External Aerodynamics of an Ahmed Body using Ansys Fluent

Ahmed Body Challenge

Objective:

Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.

For this challenge, you will have to provide answers to the following questions:

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.
3. The vector plot clearly showing the wake region.
4. Perform the grid independently test and provide the values of drag and lift with each case.

Q1. Describe Ahmed's body and its importance.

The Ahmed body is simplified car body use in 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 automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of 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.

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

When air strikes the Body is becoming almost stagnant which results air exerting very high pressure on the front portion of the body. The airflow then gets divided between the upper and lower surface of the vehicle. The higher pressure air on the front surface accelerates as it travels over the curved nose surface of Ahmed Body, causes the pressure to drop. This lower pressure creates lifts over the roof surface as the air passes over it. As the air continues to flow and make its way to the rear, a notch is created by the rear slant owing to flow separation, leaving a vacuum or low-pressure space that the air is not able to fill properly. The resulting lower pressure creates the lift that then acts upon the surface area of the rear slant

Q3. Explain the significance of the point of separation.

The point of separation is that point at which the velocity gradient changes from positive into negative value. In this point of separation the velocity profile du/dy is 0 at the wall. If the adverse pressure gradient acts over a sufficiently extended distance, the deceleration in the flow will be sufficient to reverse the direction of flow in the boundary layer. Hence the boundary layer develops a point of inflection, known as the point of boundary layer separation, beyond which a circular flow pattern is established.

Steps in Simulation:

Open Ansys Workbench. Drag and drop Fluid Flow (Fluent) onto Project Schematic.

Here, we will see Steps we need to follow to solve a particular enginnering system from a CFD standpoint.

1. Geometry: Spaceclaim, where we import the geometry.
2. Mesh: Ansys Mesh, where Boundary naming, element sizing and mesh generation with required grid size will be done.
3. Setup: Ansys Fluent, where actual case setup will be done.
4. Solution: Ansys Fluent, where runtime results visualization done.
5. Results: CFD-Post, where Post Processing of Results will be done.

CAD Cleanup:

Import Ahmed Body geometry into Spaceclaim, Spaceclaim is CAD cleanup software that can be used to prepare CAD models for simulation setup. It could be for CFD or FEA.

Full Body Geometry

Full Body Geometry

As we can see above figure is the Solid model. As we want to simulate the flow outside of the Ahmed Body. So we need to create a computational domain.

Assigning dimension to the Computational Domain around the Ahmed Body as follows:

Inlet (left side) = 2m

Outlet(right side) = 5m

Bottom(symmetry)= 0m

Top(symmetry) = 1m

Width =0.5m (on both the sides)

Create the planes normal along x, y and z axis.

Mesh Generation and Refining:

Now, we want to refine the Mesh only in the specific region around the car body instead of refining the entire our Domain. So we will create a small box inside which our Ahmed Body will be placed.

Dimensions of the Small Box will be as follows:

The following distance will be from the center of the car body.

Inlet = 0.5m

Outlet =1m

Top= 0.5m

Bottom = 0m

Now generate the Mesh to see how the Baseline Mesh (Small box and Ahmed Body) will look like. After this, our Computational domain(large box) and Baseline Mesh(small box) are overlapping, which is undesirable. To merge both the boxes, go back to SpaceClaim, click the plane option, create 3 planes by clicking on the origin. Now select first plane and click on “Section Mode”. Now we can see hatching lines inside the small box as crosshatch.

To avoid interference, go to “Prepare” and click “right tick” option.

Now, click on “Mode”, hide all the planes except “FFF” plane.

Enable only first enclosure.

Select first plane, click “Section mode”.

Now disabled the first Enclosure and by enabling the second Enclosure we can see that our Ahmed Body has been hidden.

Click “FFF” plane to see set the “Share topology” option as “Merge”. Then only our both the boxes will share information with each other.

Close the window.

Open the Mesh again from the Ansys Workbench.

Generate the Mesh.

To assign the highest quality Mesh,

Right click on Mesh, click Method, select Body, hide Section Plane 1, click apply on Geometry.

In definition, select Method, select Multizone, assign element size from 413mm to 100mm to get evenly size Mesh at the Outer region.

Generate the Mesh.

Now we can see that, the pillars at the bottom of the Ahmed Body are not fully circles. So we need to make them round by assigning element size of 10mm.

Assign Inflation layer due to Boundary layer of the body over which flow of air takes place. Assign Total layer thickness as 12.5mm.

Close the Mesh window.

Fluent Setup:

Setting up physics: Select k-epsilon turbulent model

Solver: Density-based Solver because it gives good compressibility effect.

Assign Time as Steady-State Solver.

Material Selection: air.

