Project

Aerodynamic Simulation of Flow over an Ahmed Body

Ahmed Body & 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.
The Ahmed body represents a simplified, ground vehicle geometry of a bluff body type.
Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies.
The airflow around the Ahmed body captures the essential flow features around an automobile.

Geometry of Ahmed Body:

To simulate a flow over this geometry, we have to create a volume around the body.
For greater accuracy and better results two enclosures are created around the body.
The simulation of the flow will take place with in the enclosure.

The total geometry is split into symmetrical half.
The entire simulation is conducted on a single half with symmetry boundary condition.
This will reduce the time of simulation.

Name selection of the Geometry:

Inlet: Outlet:

Symmetry-Wall:

Car-Wall:

Computational Meshing:

as the outer enclosure contains another cubical enclosure inside, it can be meshed with multizone method for higher accuracy.
The inner enclosures are Meshed with acceptably smaller meshes, as you can see in the figure below.

For better calculations near the car wall, inflation layers are introduced.
These meshing form layers around the walls to generate the mesh more even for the flow to be simulated.

Grid independency test :

Let us select the best suitable Mesh by doing grid independency test .
The Drag & Lift values are taken for different mesh elements and compared with each other to get the best simulation mesh.
This is done by creating a parameter table in which different cases are calculated.

From this table we refined the mesh as small as possible to calculate the drag coefficient.
The refining is done upto the tendency of the system.
here we can see that the value of drag coefficient getting converged. It can be seen clearly from the graph below.

We can see that the coefficient of Drag is still converging.
But as the value is given as 0.33 in the question, we will be considering the respective mesh element.

Setup & Simulation:

The simulation is calculated based on density.
The material properties of air is taken as the flow material.
The inlet velocity is considered as 25 m/s as per the problem.
To take the value of drag coefficient, the reference values are changed in reference to the simulation setup.
The simulation is done for 1000 iterations.

Velocity Contour:

Pressure Contour:

Reason for Negative Pressure:

As the flow of the fluid hits the body, there is a decelaration in the flow.
Thus the flow tends to move along the curvy edges of the body.
This creates a high pressure at the center of the frontal area, and low pressure at the curvy edges, so that the flow takes place from high pressure to low pressure.
We can also see some negative pressure at the rear end of the body.
This happens when the flow reaches the end of the body & the momentum of the flow is disturebed.
The direction of the flow falls down creating a region of negative pressure.
This point of Downfall is called as Point of Seperation.

Point of seperation (or) Boundary layer seperation:

The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction.
The fluid exerts a constant pressure on the surface once it has separated instead of a continually increasing pressure if still attached.
flow separation results in reduced lift and increased pressure drag, caused by the pressure differential between the front and rear surfaces of the object.

Drag Coefficient:

Lift Coefficient:

Conclusion on drag & lift:

Wake Region:

The region of a decelerated fluid that arises behind the body around which the fluid flows and extends for some distance.
It is the boundary layer that has separated from the rear of the body.
the wave pattern on the water surface downstream of an object in a flow, or produced by a moving object (e.g. a ship), caused by density differences of the fluids above and below the free surface & Gravity.

Vector Plot :

As we can see that only some vector elements tend to bend into the wake region.
They tend to cover up the low pressure formed at the area.
The recirculating flow is formed immediately behind the Ahmed body, because of the viscosity accompanied by the flow seperation.

Ship work like this
in your own course.