Ahmed Body Challenge

Aim: To study the effects of flow over the Ahmed body and perform the grid independence test.

To Find:

• Describing 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 generate Velocity and pressure contours.
• To represent the drag coefficient plot for a refined case. ( For velocity of 25m/sec, the drag coefficient should be around 0.33).
• To show the vector plot clearly showing the wake region.
• Performing the grid independency test and provide the values of drag and lift with each case.

Ahmed's body and its importance :

• The Ahmed body is a generic car body. This was first defined and characterized in the experimental work of Ahmed.
• The Ahmed body is the benchmark model used in the automotive industry for validating simulation tools.
• This body maintains car-like geometry features.
• The air flow around the Ahmed body captures the essential flow features around the geometry.
• In vehicle aerodynamics we generally use Ahmed body to validate their numerical model because the experimental data from wind tunnel testing is available on Ahmed body.
• Once the numerical mode is validated, it is used to design new models of the car.
• It also helps to study the recirculation regions formed behind a car and the effects of various other parameters on the automobile body.

Negative pressure in the wake region :

The air that is moving around the vehicle is separated it leaves a low pressure turbulent region called as wake. This wake exhibits the formationof pressure drag which reduces the vehicle performance. When the air hits the front portion of the ahmed body, the flow separation takes places. The higher pressure air on the front surface increases as it moves over the body which causes pressure drop. This lower pressure create lift as the air flow over it. When the air flow towards the rare end, it leaves a low pressure turbulent region behind the vehicle which is called as wake region.Hence negative pressure is developed in the wake region.

Significance of the point of seperation:

The velocity gradient changing from positive to negative value is known as the point of seperation. At the point of seperation the velocity profile du/dy is equal to zero at the walls. The point of seperation is the important aspect of the flow as the wake flow is contributor for the drag force.The more closer flow to the body results in low recircualtion zone and hence helps to reduce the effect of drag on the body and maximise the efficiency of the body.

Ahmed Body Baseline simulation:

Geometry :

Creating the Enclosure (first enclosure) :

• Using the enclosure option, enclose the ahmed body in ordr to create the mesh outside the ahmed body.
• Open spaceClaim > go to Prepare > select Enclosure > draw the enclosure around the actual body.
• Set the dimensions as shown below;

Meshing :

• For the base line mesh the element size is considered to be 0.4136m.
• Number of inflation layers used around the body is 5 for the Growth rate of 1.2.
• For this case, select Inflation option > Smooth Transition.
• Go to Details of Mesh > Statistics >

• In this case, we can see that mesh is also generated inside the ahmed body i.e. we have mesh where we dont want to simulate the flow. Hence we should not have any mesh inside the body.
• To fix this go to Geometry > unselct First copy of ahmed body >right click and select Supress for physics.

Setup :

• Go to Mesh > click on Check Mesh. This helps to check the quality of the mesh.
• Go to Setting up Physics > select Solver=Density-Based, Velocity Formulation=Absolute, Time=Steady State.
• Here, when we have Mach number is 0.3 or above its good to use Density-Based Solver as we can take it to good compressibility effects.

Inlet velocity = 25m/sec

speed of sound = 300m/s (Assuming)

Maximum value=1/6 i.e. Mach number=0.3

Mach Number- The ratio of the speed of a body to the speed of the sound in the surrounding medium.

• In this case we need a turbulence model, as the flow is turbulent.
• Go to Setting up Physics > select Energy > also go to Models > go to Viscous > Select k-Epsilon Model(Baseline mesh).
• Go to Materials (use default values provided)

Material Name - Air

Density - 1.225kg/m^3

• Go to Boundary Conditions

At inlet,

Velocity=25m/s.

Temperature=300K.

• Go to Solution > select Hybrid initiallization.
• Enter the number of iterations and press Calculate.

case-1 (Baseline mesh - with first enclosure)

Mesh element size=0.4136m

Nodes=16804

Elements=85081

Residuals :

Coefficient of drag:

• Coefficient of drag = 0.44

Coefficient of lift:

• Coefficient of lift = 0.27

Velocity Contour:

Pressure contour:

Velocity Vector Plot :

Creating the Enclosure (second enclosure) :

• In this case we want the flow around the Ahmed body. To get the accurate results finer mesh is generated.
• Here for refining the mesh, another enclosure is created to the ahmed body as shown in the figure.

• Once the enclosure is created for ahmed body create a baseline mesh.
• If we create a section plane and check the internal geometry. It is observed that initially we dont have a mesh inside ahmed bodybut we can see the mesh from bigger box and the smaller box get overlapped, which is not correct.
• Here, initially there was a huge interference between the outer and the inner enclosure so that the mesh gets overlaped which is not acceptable. To have some data exchange between both the blocks we need to use Share Topology = Share. With the help of Interference Tool we can delete the interference region between both the blocks.
• In order fix this problem, uisng plane tool create planes by clicking on the origin. Selecting the side plane and select Cross-section Mode. Now we can see the cut section of geometry. Its observed that the meshing is overlaped. To over this go to Prepare > select Interference > click OK.
• We can see that now there is no overlap of mesh between the bigger box and smaller box.

• Outside the inner region, we have a perfect structure i.e which is made up of straight lines(2D).
• In such cases is better to use Hexahedral Mesh as this type of mesh has heighest quality compared to other meshes.
• Hence for the outer box we create a Hexahedral Mesh which is interfacing with inner Tetrahidral Mesh.

