An Icing on Flying airfoil CFD simulation is one of the most important safety analyses in modern aircraft design. The problem of airfoil icing is a serious danger to aviation. It happens when an aircraft flies through clouds that contain supercooled water droplets. These are tiny drops of water that are still liquid even though the air temperature is below freezing (0°C). When these supercooled droplets hit the cold surface of an aircraft’s wing, they freeze almost instantly. This process, called ice accretion, causes layers of ice to build up, changing the carefully designed shape of the airfoil. This new, rough shape disrupts the smooth flow of air, which can dramatically increase drag and, most dangerously, reduce the wing’s ability to generate lift, potentially leading to a loss of control.

To predict and prevent this danger, engineers use specialized software like ANSYS ICING. The ANSYS ICING CFD module is a powerful tool inside ANSYS Fluent that is built specifically to simulate this complex problem. An Airfoil Icing Fluent simulation is not simple; it must model many different physical processes at the same time. The ANSYS ICING module does this by combining several advanced models. First, it uses a Eulerian multiphase model to simulate the cloud of supercooled water droplets flowing in the air around the wing. Second, it calculates the particle trajectories to see exactly where these droplets will hit, or impinge on, the airfoil surface. Third, it uses a sophisticated freezing model to determine how the droplets freeze. Some may freeze instantly, creating a rough, milky-white rime ice, while others might not freeze right away, running back along the wing surface as a thin film of water before freezing into a clear, dense glaze ice. Finally, to make the simulation realistic, it uses a dynamic mesh morphing technique. This means that as the simulation calculates the ice building up, it physically changes the shape of the airfoil in the model. This is critical because the new ice shape changes the airflow, which then changes where the next droplets will hit. An Airfoil Icing CFD study with ANSYS ICING allows engineers to see this entire process on a computer, helping them design and test ice protection systems, like heaters or de-icing boots, to ensure the aircraft can fly safely through these dangerous conditions and meet strict government safety regulations.

Figure 1: A conceptual diagram showing the three key stages of the airfoil icing process: droplet impingement, heat transfer at the surface, and the resulting ice accretion that changes the airfoil shape.

Simulation process:  Fluent-ICING Workflow, Modeling Droplet Impingement and Accretion on a NACA 6409 Airfoil

The simulation process for this Icing on Flying airfoil CFD study was built around a detailed 3D model of a NACA 6409 airfoil. To prepare this geometry for simulation, a high-quality structured mesh was created using a blocking strategy. This method produces a very organized grid of 1,338,000 cells, which is ideal for accuracy, especially near the airfoil surface where the ice grows and the airflow is most complex. This mesh was then imported into the ANSYS ICING module within ANSYS Fluent.

Inside the ANSYS ICING module, the specific conditions for the icing event were defined. The simulation was set up to model a cloud of supercooled water droplets with a single, uniform diameter of 20 microns; this is known as a monodispersed particle distribution. The Liquid Water Content (LWC) of the cloud was set to 0.001 kg/m³, which represents a moderate icing condition that is known to be dangerous for aircraft. To make the simulation realistic, a specialized droplet drag model for “Water” was selected. This is a critical step because this model correctly calculates how the tiny water droplets move through the air. An incorrect drag model would cause the simulation to predict that the droplets hit the wrong part of the wing, or miss it completely, leading to inaccurate results. This complete setup creates a highly accurate virtual environment to test how ice will form on the wing.

Figure 2: 3D computational domain and the structured mesh with 1,338,000 cells used for the NACA 6409 airfoil CFD simulation, showing refined cells near the leading edge to accurately capture ice growth

Post-processing: Ice Accumulation Distribution and Mass Caught Patterns on NACA 6409 Airfoil

The simulation results tell a complete story of cause, consequence, and the final aerodynamic damage. By analyzing this story, we can deliver a clear engineering verdict on the level of threat these icing conditions pose to the aircraft. The ice.Mass Caught contour in Figure 3 shows us where the icing attack begins. This contour is the “blueprint” for the ice formation, showing the rate at which supercooled water droplets are hitting the airfoil. The data is clear: the attack is focused entirely on the front of the wing. The leading edge shows a dark red color, indicating the maximum impingement rate of 0.044 kilograms of water hitting every square meter, every second. This happens because the 20-micron droplets have inertia and cannot follow the air as it curves sharply around the leading edge, causing them to impact it directly. As we move back along the top surface, showing the impingement rate drops quickly. The bottom surface is almost entirely blue, meaning a rate of near zero. This is because it is in the “shadow” of the leading edge. From an engineering viewpoint, this contour provides the first critical piece of information: any ice protection system must be focused on the leading edge, as this is where 100% of the threat originates.

