External flow simulation over an Ahmed body.
AIM: To simulate external flow conditions over an Ahmed body for refined case.
To perform a grid independence study
INTRODUCTION:
Ahmed body and its importance:
Ahmed body is a simplified form of an automobile body that resembles all the essential flow features around an automobile. This body allows us to capture all the characteristic features relevant to an automobile. Ahmed body can be useful for developing simulation models and validating it experimentally.
PRE-PROCESSING:
Geometry:
The STEP file of Ahmed body was imported into space claim to create a geometry setup for the simulation. The Ahmed body was split about the symmetry axis using split body option. Enclosure option was selected to create an outer enclosure around the geometry. In order to refine the regions around the Ahmed body further, an inner enclosure was setup near to the body. Interference check was run to check for interferences. And finally, share topology was set to share.
Mesh:
In Mesher, multizone meshing method was used with hexadominant mesh on the outer enclosure and tetrahedral mesh on the inner enclosure. Face sizing was given to the circular support pillars of the Ahmed body. And with y+ value as 30 for k-epsilon model, 6 inflation layers were set up by calculating the first layer height.
POST-PROCESSING:
RESULTS:
(Refined case)
Total number of elements = 479721 elements
Outer enclosure element size = 0.076 m
Inner enclosure element size = 0.026 m
Face element size = 0.0026 m
First layer height = 4.4452e-4 m
Inflation layers = 6
Velocity contour
Pressure contour
Vector plot
Drag plot
Lift plot
Observation table:
| Total number of elements | Coefficient of drag | Coefficient of lift |
| 94145 | 0.35 | 0.23 |
| 160489 | 0.33 | 0.22 |
| 330487 | 0.31 | 0.19 |
| 479721 | 0.30 | 0.25 |
Negative pressure in the wake region
When air hits the Ahmed body, it creates a high-pressure point at the front and low-pressure point at the rear due to air separation. This low-pressure region is known as the wake. This wake is responsible for pressure drag, which reduces the performance of the vehicle.
Point of separation
The point of separation is a point where the fluid flow, due to shear stress, no longer follows the contour of the body and gets detached from the surface of the body. This separation gives rise to vortices and eddies.