Properties of Material:

Density = 1.22 kg/m^3

Viscosity = 0.0000181 kg/m.s

Setting Boundary Condition:

Inlet= 25m/s

Temperature = 300kelvin

Outlet= 0 Pascal. (Ambient Pressure)

Car-wall = wall (No slip boundary condition)

Symmetry = symmetry

Assigning different Flow quantities:

• Create Pressure and Velocity contour
• Create the cut Plane in z-direction to see velocity and Pressure profile.
• Create the report Plot for drag and Lift Coefficient.
• Add Animation for Pressure and Velocity.
• Hybrid Initialize and run the Simulation.

CFD Result:

• Select car wall, assign color variable Pressure, click apply.
• Create cut-plane-z to see velocity and Pressure profile.
• Create the Vector plane in cut-plane-z to see the vector plot.
• To obtain the velocity at two different point in the cut-plane-z, create two points at some distance along cut-plane-z.
• Now to see this velocities distribution go to Insert, select Chart, go to data series, assign location as line 1
• On x-axis variable, y (coordinate)
• On y-axis velocity veriable, velocity(u)
• Similarily, assign second point at some distance.

Baseline Setup: The Baseline is a less refined mesh which use to check the solution or to get an overview of the simulation.

Nodes =16713

Cell counts = 84836

Mesh:

Residuals Plot:

Drag Coefficient (cd):

C0effocoent of Drag = 0.41

Lift coefficient(cl):

Coefficient of Lift: 0.21

Velocity Contour:

Pressure Contour:

Vector Plot:

Velocity distribution plot:

Inference from Velocity Distribution line Plot:

The above velocity line plot shows the velocity distribution at two lines at some distance from the rear end of the Ahmed body. The velocity increases as the vertical distance increase because, after the velocity loss at the rear end of the body, it regains its momentum due to rejoining the streamline (from y=0 to 0.8) and further that vertical distance the velocity is constant which represents natural streamline velocity.

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Refined Mesh Setup:

Case 1: Nodes=32006 Cells counts(elements) = 102946

Mesh Element size= 100mm

Body Sizing = 50mm

Face sizing = 5mm

Mesh:

Residuals Plot:

Drag Coefficient (cd):

C0effocoent of Drag = 0.3416

Lift coefficient(cl):

Coefficient of Lift= 0.2093

Velocity Contour:

Pressure Contour:

Vector Plot:

Velocity distribution plot:

Inference from Velocity Distribution line Plot:

The above velocity line plot shows the velocity distribution at two lines at some distance from the rear end of the Ahmed body. The velocity increases as the vertical distance increase because, after the velocity loss at the rear end of the body, it regains its momentum due to rejoining the streamline (from y=0 to 0.45) and further that vertical distance the velocity is constant which represents natural streamline velocity.

Case 2: Nodes=53328 Cells counts(elements) = 171911

Mesh Element size= 75mm

Body Sizing = 40mm

Face sizing = 5mm

Mesh:

Residuals Plot:

Drag Coefficient (cd):

C0effocoent of Drag = 0.3195

Lift coefficient(cl):

Coefficient of Lift= 0.0385

Velocity Contour:

Pressure Contour:

Vector Plot:

Velocity distribution plot:

Inference from Velocity Distribution line Plot:

The above velocity line plot shows the velocity distribution at two lines at some distance from the rear end of the Ahmed body. The velocity increases as the vertical distance increase because, after the velocity loss at the rear end of the body, it regains its momentum due to rejoining the streamline (from y=0 to 0.42) and further that vertical distance the velocity is constant which represents natural streamline velocity.

Case 3: Nodes=125854 Cells counts(elements) = 391477

Mesh Element size= 50mm

Body Sizing = 30mm

Face sizing = 5mm

Mesh:

Residuals Plot:

Drag Coefficient (cd):

C0effocoent of Drag = 0.322

Lift coefficient(cl):

Coefficient of Lift= 0.1533

Velocity Contour:

Pressure Contour:

Vector Plot:

Velocity distribution plot:

Inference from Velocity Distribution line Plot:

The above velocity line plot shows the velocity distribution at two lines at some distance from the rear end of the Ahmed body. The velocity increases as the vertical distance increase because, after the velocity loss at the rear end of the body, it regains its momentum due to rejoining the streamline (from y=0 to 0.35) and further that vertical distance the velocity is constant which represents natural streamline velocity.

Effect of Mesh refinement on the coefficient of drag and lift:

Mesh size (mm)	Body size (mm)	Face size(mm)	Coefficient of Drag (Cd)	Coefficient of Lift (Cl)	Nodes	Elements
100	50	5	0.3416	0.093	32006	102946
75	40	5	0.3195	0.0385	53328	171911
50	30	5	0.322	0.1533	125854	391477

Conclusion:

• As the Mesh becomes finer, the wake region gets more visible as the number of cells increases which increases the accuracy of the solution as well.
• The value of the Coefficient of Drag is not getting much affected by the Mesh quality. Instead it depends on the Reynolds number and Y plus value from which the total thickness of the inflation layer has been calculated which is useful to reduce the Drag coefficient.
• The velocity distribution is more accurate as the Mesh becomes finer.
• However, the computational time increases by increasing the refinement of the mesh.