Ahmed Body

Objective:

The main aim of this project is Simulating Ahmed Body.

• Describe Ahmed body and its importance
• In order to refine the mesh around the area of interest. Create an enclosure around it and refine the mesh locally. Choose appropriate dimensions for the second enclosure and use inflation layers around the body. Perform a grid dependency test on this setup by varying the mesh appropriately (You can choose your own values). Use three different setups containing different element sizes. Try to bring the final setup as close to the academic limit as possible. Plot the velocity profiles for the three setups using a midplane cut plane.
• In the post-processed results, explain the variation in pressure as to why it goes negative.

Describe Ahmed body and its importance:

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 Ahmed [1]. 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 invalidated, it is used to design new models of the car.

The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model while maintaining car-like geometry features. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, the height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees.

The front of the body is placed at a distance of 2 car lengths (2L) from the flow inlet. To reduce the computational cost, the asymmetry 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 in addition to the velocity field. 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.

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.

Geometry:

User Data:

Velocity: 50 m/s

Material: air

Wall-boundary: no-slip

Solver: Steady-state, Density-Based solver

Calculating Reynolds Number:

`Re = (rho * U * L)/mu`

`rho = 1.2 (kg)/m^3`

`mu = 1.85 * 10^-5 (kg)/(m-s)`

`Re = (1.2 * 50 * 0.288)/(1.85*10 ^-5)`

`Re = 960000`

Re number is very high so it is Turbulent flow and the first partice inside log-law region.

First Layer thickness:

`delta = (0.346*x)/(Re^0.2)`

`delta =`total thickness

`x = (delta * Re^0.2)/(0.346)`

x = 0.12M

2.

Base Mesh:

Using Ansys Base Mesh Regarding geometry size it will create automatically.

Velocity-Distribution:

Pressure distribution:

Case 1:

Element size: 50 MM, Base mesh: 50 MM and inflation layers: 5

• Nodes: 160150
• Elements: 288948

Mesh:

Residual Graph:

Velocity Distribution Diagram:

Pressure Distribution:

The velocity and pressure counter provide less accurate values, as compared to other cases. As per the above figure, the velocity counter gives how the velocity distributes around the body. As observing the velocity vector plot, before the body as well end of the body having low velocity and observing the pressure contour in front of the body having high pressure because the velocity is high at the same point.

Case 2:

Element size: 50 MM, Base Mesh: 40 MM and inflation layer: 5

• Nodes: 182303
• Elements: 407653

Mesh:

Residual plot:

Velocity Distribution:

Pressure distribution:

In this case, We are refining the mesh comparing to the first case. As per seen the above results will give more accurate values.

Drag Force:

As per the above figure, the fluid flows over a body developed a drag force at in front of the body developed high drag force due to shear stress and center of the body having low friction between the body and fluid.

Case 3:

Element size = 100 MM, Base Mesh = 33 MM and inflation layers= 5

• Nodes: 116963
• Elements: 504211

Mesh:

Residual plot:

Velocity Distribution:

Pressure Distribution:

Comparing velocity distribution from three cases:

Comparing the velocity distribution around the Ahmed body by three different kinds of Mesh. By using a line plot in CFD post processer predict the velocity at a certain distance from Ahmed body and mentioned below table.

Grid Dependency Test:

A mesh convergence test should mean computing the solution on successively finer grids. The difference between the two refinements is usually taken as a measure of the accuracy of the coarser of the two. (Note that unless you have proven that your scheme will converge there is no guarantee that either of the two is close to the solution.) It will never show independence unless the difference is of the order of machine zero. On the contrary, if two refinements hardly differ it means that there is no convergence and something is likely to be seriously wrong. Grid independent test appears to mean the same thing but the name is non-sensical since no, or at least very, very few, numerical entities are ever grid-independent.

• We have done simulation using three different cases as mentioned above.
• There are thee mesh refinement from case 1 to case 3 to check the grid dependency test.
• The third case gives smoother and more accurate values as mentioned above comparing table.
• When the element size increases it\'s required more time to simulate but it gives smoother values.
• When mesh refinement done it helps to get true values.

3. Why the pressure goes negative?

1. Thee pressure goes low where the velocity increases in the flow around the body.
2. At the end of the Ahmed body, we are having low pressure that causes to flow separation.
3. Pressure could also become negative to overcome the shear stress.
4. Due to flow recirculation, the pressure goes to negative.
5. Where the pressure negative, the flow gets accelerated.