Ahmed Body Challenge

REPORT:

AIM:

To simulate for the case of ahmed body and check the drag and lift values and also check the grid independence of the solution.

OBJECTIVES

• To Describe Ahmed's body and its importance.
• Explain the reason for the negative pressure in the wake region.
• Explain the significance of the point of separation.
• To plot the Velocity and pressure contours.
• Perform the grid independency test and provide the values of drag and lift with each case.

THEORY:

AHMED BODY:

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. Once the numerical model in validated, it is used to design new models of the car.

The Ahmed body was described originally by S. R. Ahmed in 1984 [1]. Three main features were seen in the wake:

1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

2. The B recirculation region that is formed due to the separation at the base of the model.

3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.

The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30° the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.

DRAG COEFFICIENT:

In Fluid dynamics, the drag coefficient (commonly denoted as: , or ) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment, such as air or water. It is used in the drag equation in which a lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area.

LIFT COEFFICIENT:
The lift coefficient (C_L) is a dimensionless Coefficient that relates the lift generated by a lifting effect to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a complete foil-bearing body such as a fixed-wing-aircraft. C_L is a function of the angle of the body to the flow, its Reynolds number and its Mach Number. The section lift coefficient c_l refers to the dynamic lift characteristics of a Two-dimensional foil section, with the reference area replaced by the foil chord.

PROCEDURE:

CASE 1: (236864 CELLS)

GEOMETRY:

SPACE CLAIM:

• Upload the ahmed body geometry un space claim.

• After you upload your ahmed body it looks like the above geometry.
• click on Prepare->Enclosure option to create a enclosure over the ahmed body.
• Now to get a accurate results we should create a inner enclosure known as local mesh area with the above option.
• After creating the two enclosures we need to cut the geometry into two halfs.
• Now click on Design->Plane option to create a plane and place it in the origin.
• Now click on Prerpare->Split by plane option to cut the geometry.
• Delete the half of the geometry.

• After the above process we can create the above geometry.
• Now click on Prepare->Interface option.
• Now close the space claim. The geometry automatically gets saved.

MESH:

• In Mesh we need to name the geometry by using the option named selections.
• Now we need to make the inner enclosure more finer so we need to use the body sizing option. Now click on Mesh->Insert->Sizing option.
• Now select the inner enclosure body and give the element size.
• Now we have refine the circles or the support below by using the edge sizing option.
• Now click on Mesh->Insert->Sizing option.
• Now select the two edges and give the no.of.divisions.

• Now we have to create the inflation layers around the boundary layers to get accurate results near the wall.
• we can create the inflation layers with the help of the y+ value. We are using K-Omega so our y+ values should be between 0 to 5.
• To determine the first layer thickness we shoul use the following y+ calculator.
• With the help of y+ calculator available online we can find the values

Now click on Mesh->Insert->Inflation Option and give the above value in the first layer thickness and click on generate mesh.

• From the above steps we get the following inflation layer.
• Now close the Mesh.

SETUP:

• In the general settings click the pressure based and steady state in the solver.
• click on physics->viscous->K-Omega
• Now provide the reference values to get the accurate drag Coefficient.

• Select the Materials
• Now click on Physics->Boundaries Option and click on inlet and give the inlet velocities
• Now click on solutions->Methods option and give coupled.

• Now click on Solution->Definition->New Option and select drag and lift to plot the graphs
• Now click on Initialize.
• After initializing click on Results->Contour->New and create a velocity contour.
• Now click Solutions->create->Solution animation option to create a animation.
• Now click on Calculate.
• Run the simulation till your results converge.

RESULTS:

RESIDUAL GRAPH:

DRAG COEFFICIENT:

LIFT COEFFICIENT:

POST PROCESSING:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

VECTOR PLOT:

RESULTANT VALUES:

DRAG COEFFICIENT: 0.4971

LIFT COEFFICIENT: 0.2713

CASE 2:(309907 CELLS)

GEOMETRY:

SPACE CLAIM:

• Upload the ahmed body geometry un space claim.
• Follow the same steps as the case 1 to prepare the geometry
• Close the spaceclaim, It automatically saves the geometry.

MESH:

Follow the same procedures as mentioned above for the case 1

The only change is that the size of the cells in the inner enclosure or the local mesh is reduced to 0.035 by body sizing option.

