Week 3 - External flow simulation over an Ahmed body.

Ahmed body

Ahmed body has a profile of a significantly integrated automotive. It has a configurable rear end with an upper slanted surface, similar to a "hatchback" vehicle, and a blunt nose with rounded edges attached to a middle portion with a box-like appearance. The model is supported by circular-sectioned legs or platforms rather than wheels. Despite ignoring a number of details of an actual vehicle, the Ahmed body produces the key aspects of flow around a car, namely: flow impingement and displacement around the nose, relatively uniform flow around the middle, and flow separation and wake generation at the rear.

            Ahmed body is important because its results are available easily because of a lot of experimentation that is done over it. So it will give a lot of data to validate the simulation results.

Reason for the negative pressure in the wake region

The pressure is negative at the wake region because of the relative pressure that drops in this region due to lack of regular streamlined velocity vectors that’s the cause of air to pass from them without interacting wake region, which generated somewhat vacuum and then due to that air came their and wake region generated.

Significance of the point of separation

The separation point is where a component flowing via the air ceases "sticking" to the air. Pressure drag is the term for the force that air molecules exert against an object as it moves through the atmosphere.

 

Introduction:

 

In the field of fluid dynamics where predicting the right results with right assumptions is a really big deal. For resolving this CFD is a tool that is doing the same via solving several governing equations like conservation of momentum – Navier stokes equation, conservation of energy etc. at the same time. It is a sub-part of fluid dynamics that visualise the flow of fluid and its relative physics. So, we are solving the fluid challenge of Ahmed body with Ansys software by starting from pre-processing and ends with post-processing by doing solving in between both of them. This is a problem that is basically based on the analysis of the velocity and flow pattern of the fluid over the Ahmed body. On the basis of the velocities of the fluid flow and the pattern that it follows around the geometry, we have to validate the same with present research data from several research papers.

 

Procedure

To get the desirable results we go through 3 major steps in the same sequel and based on the steps we followed during whole setup we got the results.

1. First of all, open the workbench
2. Drag the Fluid Flow (fluent) part in the Project schematic window
3. So, via going through Geometry option open the Space claim
4. Go to the open – new – go to geometry location – select open command
5. By following step 4 we will able to open the geometry in the space claim
6. Now choose the required volume from the whole geometry to do further process efficiently.
7. Check for any overlaps and the intersections that cause problem in the meshing.
8. After verifying the model and then close the window.
9. Now open Ansys Meshing and let the geometry upload.
10. Then firstly name all the parts of the model.
11. Then from the Tree right click on mesh and tap "Generate mesh"
12. Set the sizing of the meshing from its detail and then repeat the step 11.
13. After verifying the mesh, close the Ansys Meshing window.
14. Now open Ansys fluent window by clicking over ‘Setup’ and let the geometry upload.
15. For verifying the check click of ‘Checks’ option and see the results in console.
16. Go to the General and set the case accordingly as mentioned in the challenge
17. As we are dealing with temperature so mark the Energy option and chose the viscous flow option to define the fluid flow.
18. After this set the Boundary conditions.
19. Generated the graphs that we want to observe during the case run.
20. With this go to the results and create a plan in from the centre of the body and allow to show the velocity contour pattern over it during the case run time.
21. In addition to this, from ‘solving’ section also select animation for the same velocity contour.
22. The initialize the case
23. Set the total no. of iteration.
24. Click calculate to make simulation run
25. As the simulation completed close the tab and do the postprocessing as analyse the contours and plots.
26. For this select the plane slice that is situated in the centre of the geometry and over it we have to perform the contour analysis.
27. Close the tab after extracting useful results.
28. Save the case and cut the software.

Results:

 

• Velocity and pressure contours

The plots and the contours of the velocity and the pressure over the Ahmed body  at 25m/s inlet velocity of the air is placed in the attached word file. As the plot of the velocity is observed over a line which is at the 2m distance after the body. After plotting the graph, it is observed that in the starting phase the flow is highly unstable and having high fluctuation but that was stabilised also and showing the same velocity at which the air is entering form the inlet. On the other hand, we take normal and vector contour to analyse the flow thoroughly and we that the high resistance due the Ahmed body at the centre of the frontal side the velocity is minimum because of no way to fluid flow and on the other side at the edges it is maximum because all central blocked flow escapes from the edges only. Other then this due to the not much air-dynamics friendly body shape the rear portion feels a high pressure drop and due to that the air for some vortex kind of pattern which pulls the body backword. In addition to this the velocity of the flow make some streamline contour in the back that stabilised at 25 m/s of velocity after a certain distance. Velocity at the corners is seen maximum. Just opposite to that pressure is maximum at the frontal portion of the body due to resistance and minimum at the body-corners due to relative pressure drop.

• Coefficient of drag:

As the plot of coefficient of drag is attached in the word file it is coming to 0.31 and it is totally depends on the body shape, relative velocity, etc.

• Wake region:

This is the area where the flow gets disrupted (often turbulent) and is brought on by the movement of the fluid around Ahmed body. The relative portion where the maximum wake is observed in attached in the same word file. As the pressure drop is there in the back side of the solid relative to the vectors near to the section that cause turbulence in the section and as in the frontal side the fluid path is screened that cause resistance. Due to both of the phenomenon wake region is generated near to the Ahmed body. This can see clearly in the word file.

• Mesh Independent Study:

As I do the mesh independent study for the Ahmed body with current mesh size and some coarser mesh size and the result is following:

Parameter	Cell count	Coefficient of drag
Case 1	161186	0.334
Case 2	350012	0.321
Case 3	483012	0.318

 

Conclusion:

The post-CFD results revealed inaccurate results in the region of mesh overlap based on a baseline simulation. The best findings, which closely match the experimental data, come from the grid independence test. The error is greatly diminished by adjusting the mesh size with the y+ component. To provide an accurate forecast of velocity and pressure contour plots, the y+ is selected within a stable range. As y+ is raised, flow separation and the presence of swirls become more pronounced. Although no wake formation is correctly captured during baseline meshing, the flow is properly captured after mesh refinement.

Word file with all snapshots is attached below.

Thankyou

Anubhav