An Impinging Jet Spray CFD simulation is a critical tool for engineers designing systems where a liquid must be broken down into millions of tiny droplets. This process, called atomization, happens when fast-moving jets of liquid either crash into a surface or into each other. The violent collision causes the liquid to shatter into a fine spray. This is essential in many technologies, from fuel injectors in car engines, where a Jet Spray fluent simulation helps design nozzles that mix fuel and air perfectly, to industrial cooling systems that use water spray to cool hot steel. A Jet Spray CFD study is also vital for designing fire sprinklers, agricultural sprayers, and even the nozzles used for high-quality paint finishing.

Simulating this process in ANSYS Fluent is very challenging because the physics changes completely from start to finish. It begins as a solid, continuous stream of liquid, but it ends as a cloud of separate, individual droplets. To solve this, engineers use a special hybrid technique called the VOF to DPM model. The simulation starts with the Volume of Fluid (VOF) model. The VOF model is an expert at tracking the surface of the continuous liquid jet, capturing exactly how the air tears at its surface and causes it to wobble and deform. As the jet breaks apart into large pieces, the VOF model follows them. However, it would be impossible to use VOF to track millions of tiny final droplets—the computer power needed would be too great. This is where the conversion happens. Once a piece of liquid becomes small enough, the VOF-TO-DPM Fluent model automatically changes it from a VOF liquid shape into a Discrete Phase Model (DPM) particle. The DPM model is very efficient at tracking thousands or millions of individual droplets as they fly through the air. This powerful VOF To DPM Ansys method allows a single simulation to capture the entire process: the smooth jet, the violent breakup, and the final dispersed spray cloud, giving engineers a complete picture of their spray system’s performance.

Figure 1: Experimental photograph showing a real-world fuel spray impingement event, which this Impinging Jet Spray CFD study aims to simulate.

Simulation process: A VOF-to-DPM Model with Adaptive Mesh Refinement in Fluent

The simulation process for this Impinging Jet Spray CFD study began with creating the geometry of two angled nozzles designed to make their liquid jets collide in mid-air. A large computational domain was built around them to ensure the spray could develop freely without being affected by the boundaries. The initial mesh was created using the powerful pre-processor ICEM CFD to generate a high-quality structured grid of hexahedral cells. This organized mesh provides a very good foundation for the simulation. The most important innovation in this setup, however, was the use of adaptive mesh refinement in ANSYS Fluent. This means the simulation starts with a relatively coarse mesh, but as it runs, Fluent automatically makes the mesh cells smaller only in the specific areas where the liquid jet surface is located. As the jets move and break up, this zone of fine mesh moves with them. This AMR technique is absolutely essential for VOF to DPM CFD simulations because it provides very high accuracy to capture the jet breakup, but only where it is needed, which saves a huge amount of computational time.

The ANSYS Fluent solver was set up to run a transient (time-dependent) simulation, because atomization is a very fast process that happens over time. The physics was modeled using the k-ω SST turbulence model combined with a Stress-Blended Eddy Simulation (SBES) submodel. This advanced hybrid approach allows the simulation to use efficient RANS calculations in simple flow regions and switch to a more accurate Large Eddy Simulation (LES) in the chaotic jet collision zone. This is critical for correctly predicting the turbulent eddies that are responsible for tearing the liquid jets apart. The multiphase physics was handled by the VOF to DPM model. First, the VOF model was used to track the large, continuous liquid jets as they left the nozzles. Then, based on specific criteria, Fluent automatically converted the small, broken-off pieces of liquid into DPM particles. The DPM was set up with two-way coupling, which is very important because it means the droplets are not just moved by the air, but the millions of droplets also push the air, changing its velocity and turbulence. Finally, unsteady particle tracking was enabled, ensuring that the droplets react realistically to the fast-changing turbulent air, leading to an accurate prediction of the final spray pattern.

Figure 2: Mesh from the Fluent simulation after adaptive mesh refinement has been applied. The contours show how the mesh cells have been automatically made smaller (refined) only in the areas where the liquid-gas interface exists, ensuring high accuracy with less computational cost

Post-processing: Analysis of Atomization

The contours generated by the ANSYS Fluent simulation provide a complete, time-resolved story of the impinging jet atomization process. By analyzing this data chronologically, we can forensically investigate the entire cascade of events, from the initial liquid jets to the final dispersed spray cloud, and understand the complex physics at each step.

