Geometries called bluff bodies, like the Ahmed body, have forms that aren’t smooth, which causes complicated flow patterns and strong drag forces. Because of these features, bluff bodies are very important to study in many fields, such as civil engineering and automotive design, where lowering drag can improve performance and fuel economy. For example, the Ahmed body is a good example of these effects because it has a simple but typical shape. To make vehicles and structures that are open to fluid flow more efficient, it is important to understand and improve the aerodynamic behavior of bluff bodies. The main topic of this report is how Computational Fluid Dynamics (CFD) simulations using ANSYS Fluent can be used to look into ways to lower drag, especially by adding flaps. This study is part of the sixth Session in the ANSYS Fluent course for Beginners.

Figure 1: Adding flap to the bluff body to reduce drag

Simulation process

In the design modeler software, the geometries were created within an extended 3D domain to accurately represent the system under study. The geometry was then meshed using tetrahedral elements to ensure a robust and flexible representation of the physical space. A boundary layer was implemented around the bluff body to capture the effects of flow separation and turbulence, with a denser mesh configuration near the body to enhance the resolution in critical areas. Additionally, a flap was incorporated into the design, positioned at a 10° angle to investigate its aerodynamic effects. This comprehensive approach facilitates detailed analysis of the fluid dynamics.

Figure 2: Grid over bluff body CFD Simulation

Post-processing

The results of the CFD modeling show that the flap works to lower the drag force on Ahmed’s body. The baseline configuration without the flap had a drag force of 0.00109075 N. The configuration with the 10° flap, on the other hand, had a drag force of 0.000831595 N, which is a decrease of about 23.8%. This better aerodynamic performance is due to a number of flow processes that can be seen in the visualization. The flow visualization images make the gains in aerodynamics very clear. When we look at the flapped setup (right images), the wake structure is smoother and the vortical patterns are better organized than in the baseline case. The streamlines in the bottom pictures show that the flap does a good job of guiding the flow, which makes the wake region smaller and less intense. This is shown by the blue low-velocity zones that are closer together and the changed pattern of vortex creation. The flap seems to slow down the separation of the flow and weaken the wake vortices, which directly leads to the measured decrease in drag.

Figure 3: Vortex formation and wake structured a) without b) with 10° flap installed on bluff body