External Aerodynamics Flow Simulation over Ahmed body using Ansys fluent

Ahmed body:

An Ahmed body is a simplified car model. The air flow around the Ahmed body captures the essential flow features around an automobile. It was first defined and characterized in experiment by SR Ahmed. The biggest advantage of this model is its simplicity in shape and construction but it has all silent features which are relevant to automobiles in industry.

Figure: Ahmed body

The Ahmed body has a 1.044 m of total length, 0.389 m of width and 0.288 m of height. It has 0.05 m cylindrical shaped legs to support and the rear surface has 20 degree slant of fall.

The flow region around car presents the major contribution to a car’s drag, and which poses severe problems to numerical predictions and experimental studies as well, is the wake flow behind the car. The location at which the flow separates determines the size of the separation zone, and consequently the drag force. Clearly, a more exact simulation of the wake flow and of the separation process are essential for the correctness of drag predictions. However, a real-life automobile is a very complex shape to model or to study experimentally. Therefore the Ahmed body is best to use. This also helps to achieve efficient vehicles with minimized fuel consumption, CO2 emissions, and noise, while keeping a good driving stability.

Methodology:

Step: 1

First open the Ansys workbench and select fluent

Step: 2

Open space-claim and give the enclosure to Ahmed body. In the baseline simulation one enclosure is provided to model while in others two enclosures are provided which helps to give the good result which we can see in results section. Ahmed body is considered as suppress for physics.

Figure: One enclosure outside body

When the two enclosures are given to model then interference is checked for both and the share properties of the both enclosures are selected.

Figure: Two enclosure outside body

Step: 3

The meshing is divided in the two parts:

1. One Enclosure
2. Two enclosures

1. One Enclosure:

In this, meshing is given to enclosure which is given below:

Figure: Baseline mesh

2. Two enclosures

As discussed previously, there are two enclosures where multi-zone method is selected thus it will give hexa meshes which give more accurate results. Body sizing is given to small enclosure which helps to give small meshes near Ahmed body. Face sizing is selected for bottom cylinders. Inflation layers are also given to Ahmed body. Here, we will use k-epsilon model therefore we must use Y+ value between 30 to 300. Here, we use Y+ value as 100 thus first boundary layer thickness is nearly 0.0014 m.

Figure: Multi-zone meshing

Y+ value:

In this meshing, different parameters are selected for grid refinement. Hence, three cases are studied which are given below table:

	Baseline Mesh	Case :1	Case:2	Case:3
Enclosure-1 mesh size	0.1	0.1	0.08	0.05
Enclosure-2 mesh size	-	0.05	0.04	0.03
face sizing	-	0.005	0.004	0.003
Total thickness of layer		0.0034	0.0041	0.005
No of inflation layers	-	5	6	7
No. of nodes	28322	54460	93541	238852
No. of elements	141990	160054	272464	613204

Step:4 Set- up

In the set-up, the following steps are done:

Type: Steady-state Simulation

Type: Pressure Based (M<0.3)

Velocity formulation: Absolute

Energy option is selected

Model: K-epsilon Standard (Y+ = 100)

Wall function: Standard wall function

Inlet velocity: 25 m/s

Initialization: Hybrid

Reference value:

Figure: Reference value

Step: 5 Results

In this section, Residuals change, Coefficient of drag, Coefficient of lift, velocity and pressure contour, velocity plot in wake of different cases are given below:

Baseline mesh:

Figure: Residuals Change

Figure: Coefficient of drag

Figure: Coefficient of lift

Figure: Velocity contour

Figure: Pressure contour

Case: 1

Figure: Residuals Change

Figure: Coefficient of drag

Figure: Coefficient of lift

Figure: Velocity contour

Figure: Pressure contour

Figure: Velocity in wake region

Case: 2

Figure: Residuals Change

Figure: Coefficient of drag

Figure: Coefficient of lift

Figure: Velocity contour

Figure: Pressure contour

Figure: Velocity in wake region

Case: 3

Figure: Residuals Change

Figure: Coefficient of drag

Figure: Coefficient of lift

Figure: Velocity contour

Figure: Pressure contour

Figure: Velocity in wake region

Comparison of different cases:

The values of coefficient of drag and lift for different cases are given in following table:

	Baseline Mesh	Case :1	Case:2	Case:3
Cd value	0.3726	0.3711	0.3509	0.3274
Cl value	0.2265	0.2733	0.2904	0.2842

In this section, the plots and contours of different cases are compared which are given below:

Comparison of Velocity contours:

Comparison of Pressure contours:

Comparison of Velocity plots:

Separation point:

When a fluid flow over plate or body there is small laminar boundary layer is developed near body. Within this layer the velocity start to decrease because of friction and this will move because of shear stress. In other words, the first layer faces the friction thus it will take kinetic energy from other boundary layer to overcome the effects of friction. Thus, it will result in velocity gradient in y-direction. Thus, `(du)/dy`> 0 and `(dp)/dx`< 0 in this region. After some length, pressure will reach at minimum. Then, after velocity decrement moves towards to become zero and pressure start to increase. At some point, velocity gradient and shear stress become zero and pressure is also high. But, at point the layer detached from surface because of zero velocity and pressure difference. After this point, `(du)/dy`< 0 and `(dp)/dx`> 0 which another layer under which the turbulent eddies are formed.

Figure: Separation Point

Negative Pressure in Wake region:

At separation point, velocity and shear stress become zero but pressure is high enough to detach layer from body. But, fluid have to move against adverse pressure gradient and also have negative velocity gradient and shear stress which cause reversible highly turbulent flow which will result in Van-Karman vortices which has very low pressure at rear end and that is responsible for drag force.

Conclusion:

Ahmed body is a simple body which helps to improve automobile complex structure.

The multi-zone method in meshing increase computation time but also give good result as compared to others.

The values obtained from grid refinement shows that refinement gives more accurate results.

A point separation is a very important in external flow simulation.

Negative pressure occurs in wake zone because highly turbulent disturbed vortices.