• Here a multizone with quad-dominant elements is used. We consider element size(enclosure-1) is 0.1m and maximum face size is also equal to 0.1m. Similarly by using body sizing for enclosure-2 consider element size is equal to 0.05m.
• Now go to Insert > Sizing > select only the Ahmed Body > refine the Meshing(Element size). Setting the element size higher or lower mainly depends on the computational capacity and the software licensing.
• After the successful generation of mesh, check the complete geometry.
• On the bottom side of the geometry, we can observe that the cylindrical pillars of the ahmed body are changed to polygons. Hence this should be changed to cylindrical.
• Go to Sizing > select a bottom face of ahmed body and refine the mesh. Even then if the pillars look similar, go to Single Select tool and select the pillars(4-faces) and click Apply.
• Now just reduce the Element size and generate the mesh again.

• We know that at each and every boundary, a boundary layer is applied. As we have irregular cells centers created on the boundary, we should create a set of cells which follow the shape of the surface very closely.
• Hence inflation layers are created along the boundary. To constrain the mesh correctly use Inflation option= Total Thickness.
• Before refining, we need to calculate the Total thickness using the y+ value for a given velocity.area,density and the viscosity value. Here the y+ value should satisfy the standard wall functions- turbulence model.

Pre-processing:

Set up physics and solving:

• Go to Setting up Physics > select Energy > also go to Models > go to Viscous > Select k-omega sst model(Mesh refinement).
• Go to Boundary Conditions

At inlet,

Velocity=25m/s.

Temperature=300K.

• Go to Solution > select Hybrid initiallization.
• Select the reference values at the inlet for the calculation of drag and lift.
• Enter the number of iterations and press Calculate.

Case-2 (Baseline mesh -with two enclosures)

Mesh element size=0.1m

Inflation=5

Body Sizing=5.e-002m

Face sizing=5.e-003m

Total thickness=1.244e-002m

Nodes=55329

Elements=178408

Residuals :

Coefficient of drag:

• Coefficient of drag = 0.38

Coefficient of lift:

• Coefficient of lift = 0.262

Velocity Contour:

Pressure contour:

Velocity Vector Plot :

y+ wall function :

Chart:

• From the above resulted contours it is observed that the wake region is not much clear and accurate. It is clearly observed that the drag in this case doesnot match the required values. Hence, we can say mesh is also not correct. So we need to perform the mesh refinement to check the perfect mesh that is suitable for the problem.

Mesh Refinement :

Mesh refinement is bascially done to find out the finest mesh that fits the body by providing the accurate drag and lift values. It also depends on the software we are using as we have restrictions for the elements in student versions. On gradually varying the y+values we find the total thickness. Based on this, modifications are done for the mesh parameters i.e. bodysizing, face sizing, number of inflation layers, and the maximum thickness for the inflation(Total thickness). By selecting the correct value we can obtain the coefficient drag close to 0.33 in the max.refined case at velocity 25m/s. In this case K-omega method is used. Refinement is done till the maximum no of elements that can be performed in software version.

The Y+ value is used as a reference mark from which we can conclude if the grid is refined or it should be coarsened further. Y+ values are restricted between 30-100 so that we get optimum result. Hence we generate wall Yplus vs Position plot , extract data and understand the obtained result.

Case-3

Mesh element size=0.1m

Inflation=5

Body Sizing=5.e-002m

Face sizing=5.e-003m

Total thickness=1.244e-002m

Nodes=55329

Elements=178408

Residuals :

Coefficient of drag:

• Coefficient of drag = 0.37

Coefficient of lift:

• Coefficient of lift = 0.248

Velocity contour:

Pressure contour:

Vector representation of velocity:

y+ wall function :

Chart:

Case-4

Mesh element size=0.05m

Inflation=8

Body Sizing=4.e-002m

Face sizing=5.e-003m

Total thickness=1.504e-002m

Nodes=186460

Elements=411045

Residuals:

Coefficient of drag:

• Coefficient of drag = 0.34

Coefficient of lift:

• Coefficient of lift = 0.274

Velocity contour:

Pressure contour:

Vector representation of velocity:

y+ wall function :

Chart:

Results:

Case	Coefficient of drag	Coefficient of Lift
Case 1	0.44	0.275
Case 2	0.38	0.262
Case 3	0.37	0.248
Case 4	0.34	0.274

Conclusion:

• Grid dependancy test helped to find out the finest mesh that is suitable for the Ahmed body.
• The velocity contour and pressure contour plots for all the cases are plotted. Also the vector plot is generated to show the wake region at various refinements of mesh.
• From all the above cases it is found that k-omega model gave the accurate results when compared to the k-epsilon model but took more number of iterations to converge the solution.
• As the mesh becomes finer, the wake region is more clear. As the number of cells increases, the results obtained were more accurate.
• The velocity distribution seems to be accurate when the mesh becomes finer and finer. The computational time seems to be increasaed on refining the mesh.
• The value of coefficient of drag depends mostly on the Reynolds number and the y+ value for which the Total Thickness of the inflation layer has been calculated. Hence this helped to reduce the drag coefficient and achieve the required output.
• When the air flow towards the rare end, it leaves a low pressure turbulent region behind the vehicle which is called as wake region.Hence negative pressure is developed in the wake region.
• The more closer flow to the body results in low recircualtion zone and hence helps to reduce the effect of drag on the body and maximise the efficiency of the body.