Week 3 - External flow simulation over an Ahmed body.

Ahmed body: The Ahmed body was first created by S.R. Ahmed. It is an important benchmark for aerodynamic simulation tools. It will be clearly shown in the sketch below that it has a length of 1.044 meters, the height of 0.288 meters and the width of 0.389 meters. It also has a 50 millimeter cylindrical legs attached to the bottom of the body and the rear surface which has a slant that falls of at 35 degrees.

The flow for the model is turbulent which is based on velocity and body length. For the default mesh size in ANSYS-FLUENT we will use k-ε">k−ε model and for the later stage k-ω">k−ωSST model is used.

Importance of Ahmed body

The drag force is an important factor considering fuel consumption. As the drag force increases the fuel consumption also increases which in turn becomes expensive for the car owners. As the burning of fossil fuels becomes an issue, the manufacturers are pressing for fuel-efficient cars. One of the main factors considering the car is fuel-efficient or not is the car's geometry.

Complex shaped cars are very challenging to model and its difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools.

Negative pressure in the wake region

When the air moving over the vehicle is separated at the rear end, it leaves a large low-pressure turbulent region behind the vehicle which is known as the wake. This contributes to the formation of pressure drag which affects the vehicle performance.

When the vehicle is moving at a certain velocity the fluid imparts viscous force to the vehicle as long as it is in the viscous sublayer of the boundary. When the fluid reaches the rear end of the vehicle it gets detached from the surface of the body causing negative pressure in the wake region. The movement of the fluid around the vehicles depends on the geometry of the car and the Reynolds number.

Significance of point of separation

The point of separation will occur when there is a discontinuity in the surface or a region of the negative pressure gradient (a sharp end or rear portion of the car).

When the flow is attached to the body of the vehicle the viscous force will increase which will increase the pressure and a decrease in velocity before the rear end of the vehicle. As soon as the air encounters the sharp turn causing the velocity to decrease resulting in the negative pressure region which is in the wake region. The recirculation region happens beyond the point of separation.

If the rate of change of pressure is large the mixing process will not occur and the boundary layer flow stops.

The above figure shows that at the point of turning the flow gets separated from the boundary layer and there is a region of recirculation after which the flow attaches itself to the boundary layer.

In order to reduce the separation of fluid from the boundary layer, it is advisable to have a smooth edge to reduce the effect of drag on the vehicle.

Modeling approach

Baseline simulation approach using k-ε">k−ε model.

• Ahmed's body is loaded into the Spaceclaim.
• A single enclosure is drawn across Ahmed's body.
• Now the geometry is loaded for meshing.
• A baseline simulation is set up with the default element size of Ansys.
• A named selection is created for the enclosure viz; inlet, outlet, symmetry, car-wall.
• To create the named selection for the car the enclosure faces are hidden and box select is selected from the meshing toolbar and (car wall) name is given to it.
• Ansys Fluent window is opened and the geometry is loaded.
• Steady-state, the density-based solver is selected.
• Reference values are updated as per the prescribed conditions.
• In the inlet boundary condition, inlet velocity is 25 msec">msec and 300">300 K">K temperature are set.
• Drag-coefficient plot and lift-coefficient plot is created by setting the reference as (car walls).
• Hybrid initialization is selected.
• The solution is initialized by hitting t=0">t=0.
• The cut-plane-z is created along Ahmed's body.
• The velocity contour is created along the cut-plane-z.
• The animation video is created for the velocity contour.
• the simulation is executed for 500 iteration`

Simulation with a finer mesh using k-ω">k−ωSST">SST.

• The first simulation ran successfully.
• The geometry is loaded into the SpaceClaim.
• The region of local refinement is created along Ahmed's body.
• As the body is perfectly symmetric, the simulation is executed by considering the only half body. This is the best practice where we can save on the number of cells and get the results faster as well.
• The geometry is loaded for meshing. The multizone method is selected for the outer enclosure to provide hexahedral mesh which provides accurate results in simulation.
• The element size for the first enclosure will vary as (100mm, 90mm, 80mm), the element size for the second enclosure will vary as (50mm, 45mm, 40mm), the element size of the leg of Ahmed's body will be 5mm, the first layer thickness, inflation layer, and the maximum thickness will be varied accordingly to imitate the experimental results.
• The rest procedure is the same as above, the necessary contour plot and graphs have been uploaded respectively.

