Simulation of external flow simulation over an Ahmed body using Ansys Fluent
Ahmed Body and its importance
The Ahmed body is a bluff model body with basic aerodynamic properties of a vehicle, which was developed for investigating the influence of the slant angle at the back on the flow field and on the resulting aerodynamic forces, with suppressed interactions between the front and the rear parts.
In simple words, the Ahmed body is a generic car body (a simplified car model) i.e the flow of air around Ahmed body captures the essential flow features around an automobile.
ANSYS is a general-purpose FEA and CFD simulation software. It gives a numerical prediction of the behavior of a system. These numerical predictions need to be validated by experimental data.
People working in vehicle aerodynamics use the Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available for the Ahmed body. Once the numerical model is validated, it is used to design new models of the car.
The Ahmed body was described originally by S. R. Ahmed in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, a height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.
The figure above represents the Ahmed model.
Three main features were seen in the wake:
1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model
2. The B recirculation region that is formed due to the separation at the base of the model.
3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.
The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30°, the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low-pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.
Effect of Aspect Ratio
The aspect ratio of the rear slant had a significant effect on the wake. The wider bodies ceased to reattach at slant angles of 25°, suggesting that the critical angle lowers as the aspect ratio increases. They also provide vortex core location from experimental data that can be used for validation.
Procedure
The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy in addition to the velocity field. For this simulation, we need a larger mesh size than what is usually common to resolve the turbulent flow. More specifically, we use a finer mesh downstream of the model to capture the wake zone.
The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields.
Case-1
For body sizing 50mm and face sizing 5mm
Residuals
Drag coefficient
Lift coefficient
Velocity contour plot
Pressure plot
Velocity vector plot
Case-2
For body sizing 40mm and face sizing 4mm
residuals
Drag coefficient
Lift coefficient
Velocity plot
Pressure plot
Velocity vector
Case-3
For body sizing 30mm and face sizing 3.5mm
residuals
Drag coefficient
Lift coefficient
Velocity plot
Pressure plot
Velocity vector
Results
Grid independency test
| Cases | Coefficient of drag Cd | Coefficient of lift Cl | No of iterations for convergence |
| For body sizing 50mm and Face sizing 5mm | 0.3755 | 0.2028 | 120 |
| For body sizing 40mm and Face sizing 4mm | 0.3618 | 0.2228 | 150 |
| For body sizing 30mm and Face sizing 3.5mm | 0.3318 | 0.2236 | 180 |