Figure 3; ice.Mass Caught contour on the NACA 6409 airfoil from the ANSYS ICING simulation. It shows the rate of droplet impingement, with a maximum of 0.044 kg·m⁻²·s⁻¹  occurring at the leading edge.

The blueprint from the droplet impacts leads directly to the physical growth of ice. The simulation was run for a total time of 420 seconds (7 minutes). The ice.Accumulation contour (Figure 4) shows that a total of 16.055 kg/m² of ice has built up on the leading edge. The ice thickness contour (Figure 5) gives us the most direct and alarming view of this consequence. It shows that the ice has formed a distinct, horn-like shape right at the stagnation point, with a maximum thickness of 8.06 millimeters (about one-third of an inch). This is a significant amount of ice. The shape is not smooth; it is a sharp, rough bump that completely changes the original, clean profile of the NACA 6409 airfoil. The simulation also captures an important physical effect called “runback,” where some water flows backward for a short distance before freezing. This is why the ice accumulation zone in Figure 4 is slightly wider than the direct impact zone in Figure 3. The formation of this sharp, asymmetric “glaze ice” horn is the most dangerous physical outcome of the simulation.

Figure 4: The ice.Accumulation contour, which displays the total mass of ice that has built up after 420 seconds of exposure. The contour shows a peak accumulation of 16.055 kg/m² (dark blue) at the leading edge

Ice Thickness Growth and Velocity Field Modification from Ice Accretion

The final and most critical part of the analysis is to see how this 8mm ice horn damages the airflow. The Velocity Magnitude contour in Figure 6 shows the result. On a clean wing, the air would flow smoothly over the surface. However, with the ice horn present, the airflow is violently disrupted. We can see a much thicker, low-velocity boundary layer near the leading edge. This thickened layer is a sign of increased friction and drag. The sharp ice shape acts like a “spoiler,” disturbing the smooth acceleration of air over the top surface. The ultimate danger, which this simulation is designed to predict, is that this disruption can lead to flow separation—a condition where the airflow detaches from the wing surface, causing a sudden and catastrophic loss of lift.

Figure 5: The ice thickness contour from the Fluent ICING simulation, visualizing the final ice shape. A maximum ice thickness of 8.06 millimeters is shown concentrated at the leading edge stagnation point.

Figure 6: The Velocity Magnitude contour around the iced airfoil. It shows the airflow speed, revealing an accelerated flow ( 75-90 m/s) over the upper surface and a thickened, low-velocity boundary layer near the rough ice.

Based on all the simulation data, the verdict is clear: flying in these conditions for 7 minutes without ice protection is extremely dangerous and would likely lead to a loss of aircraft control.

For an aircraft designer or manufacturer, this simulation is not just a warning; it is invaluable, actionable intelligence:

1. It Pinpoints the Danger Zone: The simulation proves that the ice protection system (like heaters or inflatable boots) must be concentrated on the first few inches of the leading edge. This allows for an efficient and lightweight design.
2. It Quantifies the Threat: The simulation tells designers not just that ice will form, but how fast it will form (a growth of over 1mm per minute). This allows them to calculate exactly how much power a heating system needs or how often a de-icing system must cycle to keep the wing safe.
3. It Creates a Virtual Testbed: This ANSYS ICING model can now be used to test solutions. Engineers can add a virtual heater to the model and re-run the simulation to see if it prevents the ice horn from forming. This saves millions of dollars and months of time compared to building physical prototypes and testing them in expensive icing wind tunnels.