In automobile aerodynamics, simulation and validation of experimental data are critical for understanding complicated flow phenomena and optimizing vehicle design. The Ahmed body, developed by Ahmed et al. (1984), has become a major benchmark geometry for computational fluid dynamics (CFD) validation investigations because to its simple yet standard assets of real vehicle wake structures. Researchers can use powerful CFD tools such as ANSYS Fluent to effectively simulate and evaluate the intricate flow patterns, pressure distributions, and aerodynamic forces around the Ahmed body, with a focus on crucial factors such as drag coefficient prediction and wake formation. This systematic strategy of combining CFD modeling and experimental validation not only improves our understanding of vehicle aerodynamics, but also helps to produce more efficient and aerodynamically optimized automobile designs. Thus, our objective in the current study is to validate our CFD results against benchmark Ahmed`s paper [1]. This is the session 6 of ANSYS Fluent Course For Beginners.

• Reference [1]: Ahmed, Syed R., G. Ramm, and Gunter Faltin. “Some salient features of the time-averaged ground vehicle wake.” SAE transactions(1984): 473-503.

Figure 1: Horseshoe vortex system in wake (schematic) [1]

Simulation Process

The bluff body geometry model is initially designed using designed modeler software, following the schematic drawing given in the reference paper of Ahmed [1]. It then meshed considering Body Of Influence tool in ANSYS meshing. It resulted in about 6 million tetrahedron elements. The model requires appropriate consideration of reference values, plus turbulence model.

Figure 2: Schematic of Ahmed body geometry model [1]

Post-processing

The CFD simulation results show excellent agreement with the experimental data, with the predicted drag coefficient of 0.27 being only 3.8% off from the experimental value of 0.26. The velocity contours show unique flow features around the Ahmed body, especially in key places. At the front end, a stagnation point is clearly apparent where the flow initially meets the body, resulting in flow separation and acceleration across the rounded front edge. The flow remains greatly attached to the body’s top surface, maintaining an even high-velocity zone  until it reaches the slant angle at the rear.

Figure 3: Velocity distribution and streamlines of Ahmed Body CFD Simulation by ANSYS Fluent

The most detailed flow events occur in the wake region, where the simulation captures the complex flow structures that characterize the Ahmed body. The sloped rear surface creates a strong longitudinal vortex system, as seen in the streamlines image, which is critical for accurate drag prediction. The wake structure depicts a major recirculation zone directly behind the vertical base, with lower velocity zones indicating a wake absence. The flow separation at the sharp edges of the slant angle generates a shear layer that interacts with the ground effect, resulting in a well-defined wake structure that extends approximately one body length downstream. This wake pattern, together with an appropriate forecast of the drag coefficient, demonstrates the numerical setup’s reliability and the simulation’s ability to capture the basic flow physics of the Ahmed body design.