Week 3 - External flow simulation over an Ahmed body.

AIM: Run the External flow simulation over Ahmed body.

OBJECTIVE: 

Q1. Describe Ahmed's body and its importance.

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

Q3. Explain the significance of the point of separation. 

Expected Results:

1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. ( For a velocity of 25m/sec).

• The vector plot clearly showing the wake region.

• Perform the grid independency test and provide the values of drag and lift with each case. 

THEORY: 

A1.

The Ahmed body is a simplified car body used in the automotive field to study the impact of the flow pattern on the drag. The external aerodynamics of the car defines many major traits of an automobile like stability, comfort, and fuel consumption at high speeds. The flow around the vehicle is characterized by high turbulent and three-dimensional flow separations as well as there is a growing need for more insight into the physical features of these dynamical flows. The Ahmed Body is a simplified car, used in the automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of a round front part, a moveable z slant plane in the rear of the body to study the detachment phenomena at different angles, and a rectangular box which link the front part and the rear slant plane. The principal objective to study such a simplified car body is to tackle the flow processes involved in drag production. Through perceiving the mechanisms involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximize vehicle performance.

Important of Ahmed body:

 

An Ahmed body represents a simplified vehicle volume which helps understand the fundamental physics defining by the engineers the lift and drag forces and coefficients at certain characteristically velocities, pressure differences and seeing and analysing the vortices, turbulence, and Reynolds number obtained. ANSYS is a general-purpose FEA and CFD simulation software. It gives a numerical prediction of the behaviour of a system.

A2.

The path lines showing the exact circular motion or vorticity generation just at the rear end of the car. The separation takes two regions of distribution. One at the upper part of the rear and the other at the lower part of the rear end.

The path lines show about the nature of the fluid interaction with the rear end of the Ahmed body which here shows that the fluid just after leaving the contact from the slope still possess the momentum in it and the fluid particles near to the surface almost show the no-slip region which means the very least possible velocity of the fluid particles is in contact with the slope and base of the rear part. Thus the fluid particles away from the rear region having the higher pressure beside the wake region try to approach from the higher pressure region to the lower pressure region thus creating a circularity exactly in the wake region created.

The body penetrating the fluid creates a region of emptiness just beside its end due to the nature of the geometry of the body here which can be seen least in case of a perfectly streamlined shaped body. This emptiness created is a low-pressure region towards which the fluid column exactly behind the wake region which of atmospheric pressure tries to fill the space thus dragging the fluid column with it thus creating the region of vorticity. This also results in the huge pressure gradient in the region exactly rear of the body creating the –ve pressure at the end. Also, this is the layman’s reason why there is negative gauge pressure in the wake region.

A3. 

The fluid separation point can be defined as a point where the dv/dy i.e. gradient of velocity goes zero along with the dp/dx = 0, for the specific point. In more simplified term we can say that the point where the velocity gradient goes zero and the pressure gradient along the x- direction goes zero the point of separations starts occurring or the negative pressure gradient starts forming after that point. The point of separation cannot be understood just by single contour in hard and fast way. It can be supportively studied with the help of the dp/dx plots or velocity plots w.r.t the Y direction and other such plots in which we can see the nature of such physical properties like dp/dx=0, and dv/dy=0 experiencing in a particular region of fluid and body interaction.

PROCEDURE: 

• Geometry Setup in Space Claim-

1. Load the Ahmed body model into SpaceClaim and convert the units from millimeters to meters.

 

2. Go to prepare and click on the enclosure and select the Ahmed body. Enter the enclosure dimensions as shown in the below snap. And uncheck the symmetric option to feed different dimensions for each side before giving the value.

 

 

 

 

 

 

3. Once the enclosure is done, we can create another enclosure close to Ahmed body same as the above steps, so that we can refine the mesh in that enclosure and still keep the total cell countless.

 

 

4. After both enclosures is created, Select a design and choose planes and bring the cursor near the origin and left-click. we get 3 planes, now we can check interference (by selecting a plane and applying cross) to avoid overlapping of meshing in the 2nd enclosure by applying check interference and deleting the overlap. This is automatically done by Space claim when the interference option is selected inside prepare.

4. Now as in above image the boundary layer is not uniform, to make it uniform, select total thickness instead of first layer. Calculation for layer is first layer is 5, then next is 5 times of 1.2 (growth rate) and total 6 number of layer is required. So change maximum layer as 12.5.

5. Now ensure the Share topology should set to share so that both the enclosure individually can share the information.
6. After clearing the interference, we can now split the body in half using the split body command in design, but before that, we have to create a XY plane at the origin to generate 2 equal halves. Hide the unnecessary planes and use the XY plane. Choose split body and choose the enclosure needed to split, select the plane and choose the part needed to be removed. Similarly, we can achieve split on both the enclosures and the body.

 

7. After achieving the split, hide Ahmed body, suppress the physics.

 

• Meshing-

1. At first generate the mesh.

Now to work on meshing part, right click on mesh and select method from insert and then select the outer body through body selection tool & then apply. Now select the multizone method so that it can create hexahedral mesh.

 

 

 

 

 

1. Setting the mesh setup, double click on multizone to change the setup as follows-
1. Case 1-
1. Element size outer Box- 100mm
2. Element size inner Box- 50mm

To avoid this polygon at the bottom face of cylinder, select the mesh< sizing

 

 

 

3. Now create a wall boundary layer mesh- Go to mesh< Inflation

 

 

 

 

 

 

4. Now as in above image the boundary layer is not uniform, to make it uniform, select total thickness instead of first layer. Calculation for layer is first layer is 5, then next is 5 times of 1.2 (growth rate) and total 6 number of layer is required. So change maximum layer as 12.5.

 

 

 

 

 

 

• Fluent Setup-

1. Update the mesh before opening the fluent setup and mesh checking should be done
2. Take the k-epsilon turbulence model because we are taking the inlet velocity 25m/s which is smaller

 

3. Consider the pressure density solver 
4. Take 25m/s as a velocity for the inlet boundary.

5. Set the Reports for drag coefficient and lift.
6. Then initialize the solution
7. After initialize we should go to the post process to create a cut plane to see the velocity profile on it.

Go to contour to set the contour graphs for velocity on cut plane z.

 

 

 

8. Calculate for 500 iteration.

 

• Residual

Lift Plot-

 

 

Drag Coefficient plot-

 

 

 

 

• Velocity Contour-
• 

 

• Pressure Contour-

Velocity Variation-

 

 

Grid independence test

	number of cells	CdCd	CLCL
case 1	94740	0.3467	0.227
case 2	144529	0.348	0.245
case 3	293485	0.3184	0.2386

 

From the above table with the refinement of the mesh drag coefficient approaches closer to the analytical values that are because more cells are assigned at the boundary layer which results in more accurate results of drag coefficients.

 

 

• Result-

1. High pressure is formed at the frontal area of the Ahmed body due to the loss of kinetic energy of the air.
2. The adverse pressure gradient is created near the wake regions because of the flow separation which is occurring because of the geometry of the model. As a result flow separation alters the pressure gradient which in turn affects the drag and lift coefficients.
3. In order to reduce the size of the wake region, detachment of the fluid should be farther away from the body which can be achieved by an increase in the slant angle. 
4. With the increase in mesh size, the numerical results approach closer to experimental results.

 

 

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