Ahmed Body Aerodynamic Study, ANSYS Fluent Training

Description

Ahmed Body Aerodynamic Study, ANSYS Fluent, CFD Simulation Training

Description

The Ahmed body is a simplified vehicle model that captures the essential flow features around an automobile. Also, Ahmed’s body allows us to capture aerodynamic characteristics relevant to bodies in the automobile industry.

It’s used to describe the turbulent flow field around a car-like geometry. When the numerical model is validated, it is used to design new car models. To test this geometry for ourselves, in this study, we employ a CFD solver is and then calculate the drag values.

This project is going to study an incompressible isothermal airflow adjacent to the famous Ahmed body geometry. The geometry is a 1-meter car inside a 25-meter wind tunnel. Also, we selected the maximum speed of 20 m/s for the inlet, corresponding to a 72 Km/h car speed.

To study the current problem, one must solve the flow equations in the differential form. Also, we assume the isothermal, incompressible, turbulent condition inside the wind tunnel.

As a numerical study, the initial step towards the modeling is the production of the CAD geometry. We consider the blue face as the inlet of the domain while the red face as the outlet.

For the current problem, we generate a mesh count of 624,482 elements to represent the geometry using ANSYS Meshing. Regarding the quality of the mesh, the maximum skewness of 0.84 with an average of 0.22 is satisfactory. In addition, for an interested reader, we depicted the quality distribution of mesh as below. Also, we added 10 layers for our prism elements to accurately calculate the boundary layer.

Ahmed Body Aerodynamic Study Methodology

The calculation procedure can be started when we import the mesh into the ANSYS FLUENT solver.

In this project, we consider gravity. The flow behavior is steady. The type of this project is Pressure-based.

We used the k-w SST model to simulate the fluid’s turbulence.

Conclusion

After we converge the solution, we could obtain the results through post-processing. Meanwhile, as an assurance of a valid convergence, the drag and Y-plus values were monitored during the solution iterations.

This study decided that the solution is converged when the drag force reached a constant rate, and the residuals were below 10^-5 values. Also, the maximum Y-plus value was 60 on the Ahmed body, which shows us that we have efficiently resolved the boundary layer.

Before going through the results, two important issues must be noted regarding the Ahmed body geometry. Firstly, the Ahmed body geometry is a turbulent benchmark. Ahmed’s body is employed for cases where new turbulent models have been developed, like lid-driven cavity and backward step, and its accuracy is under examination.

For the noted cases, the experimental results of drag, flow separation, and separation angle are compared to the obtained data from the new model implemented inside the CFD code. Therefore, Ahmed body allows us to validate our numerical model.

Secondly, due to the simple geometry of Ahmed body, it could be easily employed for LES/DES simulations, where high-quality elements are required. Thus, Ahmed body could inform us regarding the details of turbulence for automotive applications.

Afterward, the results regarding the pressure and the velocity field are depicted in the below figures. The maximum value of velocity was found at the corners, while the maximum value of pressure was at the middle of the front face, where the velocity was minimum. Also, we represent the streamlines to give much insight into the problem.

Finally, we have found that the drag force is 15.091 (N), which was accurate for a 1-meter car with the noted specifications.