Ahmed body steady state simulation
I. Introduction
The main objectives of automotive aerodynamics are reducing of drag, minimizing noise emission, improving fuel economy and eradicating undesired lift forces and other causes of aerodynamic instability which can arise at high speeds.
The generic ahmed body reference model has been chosen as the benchmark for carrying out computations for studying of the aerodynamic parameters.It has similar features like car and is widely used for validation of new codes in automotive industry.
The simple geometrical shape has a length of 1.044 m, height of 0.288m and a width of 0.389 m. It has also 0.5m radii cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.Ahmed body represents the simplified ground vehicle geometry of a bluff body type. It\'s shape is simple enough for accurate flow simulation and retains some important practical features relavant to automobiles.This model helps us to create turbulent flow field around the simple car using k-epsilon model.Despite neglecting number of features of a real car( rotating wheels, rough underside, surface projections etc), the Ahmed generates the essential features of flow round a car such as flow impingement and the sisplacement around the nose, relatively uniform flow around the middle and flow separation and wake generation at the rear.
As Ahmed body is simple enough to model, it can be easily used for studying various properties like drag coefficient, turbulence, lift forces,wake region, velocity magnitude at different regions of the car, pressure magnitude around the car, which can help in deciding what will be resistance, fuel efficiency etc of the car.
II. Study Objective.
To simulate the turbulent flow in and around the wind tunnel of the ahmed body and around the ahmed body to better capture the flow in the rear and in the wake region.To do local refinement of mesh inside the small enclosure and give body sizings around the ahmed body for the refinement to get well defined plots by post processing velocity and pressure contours & perform grid dependency test.
III. Geometry Creation.
In order to perform grid dependency test simulations on ahmed body was carried out in 3 different cases.
The larger and the smaller enclosure is developed in Ansys spaceclaim of appropriate dimensions which are given to capture the flow around the Ahmed body.The smaller enclosure is made for local refinement of Mesh.A check for Interference is done and its eliminated and it's topology is set to share.
IV.Meshing
The inner facing of the wall body of the Ahmed body(14 faces) are meshed further and an inflation layer is created outside the body to capture the outer bpoundaries data accurately with a clean shape of mesh.For the same reason face sizing of the legs are also given.To perform the grid dependency further refinement is done with the further cases.
1. Case A
Mesh details:
- Main enclosure - 200 mm
- Small enclosure - 70 mm
- Body sizing of the Ahmed body - 20mm
- Face sizing of leg - 5mm
- Inflation: First Layer thickness: 5 layers(Max) : First layer height- 0.57mm
- 20% Growth
- Elements - 119379
- Nodes - 34053
Cut section mesh display
Inflation layers which is negligibly noticeable because of first layer height set to 0.57mm
Legs
2. Case B
For the same geometry we further refine the mesh.
Mesh Details:
- Main enclosure - 100mm - Multizone(Hexa)(more refined than the previous case)
- Small enclosure - 70mm
- Face sizing of the Ahmed body - 20mm
- Face sizing of the legs of the ahmed body - 5mm
- Inflation: Total thickness : 5 layers - 12.44mm
- 20% Growth
- Elements - 143713
- Nodes - 70118
Cut section mesh display
Meshing inner enclosure to 70mm
Presume mode.
Face sizing of legs refined to 5mm.
3. Case C
Mesh details
- Main enclosure - 90 mm - Multizone(Hexa)
- Small enclosure - 50 mm
- Face sizing of the Ahmed body - 10mm
- Face sizing of the legs of the Ahmed body - 5mm
- Inflation: Total thickness : 8 layers- 35mm
- 20% Growth
- Elements - 206443
- Nodes - 71772
Meshing after applying hexa-Multizone.
Inflation layers.
Legs.
Final refined mesh.
V. Case Setup
- Type - Density based.
- Velocity Formulation- Absolute.
- Time - steady
- Energy : Viscous model (turbulence model):k-epsilon(2 eqn)
- Inlet velocity - 25 m/s.
- Outlet : Gauge pressure - 0 pascal(1 atm)
- Hybrid Initialization
- Material: Fluid-air
- Density - 1.225 kg/`m^3`(constant)
- Viscosity - 1.7894e-05 kg/m-s(constant)
- Lower wall - Wall Boundary condition
- Side wall - Symmetry condition
- Inlet, Outlet as other B.Cs
- Plane creation to capture velocity contour and further animation is done.
