Week 3 - External flow simulation over an Ahmed body.

Aim : To simulate the flow over ahmed body and to plot for velocity and pressure contours , the vector and drag coefficient plot .

Theory :

Ahmed body : The Ahmed body represent a simplofied. this was developed by SR Ahmed and hence named it as Ahmed Body .The aieflow around Ahmed body capture the flow Feature around the automobile . This simplified body is able to cature almost the same airflow which will occur to an exact automobile .

Importance of Ahmed body : The drag force is an important factor considering fuel consumption. As the drag force increases the fuel consumption also increases which in turn becomes expensive for the car owners. As the burning of fossil fuels becomes an issue, the manufacturers are pressing for fuel-efficient cars. One of the main factors considering the car is fuel-efficient or not is the car's geometry.

Complex shaped cars are very challenging to model and its difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools.

Solving And modelling approach :

A] Geomentry

1} Firstly import Ahmed body in spaceclaim .

2} Prepare an Enclosure for the Ahmed body

Side view :-

Front view :-

3} Final geomanetry after adding Enclosure is shown below:

4} Add a plane and view in section mode for the interference .

5} Suppress the Ahmed body for physics .

6}Close spaceclaim .

B.Meshing :-

1}First give the name selection

1. a) The name selection is given as:-

2} Open the meshing interface and click on generate mesh at default values .

1. b) Baseline mesh :-

3} Now insert a meshing method use multizone .when we we use Multizone it will automatically use hexahedral mesh where ever possible .

4 } Select the outer box and apply in geomrtry .

5} now insert sizing and select inner box as geomentry and element size as per requirement and genrete mesh .

6}Generated mesh is shown below :-

7} Add inflation layer to the car body .

C] Setup :-

1) Open fluent setup and perform mesh check to ensure mesh generated is free from errors.

2) Set Reference values.

3) Change viscous model to realizable k-epsilon model and chose standard wall function.

4) Add material type as fluid and material air.

5) Change boundary conditions

Inlet velocity = 25 m/s

6) Now perform hybrid initialization.

7) Create a definition for drag coefficient.Similarly create definition for lift coefficient.

8) Create a velocity and pressure contour at cut-plane.

9) Run calculation.

Case 1 : Baseline setup

In baseline setup outer enclosure is created in spaceclaim and model is meshed using default mesh size.

Meshed model is setup is done in Ansys-fluent and calculations are done.

We get following results;

Residuals :

Drag coefficient :

Computed Cd = 0.3666

Velocity contour :

Vector plot :

Case 2 : Refined setup

Outer and inner enclosures are created in spaceclaim. Inner enclosure is created such as it will capture the wake region.

Outer enclosure is meshed using multizones and hexa elements of element size = 100 mm are created.

To perform the grid independency test the inner enclosure is meshed using different mesh sizes such as,

1. A) Mesh size = 50 mm
2. B) Mesh size = 40 mm
3. C) Mesh size = 30 mm

1. A) Mesh size = 50 mm
2. 

Residuals :

Drag coefficient :

Cd = 0.3439

Lift coefficient :

Cl = 0.2565

Velocity contour :

Velocity contours shows the velocity distribution around the body and within the computional domain.

Pressure Contour :

Pressure contours shows the pressure distribution over a ahmed body and within the computional domain.

Vector plot :

Vector plot shows the flow direction of velocity vectors.

All of the required physics i.e wake region is captured in this case but still we cant be sure with obtained drag and lift cofficient values thus to see the effect of refinement on obtained results we go for second refinement.

1. B) Mesh size = 40 mm

Residuals :

Drag coefficient :

Cd =0.3277

Lift coefficient :

Cl = 0.2594

Velocity contour :

Velocity contours shows the velocity distribution around the body and within the computional domain.

Pressure Contour :

Pressure contours shows the pressure distribution over a ahmed body and within the computional domain.

Vector plot :

Vector plot shows the flow direction of velocity vectors.

Thus the results of drag cofficient and lift cofficient is varried from 1st case to second case. Lets see the the effect on results on further refinement.

1. C) Mesh size = 30 mm

Residuals :

Drag coefficient :

Cd = 0.3387

Lift coefficient :

Cl = 0.2823

Velocity contour :

Velocity contours shows the velocity distribution around the body and within the computional domain.

If we compare this case velocity contours with other two case then we can see that the wake region shown in this case has a more definite shape than other two cases and this is due to highly refined mesh.

Pressure Contour :

The frontal region of Ahmed body is flow impacting region which generates high static pressure. In this case the impacted region and negative pressure region can be seen accurately.

Vector plot :

The vector plot clearly shows that the recirculated region is formed behind the ahmed body which has low velocity and the air is forming eddies behind the ahmed body.

As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster as well.

Case 3 : Half Ahmed body

Residuals :

Drag coefficient :

Cd = 0.1733

Lift coefficient :

Cl = 0.1138

Velocity contour :

Pressure Contour :

Vector plot :

Conclusion

• It can be inferred that the grid independence test is performed to know the smaller cell size for convergence.
• Apart from mesh refinement, convergence depends on the Y+ value, inflation layer, it is advisable to use k−ωk-ωSSTSST as the turbulence value because it calculates the values well near the wall.
• As the setup is refined drag coefficient, lift coefficient decreases.
• Y+ plays an important role in determining the value of drag.
• The negative pressure in the wake region is primarily because there is no flow in that region. This can be further attributed to various factors such as separation and recirculation which is evident from the vector plot near the wake region.
• The point of separation occurs at the rear end of the vehicle, the separation occurs beyond this point where it can be observed that the flow recirculates and attaches it to the boundary layer.