AHMED BODY CHALLENGE

Aim: To analyze the aerodynamic behavior of the Ahmed body by estimating the aerodynamic force coefficients of the body such as Cd and Cl.

Introduction: Ahmed body is used for analyzing necessary car aerodynamics such as efficient fuel consumption, emissions, noise, and good driving stability. It is a bluff body of a simple shape. This is used as a reference for various investigations. It has the following dimensions.

Fig 1: Geometry of the Ahmed body[1]

The behind of the Ahmed body is of major importance as it contributes to the majority of the drag. So, it is really important to estimate the coefficient of drag and drag force.

Geometry and computational domain:

Fig 2: Front view of Geometry

Fig 3: Computational domain

Meshing:

For this analysis, a baseline mesh along with 7 refinements has been done with the following features. Only baseline mesh and finest refinement of the meshings are included in this report for the reference.

Fig 4: Baseline mesh of the computational domain

Fig 5: Sectional view (parallel to XY plane) of the computational domain

Fig 6: Refined mesh of the computational domain (7th refinement)

Fig 7: Sectional view (Parallel to XY plane)of the computational domain (7th refinement)

Fig 8: Named selections of the domain

Fig 9: Car wall of the domain

The following are the various details of the mesh used in this simulation run.

Fig 10: Mesh details

Fluent Setup

Turbulent model : K-e realizable

Fluid : Air

Boundary conditions

Inlet velocity : 25 m/sec

Outlet pressure: 0 Pa

Symmetry : For sidewalls

Wall : Bottom

Every simulation was run for 300 iterations. Report definitions of Coefficient of drag (Cd), Drag force (Fd), Coefficient of lift (Cl), Lift force (Fl) are defined and plots are drawn during the run.

Results and discussions:

Velocity and pressure contours of a plane parallel to the XY plane were captured. Vector plots were also captured.

Following are the contours for the baseline mesh and the most refined case.

Fig 11: Velocity contours for baseline mesh

Fig 12: Velocity vectors around the wake region for baseline mesh

Fig 13: Pressure contours for the baseline mesh

Fig 14: Plot for the coefficient of drag vs the number of iterations

Fig 15: Velocity contours for the refined mesh (7th refinement)

Fig 16: Velocity vectors for the wake region for the refined mesh (7th refinement)

Fig 17: Pressure contours for the refined mesh (7th refinement)

Fig 18: Drag coefficient for the refined mesh (7th refinement)

Following table shows the tend of Drag coefficient vs the refinement

Fig 19: Coefficient of drag vs refinement

Fig 20: Plot for Cd vs refinement

Grid independency test

For grid independency Lift and drag, forces are calculated for each run. The following table represents the values of Lift force and Drag force.

Fig 21: Lift and Drag forces

Fig 22: Lift and Drag forces vs refinement

We can observe from the above plots that the Lift force converges to a value of 13 N, whereas Drag force converges to 15 N.

Negative pressure in the wake region

• Within the wake of the Ahmed body, due to high turbulence intensities, the mean velocities approach zero. A pair of vortices are developed in this region very strongly. These counter-rotating trailing vortices are responsible for the attached flow.
• When the flow separates at the lower edge of the slant, strong underpressures are created. This results in an increase in drag.

Significance of point of separation

• A flow separates as soon as it detaches from a surface into a wake. It occurs when the flow slows and there is an increase in the pressure after passing a bluff or streamlined body.
• The flow becomes detached when the speed of the boundary layer relative to the surface is zero and reverses its direction. This results in the formation of eddies or vortices. This causes an increase in pressure drag and a reduced lift.
• In the above simulation, we can observe at 80 mm downstream (x=1.124 m) from the trailing edge of the Ahmed body there is a large and strong recirculation towards the surface. This is reduced as it moves away (increase in the x-direction). At a distance of 500 mm from the trailing edge, we can observe the cores of the trailing vortices with a higher reduction in velocities compared to the vicinity.

Fig 23: Velocity contours of YZ plane at X=1.044m

Fig 24: Velocity contours of YZ plane at X=1.124 m

Fig 25: Velocity contours of YZ plane at X=1.224 m

Fig 26: Velocity contours of YZ plane at X=1.344 m

Fig 27: Velocity contours of YZ plane at X=1.444 m

Fig 28: Velocity contours of YZ plane at X=1.544 m

References:

[1] W.Meile et.al,(2011) Experiments and Numerical simulations on the aerodynamics of the Ahmed body

[2] H.Lienhart,C.Stoots,S.Becker, Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model)