Ahmed Body Challenge

Q1. Describe Ahmed's body and its importance.

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 Ahmed. Ahmed body plays a significant role in CFD stimulation for vehicles. The ahmed body can be used to understand and compare it with relevant body. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry. This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated, it is used to design new models of the car.

Q2. Explain the reason for the negative pressure in the wake region.

The wake region has negative pressure because the zone behind the vehicle as nothing. There will be vacuum initially with the flow of fluid. When the fluid is flown over the vaccum region, the air tries to fill in that void creating vortex. Hence the pressure decreases while in the wake region.

Q3. Explain the significance of the point of separation.

The point of seperation is the point that seperates laminar and turbulant flow. boundary layer separation can lead to very significant consequences ranging from an increase in pressure drag to a dramatic loss of lift, known as aerodynamic stall. Boundary layer separation is an important issue as it induces a large wake that completely changes the flow downstream of the point of separation. Skin-friction drag arises due to inherent viscosity of the fluid, i.e. the fluid sticks to the surface and the associated frictional shear stress exerts a drag force. When a boundary layer separates, a drag force is induced as a result of differences in pressure upstream and downstream. The overall dimensions of the wake, and therefore the magnitude of pressure drag, depends on the point of separation along the wing.

Flow stimualtion over ahemd body:

The ahmed body is prepared for stimulation with Three enclosure for perfect meshing and results. The geometry of ahmed body with enclosure is done in spaceclaim with split body condition for symmetry stimulation.

Ahmed body:

Prepared Geometry:

Meshing:

in meshing each enclosure is meshed finer than the outer enclosure. This is the finest mesh possible with the computation power i have.

Meshed Geometry:

The outer enclosure's mesh size is 0.1m, mid enclosure's mesh size is 0.05m and enclosure near ahmed body is 0.01m.

Inflation:

The inflation layer is added to outline of the ahmed body so the results of coeffiecient of lift and drag will accurate. The first layer thickness is calculated using the y+ formula.

y+ = (y*ut)/v

where y is the first layer thickness = 0.43. for the inflation,the total thickness is selected under the selection of named geometry. The total thickness for 6 layers with 1.2 growth rate is 1.069mm.

Setup:

This is pressure-Based, steady state solver with k-epsilon viscous model. The force report for coefficient of lift and drag is made. Then the model is initialised with a number of iteration and select calculate.

reference value:

inlet velocity:

Result:

lift:

drag:

Coefficient of lift and drag:

Velocity contour:

Pressure contour:

Vector plot:

grid independency test:

inner enclosure Mesh size	coefficient of drag	coefficient of lift
0.05m	0.3766	0.3372
0.03m	0.3481	0.24
0.01m	0.3398	0.1703

Thus the grid independence test is done by using various mesh size in the innermost enclosure. From the results in the above table show that the finner the mesh the more accuracy is obtained.