## Ahmed Body Challenge

1. Describe about Ahmed body and its importance

•  The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools.
• Ahmed Body had some practical features relevant to automobile bodies.

Fig.1   Basic Dimensions of the Ahmed Body

• The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.05 meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.
• The flow of air around Ahmed Body captures the essential flow features around an atuomobile.
• This model describes how to calculate the turbulent flow field around a simple car-like geometry using the Turbulent flow, k-epsilon interface.
• Numerical results obtained using Ahmed Body were compared with the Analytical results and its calculation time is less.

### Modeling Airflow Over an Ahmed Body:

•   The verification Ahmed Body model has 25-degree slant and is          placed in the following domain, measuring 8.532 x 2.088 x 2.088      metres, to compute flow field.

Fig.2 Computational domain and boundary conditions for the fluid flow simulation.

• The front of the body is placed at a distance of 2 car lengths (2L) from the flow inlet. To reduce the computational cost, a symmetry plane is introduced to model half of the model.
• The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity.
• The simulation solves for the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields. For this simulation, we need a larger mesh size than what is usually common to resolve the turbulent flow. More specifically, we use a finer mesh downstream of the model to capture the wake zone.

CASE:1

• Element size = 300 mm
• Number of Nodes = 29947
• Number of Elements = 99667

Simulation Details for 3 cases:

Type of Model: k-epsilon (Turbulence Model)

Type of Fluid : Air

Inlet velocity : 50 m/s

Outlet pressure : 1 atm

Scaled Residuals

Drag coefficient

Lift coefficient

Moment coefficient

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

The velocity and pressure contours are less smoother than compared to other two cases, the computation time is less for this case and gives  less accurate nearby values. This kind of less redefined mesh can be used as a baseline mesh to check whether everything is working or not

CASE:2

• Element size = 250 mm
• Number of Nodes = 37239
• Number of Elements = 136520

Scaled Residuals

Drag coefficient

Lift coefficient

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

The velocity and pressure contours are less smoother as compared with case 3 but they are smoother than case 1.

CASE:3

• Element size = 200 mm
• Number of Nodes = 55828
• Number of Elements = 227284

Scaled Residuals

Drag coefficient

Lift coefficient

contours of velocity at symmetry

contours of velocity with AhmedBody at symmetry

velocity pathlines

velocity vectors

contours of pressure at symmetry

Pressure Distribution on Ahmed Body

The computation time is more, but the results are more accurate and closer to the true values.

Grid Dependency Test:

• Mesh is more refined from case 1 to case 3.
• A smoother mesh provides a smoother and more accurate values.
• The flow is more defined at lower element size i.e, at case 3
• Mesh refinement increases the closeness of values to the true values.

3. In the post processed results, explain the variation in pressure as to why it goes negative

• The flow around a body has maximum velocity, where as the pressure becomes negative to maintain the flow, but at the front tip of the Ahmed body has least velocity which inturn increase of pressure takes place.
• The pressure drag results depends on velocity, pressure and vorticity field, pressure becomes negative because of velocity and vortices.
• The separation of flow from the curved surface interface of the Ahmed body results in negative pressure.
• The flow gets accelerated at low pressure regions.
• The pressure turns negative to counter balance the effect of shear stress.

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