Week 3 - External flow simulation over an Ahmed body.

Aim: External flow simulation over an Ahmed body.

Objective: The objective of this project is to determine the aerodynamic forces on the Ahmed body such as drag and lift coefficient and to perform the grid independence test.

The expected results will include

1. Velocity and pressure contours.

2. The drag coefficient plot for a refined case. ( For a velocity of 25m/sec, the drag coefficient should be around 0.33).

3. The vector plot clearly shows the wake region.

4. Perform the grid independence test and provide the values of drag and lift with each case.

Abstract

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, a height of 0.288 meters, and a width of 0.389 meters. It also has 50-millimeter cylindrical legs attached to the bottom of the body and the rear surface which has a slant that falls off 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 the `k-epsilon` model and for the later stage `k-omega`SST model is used.

Importance of Ahmed's 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, manufacturers are pressing for fuel-efficient cars. One of the main factors considering whether 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's 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 the `k-epsilon` 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 (413.6 millimeters).
• A named selection is created for the enclosure viz; inlet, outlet, symmetry.
• To create the named selection for the car the enclosure faces are hidden and the box select is selected from the meshing toolbar and the (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 `frac{m}{sec}` and `300` `K` temperature is 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`.
• The cut-plane-z is created along Ahmed's body.
• The velocity contour and pressure contour are created along the cut-plane-z.
• The animation video is created for the velocity contour.
• The simulation is executed for 1000 iterations.

Simulation with a finer mesh using `k-omega``SST`.

• The first simulation ran successfully.
• The geometry is loaded into SpaceClaim.
• The region of local refinement is created along Ahmed's body.
• As the body is perfectly symmetric, the simulation is executed 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.
• 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, and 80mm), the element size for the second enclosure will vary as (50mm, 45mm, and 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.

Simulation of Ahmed's body:

• The geometry of Ahmed's body is shown below,

• Load the Ahmed body model into SpaceClaim and convert the units from millimetres to meters.

• Creating the rectangular box around the body, As the body is perfectly symmetric, we can run the simulation by considering the only half body. once the enclosure is done, we can create another enclosure closely around Ahmed's body so that we can refine the mesh in that enclosure and still keep the total cell countless.

• In this meshing we can assign the named selection for Inlet, Outlet, Symmetry wall and Car Wall, We can use the different types of meshing processes for this model.

  Here we follow three stages,

  1. Meshing will be done for the outer enclosure chosen as a method of the Multizone and this will create the Hexa mesh, The element size used for this is 0.1m.

2. For the inner enclosure we choose the body sizing method to make more refinement, the element size we use for this is 0.032m.

3. Next we select the face sizing method to select the legs of the body to capture them with better refinement, we take the element length as 0.005m.

4. Then to capture the flow over the body we make the inflation layer which helps us in capturing the flow very finer way and the mesh will also be captured along the body perfectly. We take the number of inflation layers as 6, Total thickness value as 0.0036.

• Reference values:

SETUP :

Solver type: Density based

Check the Energy equation

Turbulent Model : K-Epsilon

Fluid Material: Air

Boundary conditions

Inlet velocity : 25 m/s

Outlet pressure : 0 pa

Symmetry: For side walls

Wall: Bottom,

Creating a surface plane to observe the contours.

• Checking mesh,

• Initialize the condition using a hybrid,

• Run the simulation for 800 iterations.

Results:

1. Residual plot:

• The plane along the z-axis gets the velocity and pressure variation.

2. Velocity contour plot:

Pressure contour plot:

• DRAG COEFFICIENT:

• LIFT COEFFICIENT :

• Velocity variation:

Result:

For,

Inlet velocity : 25 m/s

Outlet pressure : 0 pa

• Total no of Cells = 379416
• Total no of `C_d` = 0.376
• Total no of `C_l` = 0.246
• From the above table, 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.

Conclusion:

• The Ahmed body's frontal region experiences high pressure because of the air's loss of kinetic energy.
• Because of the flow separation caused by the model's geometry, an adverse pressure gradient is produced close to the wake regions. The pressure gradient changes as a result of flow separation, which in turn changes the drag and lift coefficients.
• Detachment of the fluid should occur farther from the body in order to reduce the size of the wake region, which can be accomplished by increasing the slant angle.
  The numerical results get closer to the experimental results as the mesh size is increased.