A Car Aerodynamics CFD simulation is a vital tool for car designers. The way air flows around a car has a huge impact on its fuel efficiency, stability at high speeds, and even how quiet it is inside. A great example of this is comparing two versions of the same car, like the Peugeot 206 hatchback and sedan (SD). Even though they look similar from the front, their different rear shapes cause major changes in airflow. This report details a CFD analysis that visually explains why the sedan is more aerodynamically efficient than the hatchback.This analysis is a perfect example of the skills taught in our comprehensive ANSYS Fluent course for beginners, a FREE Course that provides an excellent foundation for performing your own simulations.

Figure 1: Comparing the different rear body styles of the hatchback and sedan models

Modeling the Car Aerodynamics Fluent Simulation

The simulation was performed using 3D models of both the hatchback and sedan. To prepare for the Car Aerodynamics Fluent analysis, a large virtual box, or “wind tunnel,” was created around each car model. This entire volume was then filled with millions of small tetrahedral cells, a process called meshing. We used a very fine mesh, especially behind the vehicles and close to their surfaces. This is critical because the air in these areas is very active, and we need a fine grid to accurately capture the physics. To correctly simulate the complex, swirling airflow (turbulence), we used the industry-standard k-omega SST turbulence model. This model is excellent for external aerodynamics because it accurately calculates the airflow both in the boundary layer attached to the car and in the large wake region that forms behind it.

Figure 2: The high-quality mesh generated around the hatchback model for the aerodynamic analysis.

Post-processing: CFD Analysis, How Rear-End Shape Creates Aerodynamic Drag

The simulation results provide a clear and fully substantiated story that begins with the shape of each car’s rear end, which is the primary “cause” of its aerodynamic performance. The hatchback has an abrupt, almost vertical cutoff at the back. This sharp shape is the “cause” of a massive aerodynamic problem. The air flowing over the car cannot follow this sudden drop. Instead, it separates from the surface and creates a large, swirling, low-pressure zone directly behind the car. This zone is called the “wake.” The velocity plot in Figure 3 provides perfect visual proof of this effect. The large blue area behind the hatchback shows a huge region of slow, chaotic air. The turbulent kinetic energy (TKE) plot in Figure 4 further confirms this, showing a large, intense red area which signifies a highly turbulent and energy-draining wake.

Figure 3: Velocity contours showing the much larger low-velocity wake region behind the hatchback compared to the sedan.

Figure 4: Turbulent kinetic energy (TKE) plot showing the highly intense and large turbulent wake behind the hatchback.

Figure 5: Streamlines showing the size of the wake regions, visually confirming the larger and more chaotic flow behind the hatchback.

This large, turbulent wake is the “effect” that directly harms the car’s performance. This low-pressure zone acts like a powerful vacuum, sucking the car backward and resisting its forward motion. This suction force is known as “form drag.” In contrast, the sedan’s extended trunk is the “cause” of a much better outcome. This gradual shape allows the airflow to stay attached to the body for longer and separate more smoothly at the very end. The direct “effect” is a much smaller and less turbulent wake, as clearly shown by the smaller blue and red areas in Figures 3 and 4, respectively. This smaller wake creates far less suction. The most significant achievement of this Hatchback Car CFD analysis is the clear visual demonstration that the abrupt rear shape of the hatchback (the cause) leads to massive flow separation and a large, turbulent wake (the effect), which results in high aerodynamic drag. Conversely, the sedan’s trunk minimizes this effect, proving that subtle changes in body shape are the key to designing more fuel-efficient and stable vehicles.