VI. Post Processing the results
1. Case A
Residual plot:
The solution converged at 308 iterations.
Drift coefficient plot.
`C_d` value coming at a value of 0.49 which is more or less close to 0.33.
Velocity plot:
The yellowish to orange region is of velocity ranging from 22.5 to 26/27 m/s while the lower wall captures the Boundary layer starting from nearly 0 velocity.
Velocity vector plot.
Velocity Streamline plot
The wake region Boundary layer separation, lift at the slant and the eddies generated can be clearly observed in this plot.
Pressure plot
Ahmed body pressure plot.
Animation:
2. Case B
Residual plot:
The solution converges at around 264 iterations.
Velocity plot:
The green area corresponds to 23.6 m/s.The blue area shows low velocity and high pressure region.
We can see the wake region more clearly in this refined plot.
Velocity Vector plot.
Pressure plot of the Ahmed body
Pressure plot of the XY plane.
3. Case C
Residual plot:
The solution converges at around 525 iterations.
Drag coefficient vs iteration plot.
Velocity plot:
Streamlines velocity:
Vector plot:
We can observe that the vector field lines are much smaller in the wake region but the vector lines are more in number than the previous case.
Pressure plot:
Animation:
VII. Velocity distribution at probes and Grid Dependency Test.
For all the cases probes are created at fixed locations.The yellow vertical lines are the probes.
Case A.
We can see the velocity at the front of the Ahmed body increases from 0 to 18m/s and then again decreases near the stagnation zone to a velocity of 8m/s and then increasing again.
Similar patterns are observed with the other 3 in the wake region, but depending on the probe location, the closer the probe to the centre of the wake region(:1.11m) the lower the velocity has dropped and at later probe locations the velocity drop decreases, such that the probe on 1.22m and 1.38m the velocity is not negative.The velocity did not drop at the probe location of 1.38 m in the wake region.
Case B.
Similar velocity profiles can be observed at a finer mesh and this time the velocity at the probe location of 1.22m slightly drops to negative velocity and the velocity drop is very little in this case for the probe location of 1.38m in the wake region.
Case C.
In this case it can be observed that the wake region expanded a little with the velocity drop at the first two probe locations(1.11m and 1.22m).Here in this case like the other two the velocity drop was quite to a larger extent than the previous two case for the probe location at 1.38m.
VIII. Grid Dependency conclusion
The grid dependency test allows us to see smoother velocity profile. As we make the mesh more finer i.e when we increase the cell counts(closer to the limit), we can see from the vector plots from all the 3 cases, of the velocity that it becomes more and more smoother.One thing is observable that making the mesh more finer lets us see the wake region more prominent.
From the 3 cases, we can conclude that the flow is more defined at lower element size i.e at case 3.The finer the mesh gets, the easier it gets in visualizing the contours.Mesh refinement increases the closeness of the resulting values to the true values.
IX. Why pressure gets negative.
The pressure goes low(negative) to maintain the flow when there is an increase in the velocity of the flow.It can be the result of flow separation at the curved surface interface of the body.Whenever the velocity increases the pressure around that area is decreased.The negative pressure in the Ahmed body is due to the flow around the bluff bodies.The Boundary layer separation and recirculating of the flow and also the effect of shear stress are the causes.To overcome the shear stress, pressure become negative to turn down the effect.
Separation bubbles are produced due to flow separation which results in negative pressure.As the air continues to flow and makes its way to the rear, a notch is created by the rear slant owing to the flow separation, leaving a vaccum of low pressure or (-ve) pressure where the air is not able to fill properly.
The wake region increases the thurst force.It caused the body which showed in the vector plot and the drag which extends at the rear end of the body.This makes the backflow which travels from high to low pressure region resulting in vortex formation in the wake region resulting in negative pressure. The flow becomes accelerated in those regions of negative pressure.
X. Conclusion
Drag coefficient for case A is, `C_d`~ 0.49 and formation of wake region and recirculation od velocity profile is observed slightly from the vector plot.For the Case C simulation, drag coefficient was around,`C_d`~ 0.23 and formation of recirculation region was better than the case A simulation. The wake region is more prominent in both Case B and Case C.