Ahmed Body Challenge

Aim- To capture the flow at the wake region and perform the grid independency test and study the pressure and velocity characteristics

The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of S. R. Ahmed. Although it has a very simple shape, the Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model is validated, it is used to design new models of the car.

The Ahmed body helps study the recirculation regions formed behind a car and the effects of various other parameters on the automobile body.

Wake region - The region of recirculating flow or disturbed flow behind a moving or stationary blunt body, caused by viscosity, which may be accompanied by flow separation and turbulence.

Geometry -

For the fluid domain, the Ahmed body was enclosed in a rectangular enclosure.

The enclosure is 2 m upstream, 5 m downstream, 1 m above and 0.5 m on both the sides body.

To capture flow characteristics near the Ahmed body efficiently local mesh refinement with a smaller element size has to be created.

For that, we need to create a separate enclosure. The enclosure is 1 m downstream, 0.5 m in upstream and above and 0.2 m at the sides of the Ahmed body.

As the body is perfectly symmetric, the simulation can run by considering the only half body.

It will save time in element generation and in simulation.

Whole-body is cut by half along the XY plane.

Meshing-

Since the fluid domain has divided into 2 regions, The outer region is made of hex elements because this region is rectangular in shape so the creation of Hexa elements is easy and these elements have higher efficiency in computation.

The inner region consists of tria elements with 6 inflation layers on the surface of the Ahmed body to capture the drag coefficient and flow separation perfectly.

Element Size

Inner region - 50mm

Outer region - 100mm

Setup

The steady-state solver is used with K - omega turbulence model. the k-ω model predicts well near the wall better.

Inlet Velocity - 25m/s

Oulet Preesure - 0Pa (gauage preesure)

Default Air is selected as the working fluid.

Result

solution ran for 500 iterations.

At 200 iteration the solution got converged.

Cl-0.25220927

Cd-0.41265497

Velocity plot

Velocity Vector

Velocity Streamline

From the above velocity plot and velocity vector plot, it is visible that the flow separates from the surface and creating a highly turbulent region behind the cylinder called the wake

Flow separation detachment of a boundary layer from a surface, which leads to the formation of wake region.

Pressure contour

Pressure plot

Flow Separation occurred due to the adverse change in the pressure gradient. Front of the body has the positive pressure but at the rear end of the body has negative pressure.

Case II

In this case, the mesh gets finer to see is there any significant changes in our result

Similar procedure is used to mesh the model.

The outer region with Hexa elements and inner region with the body sizing method with inflation layers.

Element Size

Inner region - 40mm

Outer region - 80mm

7 inflation layers with first layer thickness 5mm and total thickness 15mm

Statistics

Setup

Similar Setup is used for this case also.

Results

At 300 iteration the solution got converged.

Cl-0.22720709

Cd-0.38566313

Pressure Contour

Pressure plot

Velocity plot

Vector Plot

Case III

Further fining the mesh for checking the possible change in results

Using the same type of meshing method used in both the earlier cases.

Element Size

Inner region - 30mm

Outer region - 60mm

7 inflation layers with first layer thickness 5mm and total thickness 15mm

Mesh Quality

Setup

Setup will similar to the earlier cases

Results

At 200 iteration the solution got converged.

Cl-0.25220927

Cd-0.41265497

Pressure Contour

Pressure Plot

Velocity Plot

Vector Plot

Conclusion

After reviewing all the above cases

When the flow is moving from the thick to thin part of the body then there will the adverse change in the pressure and flow detaches from the surface when it travelled in such pressure gradient. The velocity of the boundary layer relative to the surface has stopped and reversed the direction. The separated flow takes the forms of eddies and vortices.

That's why the recirculating is seen behind the Ahmed body. As the total no. of elements increases we see the wake region more defined with more no. of vectors formation which helps us to understand and study the behaviour of vectors in the wake region.

The flow hits the frontal area of Ahmed body creates a high-pressure zone which is visible in contours and pressure changes when there is a change in the profile of the surface, we can see that the pressure has gone negative in some areas. It is beacuse pressure in the wake remains close to separation-point-pressure since the eddies formed as a consequence cannot convert rotational kinetic energy into pressure head resulted in the negative pressure. The pressure even drops further at the corners of the Ahmed body which we see in above contours and flow behind the body is recirculating that's why there is a pressure drop behind the body.