Ahmed Body Challenge

AHMED BODY CFD SIMULATION IN ANSYS FLUENT

Objective - To simulate flow over Ahmed Body to observe the following:

• Velocity and pressure variation
• Drag & lift coefficient
• Wake region 
• Perform mesh independency test

Theory

Ahmed Body 

Ahmed body is a simplified vehicle model that has the capability to capture airflow features around a vehicle. 

Today's automotive industry goal is to reduce fuel consumption, drag & lift coefficient, noise, instabilities to achieve all these vehicle designs and aerodynamic studies have to be performed. All these studies can be performed first on a basic vehicle model like Ahmed Body so as to validate the numerical technique(method) on Ahmed Body. Once the numerical method is validated on Ahmed's body then proceed to actual vehicle geometry. As there are many experimental results present for Ahmed's body to validate our method. 

Geometry

The inner enclosure is made for mesh refinement near Ahmed boy.

Meshing

Half of the geometry will only be simulated so as to save computational time and cost.

Inflation layer is added at the car boundary/wall to better capture the flow near the boundary.

Multizone (Hexa mesh) method is used for meshing outer enclosure.

Mesh gets finer as one moves towards the car.

  Inflation layer at car wall

                               Name selection

                               Mesh metrics

Boundary conditions

Inlet: Veloctiy Inlet: 25 m/s

Outlet: Pressure outlet

Enclosure walls: Symmetry

Ahmed body: Wall

Solution

Air is used as the fluid

k-epsilon turbulence model

Hybrid Initialisation

Results

Coarse mesh

Number of elements= 89963

                               Residual

                             Drag coefficient = 0.43

              Lift coefficient shows solution convergence

                          Velocity contour

Medium mesh

Number of elements = 350858

                               Residual

                              Drag coefficient = 0.37

                              Lift coefficient = 0.29

                         Velocity contour

Fine mesh

Number of elements= 511705

                             Residual

                              Drag coefficient = 0.35

                              Lift coefficient = 0.29

                                Velocity contour

                            Pressure contour

                      Pressure at the car wall

                Streamline, vortex formation is seen in the wake region

                       Velocity vector plot showing the wake region

Conclusion

• Negative pressure in the wake region - As the airflow separates into two at the front side, creating high pressure there, which joins back again at the downstream flow where nothing is present creating wake region, vortex formation and therefore negative pressure is the wake region.
• The point of separation plays a vital role in deciding the amount of drag to be acted by the fluid due to the external aerodynamics which is also influenced by the vehicle slant side.
• Increased drag, means more fuel consumption i.e. less efficiency.
• With the mesh refinement, our drag coefficient value got improved as can be seen in the above drag plots.
• External aerodynamics is studied on the Ahmed body, a generic model representing a vehicle.
• Negative pressure & vortex formation in the wake region, mesh refinement, drag & lift coefficients, velocity & pressure distribution is obtained which is in comparison to experimental data available.