas entrainment. Although some experimental work has been carried out in the past, the disintegration mechanisms that govern the breakup of intermittent jets remain unknown. In this paper we have carried out a systematic numerical analysis of the breakup of pulsated jets under different injection conditions. More specifically, the duty cycle (share of active injection during one cycle) is varied, while the total cycle time is kept constant. The advection of the liquid phase is handled through the Volume of Fluid approach and, in order to provide an accurate, yet computationally acceptable, resolution of the turbulent structures, the implicit Large Eddy Simulation has been adopted. The results show that the primary disintegration results from a combination of stretching, collision and aerodynamic interaction effects. Moreover, there exists a strong coupling between stretching and collision as stretching makes the pulse thinner prior to the contact between pulses. In this work, the purpose is to study the collision contribution to breakup in terms of the near nozzle pulse disintegration rate. When approaching the low duty cycle limit, this effect is significant because of the lower liquid volume of the pulse. In contrast, for a high duty cycle, the stretching effect is limited and a wide tail region remains as an obstruction for following pulses. However, the integral momentum of the pulse is maintained to a larger degree that has an adverse effect on the outcome of the collision event.

Turbulent jet flows are very common in engineering applications. One example is that of fuel injection in internal combustion engines, which is closely related to the combustion process. Because of the widespread use, the resulting emissions of such engines have a significant impact on human health and the environment. For a long time, research has sought to improve the mixing in developing turbulent jets to reduce the level of pollutants. Findings have indicated that injection unsteadiness can be used to improve the spray quality. Furthermore, it has been demonstrated that important spray characteristics can be linked to physical phenomena occurring in the region close to the nozzle. In this work, the breakup of an intermittently injected jet is investigated using numerical simulations. Cases of both single-phase and two-phase conditions are studied, characterizing the pulse breakup for different injection timing and varying fluid properties. For single-phase pulsation, mixing efficiency is shown to be connected to the generation of different secondary flow structures and their interaction. The breaking of symmetry along the pulse, responsible for the increased the mixing, is explained through a consideration of vorticity transport. This sequence shows local mixing is faster in the trailing region of pulses that are long enough to form secondary vorticies in the corresponding region. The study is extended to include effects of acceleration and deceleration during injection. The mixing rate depends on the accumulation of jet fluid within the generated flow structures. A rapid injection increase or decrease is found to promote the jet mixing and spreading by triggering jet fluid shedding and destabilization of such flow structures closer to the nozzle. Slow velocity changes promote separation of the injected fluid which instead suppresses near-nozzle mixing. Simulations of intermittent injection of liquid into quiescent gas have also been performed. Primary breakup of liquid pulses is assessed by considering the increase of the liquid-to-gas interface area and volumetric decrease over time. The disintegration process for these cases are less sensitive to the surrounding gas flow because of the higher jet inertia. Increased injection frequency and lower injection to non-injection ratio, is observed to stimulate primary breakup. This is due partly to a stretching action near the nozzle, and partly to a stronger relative influence of collision between liquid pulses.

Combustion efficiency and the formation of soot and/or NOx in Internal- Combustion engines depends strongly on the local air/fuel mixture, the local flow conditions and temperature. Modern diesel engines employ high injection pressure for improved atomization, but mixing is controlled largely by the flow in the cylinder. By injecting the fuel in pulses one can gain control over the atomization, evaporation and the mixing of the gaseous fuel. We show that the pulsatile injection of fuel enhances fuel break-up and the entrainment of ambient air into the fuel stream. The entrainment level depends on fuel property, such as fuel/air viscosity and density ratio, fuel surface-tension, injection speed and injection sequencing. Examples of enhanced break-up and mixing are given.

The paper considers the effects of intermittent injection of a fuel spray on the initial break-up and mixing of the fuel with the surrounding "ambient" fluid. The two fluids may have the same phase or different phases. The aim of the analysis is to describe the physical process and indicate the mechanisms that control the mixing under different flow conditions (time-dependent injection and its frequency relative to the time scales of the flow) and fluid properties (Schmidt number in the single phase case or the Weber number for liquid fuel case. The computations use Large Eddy Simulation (LES) for accounting for turbulence and either Volume Of Fluid (VOF) for the initial break-up and Lagrangian Particle Tracking (LPT) with droplet break-up model in the case of liquid droplets injected into the ambient gas. The results show that depending on the physical properties of the fuel and ambient gas, the initial break-up and turbulent mixing can be enhanced substantially with intermittent injection. Most of the mixing is driven by the suction of ambient fluid at the tail of the injected fuel (as was shown experimentally and in a one dimensional model by Musculus and co-workers).