[Nishanth]

Hey, I'm analysing an automobile on Ansys and it has a tail extension at the rear. The problem is I'm getting a flow separation if I take a side view and perform an analysis 2D but I'm not getting a flow separation if I perform my analysis 3D. What might be the reason for this?

Please view the attached images for more information.

P.S: I used k-omega for the analysis.

[vinerm]

A 2D planar essentially means planar, i.e., no variation of anything along the z-coordinate. In 3D, even if you maintain the assumption of no variation in geometry along the width, i.e., you maintain exactly same profile end-to-end going along the width, the end walls do not exist in 2D. The flow stays attached to these walls and make it different from 2D. And if the profile is not same, such as the case in real vehicles, then obviously, you don't expect the results to be same.

[LuckyTran]

I see separation for both 2D and 3D. Why are you saying the 3D one is not separated?

[Nishanth]

Originally Posted by vinerm

A 2D planar essentially means planar, i.e., no variation of anything along the z-coordinate. In 3D, even if you maintain the assumption of no variation in geometry along the width, i.e., you maintain exactly same profile end-to-end going along the width, the end walls do not exist in 2D. The flow stays attached to these walls and make it different from 2D. And if the profile is not same, such as the case in real vehicles, then obviously, you don't expect the results to be same.

But will there be so much difference? In 2D it is separating at the starting of the extension and in 3D it separates at the end of the extension!

[Nishanth]

Originally Posted by LuckyTran

I see separation for both 2D and 3D. Why are you saying the 3D one is not separated?

The first 2 images are 2D, the 3rd image (which has the whole body, Nose facing left) is 3D.

In the first 2 images, there is a separation at the starting of the tail extension but for the 3D there is no separation or slightly separates at the end.

[AlexanderZ]

you may make "planar" geometry in 3D to check the difference between 2D and 3D

extrude your cross section

[Nishanth]

Originally Posted by AlexanderZ

you may make "planar" geometry in 3D to check the difference between 2D and 3D

extrude your cross section

Please read the question properly!

[vinerm]

Yes, the difference would depend on many things, namely the mesh, the angle of the side walls (close to C pillar in case of a car or van) with respect to the vertical plane. As Alexandar suggested, if you want to compare, then create a mid-surface using Surface option in Fluent and plot the contours there. Assuming the mesh resolution is similar and both cases are well converged, the closest match with 2D would be at the central vertical plane of the vehicle.

[AlexanderZ]

Originally Posted by Nishanth

Please read the question properly!

thanks for advise

[NonStopEagle]

Originally Posted by Nishanth

Hey, I'm analysing an automobile on Ansys and it has a tail extension at the rear. The problem is I'm getting a flow separation if I take a side view and perform an analysis 2D but I'm not getting a flow separation if I perform my analysis 3D. What might be the reason for this?

Please view the attached images for more information.

P.S: I used k-omega for the analysis.

Nishanth, you cannot compare the results of a 2-D planar simulation with a 3-D simulation.
Others have already pointed this out, but let me try to explain it from a physical point of view.
When you carry out a 2-D planar simulation, you assume that the body that you are simulating does not end in the lateral direction, meaning, it extends to infinity.
The implication of this is that when a low pressure region is built aft of the body, only the flow above and below the body can try to fill the low pressure region. But the flow has a certain momentum and cannot immediately fill the low pressure regions. Hence you see a separation.
In the 3-D case, your body has a finite thickness. This means that the flow at the sides of the body also flushes inwards into the low pressure region at the aft part of the body.
Totally, you have more flow trying to flush into the low pressure wake (i.e. flow from above and below and flow from the sides) The leads to a lower separation. Which is evident in the contours you posted.
Of course as vinerm suggested the numerical results depend on a lot of different things. But considering that you've them them correctly, this is the physical explanation of the process.

Hope this answer was useful

NonStopEagle