Primary Breakup – Where the Intact Jets First Shatter

The analysis begins by examining the intact liquid jets as they exit the nozzles. The water volume fraction contour (Figure 4) at the earliest time step shows two distinct, continuous streams of liquid. From an engineering viewpoint, the most important observation is that these jets travel for a distance of approximately 2-3 nozzle diameters before they begin to break apart. This “breakup length” is a critical design parameter. It is the point where the disruptive forces of the surrounding air overcome the liquid’s own surface tension, which tries to hold it together. The iso-surface contour (Figure 5, top-left) provides visual evidence of the start of this process. The surfaces of the blue liquid cones are not perfectly smooth; they show small waves and corrugations.

Figure 3: Velocity magnitude contour showing initial stage of impinging jet spray formation – two spray nozzles (gray tubes) emit high-speed liquid jets

This is the visual signature of primary breakup, which is driven by aerodynamic instabilities. As the high-speed liquid jets (60-95 m/s, shown in Figure 3) shear against the slower-moving air, Kelvin-Helmholtz instabilities form. In simple terms, this is when the speed difference creates waves on the liquid’s surface, just like wind creates waves on a lake. The simulation, using the advanced SBES turbulence model, correctly captures how these tiny waves grow larger and more unstable until they tear off the first large pieces from the main jet. The simulation’s ability to accurately predict this primary breakup length is a critical first step in validating the entire model.

Figure 4: Water volume fraction contour at 1.71×10⁻⁴ s from VOF model showing continuous liquid jets (red regions, volume fraction = 0.991) extending 2-3 diameters from spray nozzles with sharp liquid-gas interfaces (red-to-blue transitions) captured by geo-reconstruct scheme – green-yellow transitional cells (0.5-0.8 volume fraction) at jet periphery indicate interface regions preparing for VOF to DPM conversion

Figure 5: Liquid iso-surface at 2.18×10⁻⁴ s displaying chaotic spray explosion with highly irregular blue surface featuring multiple blobs, sheets, and ligaments radiating outward – dramatic morphology change proves jet collision creates extreme shear rates and turbulent kinetic energy causing violent liquid fragmentation through bag breakup (thin sheets) and shear breakup (elongated ligaments)

Secondary Breakup – The Violent Transition from Sheets to Droplets

The heart of the atomization process is the violent collision of the two jets. The iso-surface contour at a later time step (Figure 5, top-right) shows a chaotic “explosion” of liquid at the impingement point. The jets are no longer recognizable. Instead, we see a complex structure of thin liquid sheets, long, stringy ligaments, and larger, irregular blobs. This is the visual evidence of secondary breakup, where the large fragments from primary breakup are shattered into even smaller pieces.

This contour allows a skillful engineer to identify the specific breakup mechanisms at play:

• Bag Breakup: The thin, almost transparent sheets of liquid seen in the contour are characteristic of bag breakup. Here, a piece of liquid is flattened by air resistance into a shape like a parachute or a bag, which then expands and bursts into a ring of many tiny droplets.
• Shear Breakup: The long, thin, stringy ligaments are being torn apart by shear forces. These ligaments are stretched by the turbulent air until they become unstable and “pinch off” into a line of droplets.

This is the stage where the VOF to DPM model demonstrates its power. It is computationally impossible to track these complex, deforming liquid sheets with individual particles. The VOF model expertly captures the shape of these sheets and ligaments. Then, as they break apart, the model intelligently and automatically converts these small liquid parcels into efficient DPM particles. The simulation successfully visualizes the key secondary breakup mechanisms that are responsible for creating the fine spray.

The final stage of the process is the formation of the mature spray cloud, which is seen clearly in the velocity contours (Figure 3). This cloud is composed almost entirely of DPM particles, each representing a single droplet. The wide range of colors in the droplet cloud at the later time steps tells an important engineering story about the spray’s characteristics.

We can see a wide distribution of droplet velocities. The red and orange particles (95-127 m/s) are the larger, newly formed droplets that have just been created in the collision zone and still carry most of their initial momentum. The blue and cyan particles (0-60 m/s), however, are typically smaller droplets that have been flying for longer. They have been significantly slowed down by aerodynamic drag. Air resistance has a much greater effect on smaller droplets, so they decelerate very quickly. This wide velocity distribution is a critical feature of impinging jet sprays. Furthermore, because two-way coupling was enabled in the DPM model, we know that these millions of droplets are not just passive travelers; they are actively pushing on the surrounding air, transferring their momentum to it and changing the turbulence of the gas phase inside the spray cloud.