Preprocessing and the solver settings

Case-1: External flow simulation over Ahmed body by creating a single closure and using default mesh value of ANSYS-FLUENT

Turbulence model: k-ε">k−ε

Ahmed body model

Creating an enclosure across Ahmed body

Creating a plane section on the origin

The three planes are normal to the x,y,z axes respectively.

As the body is perfectly symmetric, we can simulate by considering only half the body. This is the best practice where we can save on the number of cells and get the results faster as well.

To get half of Ahmed's body, the section mode is selected in SpaceClaim and the front view plain is selected to yield the desired result accordingly.

Once the geometry part is complete, it is loaded in the meshing window to perform the baseline meshing.

Simulating with a default mesh size of Ansys-Fluent

Creating named selection to set up physics and boundary conditions.

Inlet boundary condition (velocity inlet).

Outlet boundary condition (pressure outlet)

Symmetry boundary condition

Creating named selection for car walls

To create the named selection for car walls we need to hide the faces of the enclosure by hitting function key f8.

Performing meshing with default mesh size in ANSYS-FLUENT

Number of elements: 84073

The geometry is cut into two halves by using a section plane

Thus the mesh is created inside Ahmed's body which is not good for simulation because the flow outside Ahmed's body is taken into account. It can be fixed by going back to Spaceclaim, one of the copies should be suppressed for physics.

The required geometry is then loaded into the ANSYS-FLUENT window.

Simulation procedure

Setting up of physics

Material properties

Viscous model

Hybrid initialization is selected and the solution is initialized by hitting t">t=">=0">0

Creating a cut plane to see the velocity change along Ahmed's body.

Creating contour plot along cut plane z

Results

Residual plot

Drag-coefficient plot

Lift-coefficient plot

Velocity contour

Drag-coefficient	0.3462
Lift-coefficient	0.2865

Case-2 (a) External flow simulation over Ahmed body with finer mesh in a double enclosure.

Turbulence model: k-ω">k−ω SST">SST.

First refinement

A region of local refinement is added to the Ahmed body.

Share Topology: It is set to share. It is used for sharing the information between the two enclosures and also to make the mesh uniform.

The geometry is loaded into the meshing window.

Meshing

The meshing involves the following steps

Method

Outside the box, the hexahedral mesh is used. The mesh is of the highest quality and gives higher accuracy.

Multizone method: It is used to create hexahedral meshes wherever possible and for other regions where different mesh types are used it will connect it properly.

Sizing: It is defined as increasing the number of elements by decreasing the element size. The element size of the second closure is 0.05">0.05m">m

Element size for the first enclosure: 0.1m

Maximum size for the first enclosure: 0.1m

Element size for the second enclosure: 0.05m

Element size of pillars of Ahmed body: 0.005m

Number of elements : 206067

Inflation layer: It is the process of adding layers to the boundary to cover the boundary layer thickness fully.

total thickness: 0.0125m

Residual plot

Drag-coefficient plot

Lift coefficient plot

Velocity contour

Drag coefficient	0.319
Lift coefficient	0.282

Case 2(b) : Second refinement

Turbulence model: k-ω">k−ω SST">SST.

The multizone was used for the second area. The number of element size for the outer enclosure is taken as 90">90 mm">mm whereas the element size for the inner closure is taken as 45">45 mm">mm. Face sizing was implemented for the pillars of Ahmed's body ( 5mm">5mm).

Inflation layer

Number of elements: 328398

Residual plot

Drag coefficient

Lift coefficient

Velocity contour

Drag coefficient	0.288
Lift coefficient	0.211

Conclusion

The baseline mesh gave a poor result, it wasn't able to capture the flow around the Ahmed body and the wake structure, due to coarse mesh.

The refined mesh gave better results. The plots show the wake formation accurately and the flow around the Ahmed body was also captured pretty well.

There is a variation of pressure across the Ahmed body. The pressure in front of the body is positive because that's the location where the flow first comes in contact with the body and comes to rest. At the corner, the flow accelerates, thereby decreasing the pressure.

The reason behind negative pressure in the wake region is due to flow separation, recirculating flow. It's a phenomenon that generally happens as the air moving past the body isn't able to pass through the immediate backside of the body creating a region, where there is no movement of air and thereby creating a pressure gradient.

As the wake flow is the main contributor to the drag force(pressure drag), the point of separation is an important aspect of the flow. The more attached the flow is to the body the recirculation zone will be smaller and hence reduce the adverse effect of pressure drag, improving the fuel efficiency of the body.