Flow over Ahmed Body

Ahmed body :

• It is a vehicle like geometry created by Ahmed actually to measure the effect of slant angle on drag/Lift-coefficient. Ahmed body is used to measure the flow features of a vehicle.
• It has simple shape with vehicle aerodynamic properties. Now it is used to test various turbulence models and validate numerical model.
• The rear part of ahmed body called as 'slant angle' which influences drag and lift with its critical slant angle which is around 0-30.

Importance of Ahmed Body:

• Ahmed Body plays a vital role in testing the aerodynamic properties of vehicle.
• It is also used to describe the turbulent flow field around a car-like geometry and later to design new car models with validated results.

Negative pressure in wake region and flow seperation :

• The wake is the region of disturbed flow (often turbulent) downstream of a solid body moving through a fluid, caused by the flow of the fluid around the body.
• Negative pressure is caused by seperation of flow and net pressure force is produced.
• The point on the body at which the boundary layer is on the verge of separation from the surface is called point of separation.
• Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage, for example. Flowing against an increasing pressure is known as flowing in an adverse pressure gradient.
• The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction.
• Flow separation is the detachment of a boundary layer from a surface into a wake. In aerodynamics, flow separation results in reduced lift and increased pressure drag, caused by the pressure differential between the front and rear surfaces of the object.

Simulation Procedure :

1] First we need to start the workbench and select fluid flow (fluent) solver. Then setup the steps shown in fluent solver as follows.

Geometry :

1. Open the Geometry.
2. Import the Ahmed Body.
3. Provide Enclosures to ahmed body (Inner and outer) according to given dimensions.
4. Suppress the Ahmed body.
5. Input Planes & select XY plane and view the interference range.
6. Now, go to prepare and check for interference and correct it.
7. Set the topology to share.
8. Then, split the body into half. This completes the geometry step.

Mesh :

1. Start the mesh section.
2. Name the boundaries, which are Inlet, Outlet, Symmetry, Wall, Car-wall.
3. Then, provide specific element size and generate the mesh.
4. First, we have to provide the mesh method for the outer enclosure & set the multizone method so it creates Hexagonal mesh elements over that enclosure.
5. Then, we have to add Body sizing for the inner enclosure to make it more refined.
6. Now, add face sizing to the legs of ahmed body so they become completly circular while making their mesh element size smaller.
7. Finally, add inflation layersaccording to correct y+ values. This completes Mesh section.

Setup :

1. First, we need to check the mesh from Domain section.
2. secondly, check if the boundaries hve been specified correctly.
3. Now, provide the reference values according to the problem.
4. Then, turn on the energy equation option, also set the turbulence model.
5. Select the solver according to the problem.
6. Now, set in the boundary conditions like inlet velocity, guage pressure, etc.
7. In solution section create drag and lift reports and plots, also set the solution method.
8. Now, go to the results section and create a plane at some distance normal to x,y,z axis which will be used to examine the velocity and pressure along it.
9. Initialize the problem with Hybrid Initialization.
10. Go back to result section and add velocity and pressure contours along plane. Also add velocity animation.
11. Finally simulate the problem for some amount of iteration and ovserve the Drag, Lift and Residual plots.
12. This completes the setup section.

Results :

1. Select the wall volume to make it visible. Also color it with the variable needed to be observed.
2. After that, insert the plane and color it with variable like Pressure and Velocity which shows the Pressure and Velocity flow on plane.
3. Edit the legend according to requirement. This completes the results and hence the entire simulation.

2] We will be performing 4 cases in which we will keep refining the mesh from default size upto 0.5 million elements size.

#############################{ CASE I } #############################

In this case we are using default mesh sizes with no specific mesh sizing, no inflation layers.

• Model

• Mesh

Element Size (m)	Nodes	Total mesh Elements
0.4092	8608	42693

• Reference Values :

Area (m2)	0.065741
Density (kg/m3)	1.225
Length (m)	1.044
Temperature (k)	300
Velocity (m/s)	25
Viscosity (kg/m-s)	1.7894e-05

• Output Plots

1] Drag

2] Lift

Drag	Lift
0.4231	0.2782

3] Velocity contour

4] Pressure contour

5] Vector plot

6] Line velocity contour

#############################{ CASE II } #############################

In this case we will add inner enclosure for more accuracy near ahmed body simulation. Alos we will add body sizing and inflation layers for more precision.

• Model

• Resolved interference

• Mesh

Outer enclosure element size	0.1 m
Inner enclosure element size	0.05 m
Ahmed body legs element size	0.005 m
Inflation Total thickness	0.004278814
Nodes	30924
Elements	96338

• Inflation layers

• Output Plots

1] Drag

2] Lift

Drag	Lift
0.3603	0.2347

3] Velocity contour

4] Pressure contour

5] Vector Plot

6] Line velocity contour

#############################{ CASE III } #############################

In this case we will use same model as in case II nut with more refined mesh.

• Mesh

Outer enclosure element size	0.05 m
Inner enclosure element size	0.03 m
Ahmed body legs element size	0.005 m
Inflation Total thickness	4.2788e-003
Nodes	125169
Elements	369524

• Output Plots

1] Drag

2] Lift

Drag	Lift
0.3092	0.2135

3] Velocity contour

4] Pressure contour

5] Vector Plot

6] Line velocity contour

#############################{ CASE IV } #############################

In this case again we will use same model but with more refined case than case III.

• Mesh

Outer enclosure element size	0.04 m
Inner enclosure element size	0.028 m
Ahmed body legs element size	0.005 m
Inflation Total thickness	4.2788e-003
Nodes	192757
Elements	493268

• Output Plots

1] Drag

2] Lift

Drag	Lift
0.2982	0.2896

3] Velocity Contour

4] Pressure contour

5] Vector Plot

6] Line velocity contour

#############################{ CONCLUSION } #############################

• From the velocity contours we can see that there is a formatoin of recirculation region at the back of the ahmed body.
• Also we can see that as the mesh gets refined, drag values lower down as we are refining it near walls.
• Negative lift can also be seen which means the creation of downforce. Also the lift values are decreasing.
• Y+ values are essential for correct values of drag and lift.