After the process update the mesh

SETUP:

Follow the same procedure as case 1, Use the same turbulence model and same materials

Use the same boundary conditions.

Follow the same procedures and click on the calculate.

RESULTS:

RESIDUALS:

DRAG COEFFICIENT:

LIFT COEFFICIENT:

POST PROCESSING:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

VECTOR PLOT:

RESULTANT VALUES:

DRAG COEFFICIENT: 0.47

LIFT COEFFICIENT: 0.2571

CASE 3 :(476651 CELLS)

GEOMETRY:

SPACE CLAIM:

• Upload the ahmed body geometry un space claim.
• Follow the same steps as the case 1 to prepare the geometry
• Close the spaceclaim, It automatically saves the geometry.

MESH:

Follow the same procedures as mentioned above for the case 1

The only change is that the size of the cells in the inner enclosure or the local mesh is reduced to 0.029 by body sizing option.

After the process update the mesh

SETUP:

Follow the same procedure as case 1, Use the same turbulence model and same materials

Use the same boundary conditions.

Follow the same procedures and click on the calculate.

RESULTS:

RESIDUALS:

DRAG COEFFICIENT:

LIFT COEFFICIENT:

POST PROCESSING:

VELOCITY CONTOUR:

PRESSURE CONTOUR:

VECTOR PLOT:

RESULTANT VALUES:

DRAG COEFFICIENT: 0.28

LIFT COEFFICIENT: 0.2321

TABULAR COLUMN:

CASE NO:	NO.OF.ELEMENTS	DRAG COEFFICIENT	LIFT COEFFICIENT
1.	236864	0.4971	0.2713
2.	309907	0.47	0.2571
3.	476651	0.28	0.2321

RESULTS FOR THE GRID INDEPENDENT TEST:

From the above simulations it is clear that the case 1 simulation with 236864 cells takes less time and less iteration to converge as compared to the case 3 simulation with 476651 cells. But the velocity and the pressure contours in the case 3 setup is more clear and accurate compared with the case 1. The wake region is clearly visible in the case 3 simulation compared with the other two cases. This is because the mesh is finer in the case 3 which yields us accurate results.

So it is advised to use the case 3 setup to get the accurate results.

AHMED 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. Once the numerical model in validated, it is used to design new models of the car.

1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

2. The B recirculation region that is formed due to the separation at the base of the model.

3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.

The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30° the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.

The ahmed body represents the simplified ground vehicle geometry of a fluff body type. Its shape is simple enough for accurate flow simulations and retains some important practical features relavant to automobile body.This model helps us to find the turbulent flow field around a simple car.

Reason for the negative pressure in the wake region.

• The pressure goes low to maintain the flow when there is a increase in the velocity of a flow.
• When the fluid hits the surface the flow seperation occurs, Which leads to the formation of air bubbles which are at very low pressure.
• Pressure can also be negative in order to over come the shear stress.
• It can also occur due to flow recirculation.

Significance of the point of separation.

The position at which the boundary layer leaves the surface of a solid body. It may occur with the laminar flow (laminar separation) or when the boundary layer is turbulent (turbulent separation). Separation is sometimes followed by reattachment of the flow.

The separation point is the point where the air stops "sticking" to an object that is moving through the air. When an object is moving through the air, the air molecules push back on it and resist the object's movement - this resistance is called pressure drag. As the air flows around the object, it "sticks" to the object, forming what is known as the boundry layer. This "attached flow" is flowing very smoothly. The point on the body where the boundary layer lifts off the surface is called the separation point. When the air flow separates off the surface, the pressure drag usually increases.

CONCLUSION:

• From the above three cases we determined the case 3 with 476651 cells gives us the accurate results as we can see the wake region clearly and the drag and lift values are much closer when compared with the other cases.
• It also can be said that the obtained results lie under the 305 error permissibility.
• We also found out the occurances of the negative pressure is due to the flow recirculation,and in order to overcome the shear stress and also the increase in the velocity.
• We also learnt about the significance of the ahmed body and point of seperation.

REFERENCE RESEARCH PAPERS:

Flow and Turbulence Structures in the Wake of a Simplified Car Model

Experiments and numerical simulations on the aerodynamics of the Ahmed body