Week 3 - External flow simulation over an Ahmed body.
week 3 challenege:
aim:
As the body is perfectly symmetric, we can run the simulation by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster as well.
The geometry which is provided with the challenge needs a modification which you should be able to do at this stage of a course. You need to use the 'split body' command in SpaceClaim to perform the operation and then you can use the symmetry boundary condition in fluent to perform the simulation.
Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.
introduction:
Ahmed body is a simplified vehicle body used for capturing basic yet charecteristic features seen in objects used in the automobile industry. It is used for testing new CFD solvers or testing one's ability to perform simulations as there is experimental data available about the ahmed body to compare simulation data to.
I will be performing an analysis on an Ahmed Body in ANSYS using the Fluent solver.
Geometry
This is the sketch drawing:
In this project, the angle of bend (φ">φ) is 20 degrees. The geometry can be imported from another CAD software or it can be created in ANSYS. I imported a .STEP file for this project in SpaceClaim.
Since this is a complex simulation and the ahmed body is symmetrical along the z axis, the geometry was split into two along the z axis using ANSYS's split tool.
This is the geometry after the spliting:
A wind tunnel has to be created around the ahmed body, there are multiple methods to achieve this, but the easiest is using the enclosure tool in ANSYS to generate the tunnel. The inlet of the wind tunnel is 2 metres from the front of the ahmed body, the outlet is 5 metres from the back of the body. The height and of the enclousure is 1 metre above the ahmed body while the width is 0.5m from the side of the body. The enclousure is checked for Interference and it is repaired as well.
Further enclousures may be created for mesh refinement.
This is the final geometry with three enclousures:
Make sure the original extrusion is suppressed before closing SpaceClaim.
Meshing
The base size is 0.1m. There are multiple mesh controls used to generate the mesh for this geometry, they are:
1. Mesh -> Inflation
This option is used to accurately capture the boundary layers on the surface of the body.
for an inlet velocity of 25m/s, the wall spacing is 0.42mm. ie. The size of the element on the ahmed body should be 0.42mm high. If the initial thickness is 0.42mm, number of layers are 5 for a growth rate of 1.2 => the total thickness is 12.44mm (recall geometric progession)
2. Method -> Multizone
This option is used to generate mesh in the outer region which is primarily hexahedral so we have used multizone method.
3. Sizing -> Body Sizing
This option is used to refine the larger inner enclousure of the wind tunnel. The increased size gives us more accurate results near the ahmed body. Element size is 50mm.
4. Sizing -> Face Sizing
This method is used to ensure that the legs of the ahmed body are circular and not polyhedral.
After all these controls are applied, this is the resultant base mesh:
Zooming in,
These are the mesh metrics:
As seen, most elements have their quality >0.6, hence it can be said that the mesh quality is satisfactory.
Setup
Boundaries
inlet - velocity inlet of 25m/s
outlet - pressure outlet
wall-car - wall (no-slip)
back, top and bottom - symmetry
symmetry zplane - symmetry.
Reference Values
The area was calculated using ANSYS Fluent, Results -> Reports -> Projected Areas:
This is the cross sectional area for the half ahmed body
The most important values here are: Area, Density, Length and Velocity.
Materials
The existing material(air) properties were used and were left unchanged.
We know, Reynold's Number
Re=ρ⋅u⋅D/μ
where, ρ is density (kg/m3), u is inlet freestream velocity (m/s), D is charesteristic length of the ahmed body (m) and μ is kinematic viscosity kg/(m.s)
In this case, μ=1.78⋅10-5kg/(m.s),D=1m,u=25m/sandρ=1.225kg/m3">μ=1.78⋅10−5kg/(m.s),D=1m,u=25m/sandρ=1.225kg/m3
Re=1.225⋅25⋅1/ (1.78⋅10−5 )⇒Re=1711467 or Re=1.7⋅106
Viscous
k-omega SST was used for all simulations for this project due to its excellence in solving external flows and wake regions.
Reports
Two reports were created.
1. Lift Coefficient of wall-car
2. Drag Coefficient of wall-car
Solution - Methods
Simulation of the base setup
The simulation ran for 100 iterations and the results are below:
Residuals
Drag Coefficient
The final value of drag coefficient is 17.9235, which is in agreement with experimental and simulation data.
Lift Coefficient
The final value of lift coefficient is 5.3005, the discrepancy is due to variation in reynold's number and low mesh resolution near some regions of the ahmed body.
Contours, Plots and Vectors
Note: All plots, contours and vectors were obtained from the finer mesh, due to its higher solution clarity.
1. Velocity contours
These were taken in CFD - Post.
2. Pressure Contour
These were taken in CFD - Post.
Zooming in,
There is a high pressure region at the front of the body due to how flowing air is acculumated at the front.
The pressure is negative in some regions, this is not an error in the simulation but rather how ANSYS and Fluent functions. The "pressure" displayed in ANSYS is actually gauge pressure, the gauge pressure is 0 when the total pressure is the atmospheric pressure. In this case, the "pressure" being negative is merely the pressure dropping below atmospheric in some regions.
3. Vector Plot
These were taken in CFD - Post. The background contour is the velocity contour.
Zooming in,
Switching the type of arrow to see the lines more clearly,
Two distinct regions can be spotted behind the ahmed body with two vortices (seen from this view) or recirculating regions behind the ahmed body.
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Grid Dependence Test
A Grid Dependence Test is used to ascertain whether the final values of the solution are dependent on the mesh or not. This is an important test to ensure the simulation results are correct.
In this test, the test will involve 2 different simulations - one where the mesh becomes more finer and other where the mesh becomes more coarser.
fine Mesh
These are mesh statistics:
The values of drag and lift coefficient are below:
Lift Coefficient: 5.4791
Drag Coefficient: 17.1220
even Finer Mesh:
This is the region seen in ANSYS:
Other simulation parameters were kept the same as the baseline simulation and the simulation was ran:
The values of drag and lift coefficient are below:
Lift Coefficient: 5.7413
Drag Coefficient: 17.1220
Summary of Grid Dependence Test
| Lift Coefficient | Drag Coefficient | Nodes | Elements | |
| base mesh | 5.3005 | 17.9235 | 44495 | 143437 |
| fine Mesh | 5.4791 | 17.1220 | 134759 | 226667 |
| even Finer Mesh | 5.7413 | 17.1220 | 228978 | 315050 |
The important takeaway is that drag coeffcient does not vary significantly when the mesh is made more finer. While the drag coefficient does vary when the mesh is made more coarse. This means that the grid dependence test has passed.
Conclusion and Observations
- The simulation of the Ahmed Body runs well and is in some agreement with experimental and empirical data on the body.
- There are regions behind the body where wakes are formed, circling regions of low velocity which induce a considerable amount of velocity drag on the body.
- The values of drag coefficient although there is limited data for a slant angle of 20 degrees for the Ahmed Body.
- The values of lift coefficient agree with some sources and disagree with others, this discrepancy is due to coarser mesh around the botton region of the ahmed body.
- Grid Depedence Test was conducted and the test was succesful, an additional observation was also made - the drag coefficient decreases as number of elements increases. This decrease is very small and may be considered insignificant.
=============thank you=============================