Ahmed Body Aerodynamic Study, ANSYS Fluent CFD Simulation Training

Ahmed Body Introduction

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 characteristic relevant to bodies in the automobile industry. This model is also used to describe the turbulent flow field around a car-like geometry. When the numerical model is validated, it is used to design new models of the car. To test this geometry for ourselves, in this study, we employ a CFD solver is and then calculate the drag values.

Project Description

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 select the maximum speed of 20 m/s is for the inlet, which is corresponded to a 72 Km/h car speed.

Mathematical Modeling

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. Briefly, we write the governing mass and momentum equations as follows:

Ahmed Body Geometry and Mesh

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 CFD Simulation

When we import the mesh into the ANSYS FLUENT solver, the calculation procedure could be started. The details of the solution setup are as follows:

Results and Discussions

After we converge the solution, we could obtain the results through post-processing. Meanwhile, as an assurance for 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. Like lid-driven cavity and backward step, Ahmed’s body is employed for cases where new turbulent models have been developed, 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.

Results and Discussions

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.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.