Grid Dependency test of Ahmed body

Grid dependency test of ahmed body

Ahmed Body and its Importance:

The Ahmed body is a geometric shape first presented by Ahmed and Ramm in 1984. The shape gives a model to study geometric effects on the wakes of ground vehicles (like cars).This model is used to describe the turbulent flow field around a car-like geometry.It sets a bench-mark for new cfd codes and help to validate them.This can be done by comparing new cfd code results with that of previously established experimental data.

Aim: To perform the grid independency test.

Procedure for the analysis:

Step1: open spaceclaim and import the ahmed body.

Step 2: Create the enclosures at the given distances.In this cases it is

X- =0.5m

X+= 1m

Y-=0m

y+=0.5 m

Step 3: Making sure there is no interference between the enclosure.

Step 4:Open the meshing and performing named selection process as inlet ,outlet, symmetry walls and ahmed body.Use of face sizing ,body sizing and inflation layer to ensure that mesh is as refind as possible in the further cases starting from the base mesh. Thus grid dependency test is performed on the ahmed body.

Step 5: Open fluent and setup the simulation with k-epsilon model.run the simulation.

Step 6: In post processing setup a plane as contour for velocity.

Performing the simulation for Three cases they are:

Case 1: Standard meshing with default element size.Velocity profile using mid plane cut plane:

2) Residuals:

3)Velocity Vector Distribution Representation of velocity flow at 50 m/s:

4) Velocity graph Representation:

Case 2:

• With element size 100 mm.
• Body sizing: 50 mm.
• Face sizing: 15mm.
• and multi zone hexa type.
• * Statistics: Nodes: 103924
• Elements: 354956

2)Residuals:

Velocity Vector Distribution Representation:

Chart represtation:

Case 3:

In this case face sizing is further refined to 12mm.

• With element size 100 mm.
• Body sizing: 50 mm.
• Face sizing: 12mm.
• and multi zone hexa type.
• * Statistics: Nodes: 139021
• Elements: 474353

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Velocity contour:

Residuals:

The solution coverged at about 1250 iterations.

Velocity Vector Distribution Representation:

Chart Representation:

Overall Result:

In the first case i.e the basic mesh the results are less smooth and refined.In order to get good results ,The mesh is further refined in the next case by introducing body sizing ,face sizing ,Multi zone. The contours and residuals obtained in Case 2 sis more smooth and refined ,The vectors that are obtained supports this notion.In case 3 the mesh is further improved by reducing face sizing and introducing inflation layers by doing this ,the solution convereged at 1250 iterations and the solution obtained is satifactiory. It is smooth and refined.

Grid Dependency Test:

a)There are significant changes in the 3 cases.

b)The mesh is more refined from case 1 to case 3.

c)A smoother mesh gives the smoother and accurate solutions.

d)From case 3,It can be concluded that lower element size provide more defined flow.

e) Finer mesh provides ,easier visualisation of contours.

f) Mesh refinement provides closenness to the true values.The

Why the pressure is negative?

To answer this question, Let's see the pressure graph to what extent the pressure has gone negative.

The negative pressure is quiet below zero and it can be obsereved easily.It is represented as blue color.

This can happen due to following reasons.

1) The pressure goes low to maintain the flow when the velocity of the flow increases.

2)The pressure becomes negative due to recirculation flow of the fluid.

3)It can be the due to flow sepration at the curved surface interface of the body.Separation bubbles are produced because of flow seperation which results in negative pressure values.

4)To counter the sheer stress,pressure becomes negative.

5) Sometimes in the accelerated flow regions the pressure becomes negative.