Week 3 - External flow simulation over an Ahmed body.

AIM:

To perform external flow simulation over the Ahmed body and study the wake region.

Introduction:

The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of S.r. Ahmed. Although it has a very simple shape, the Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

Despite the considerable deviation of its geometry from usual vehicles, the body represents the basic
aerodynamic properties of a vehicle, especially in the rear part. The slant angle ϕ has a strong
influence on the aerodynamic drag and lift at the back. Both the drag and lift coefficients change
abruptly at the angle of ϕ=30°. Therefore, this angle is called the critical slant angle.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model is validated, it is used to design new models of the car.

MODELLING AND PREPARATION:

Initially, the simulation was done over a standard or base model of the Ahmed body, then several cases of refined meshed models were simulated to obtain more accurate results.

Preprocessor and Solver settings:

GEOMETRY:

MESH DETAILS:

CASE 1 (BASE MODEL)

MESH:

CASE 2:

The following cases have double enclosures and are of the refined mesh cases.

GEOMETRY:

MESH:

CASE 3:

CASE 4:

CASE 5:

RESULTS:

CONCLUSION:

1. The drag value and drag coefficient decrease with an increase in refinement.
2. The Lift value and lift coefficient have an uneven trend w.r.t refinement.
3. The negative pressure behind the Ahmed body is due to an increase in velocity and vortices. (Pressure is inversely proportional to velocity)
4. The point of separation signifies the point where the flow separates or the boundary later separates and produces a wake region.