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saisanthoshm88 March 25, 2013 16:11

[CFD Post] - Surface and 3D streamlines difference
Starting from the same point I've plotted a surface stream line and a 3D stream line and found them different -

These are the steps I followed -

3D Stream line-

Inserted an arbitary point (Say Point P) in the domain and plotted a 3D streamline starting from this point in the forward and backward directions.

Surface stream line -

Inserted a XY Plane with the Z-location as the Z-coordinate of Point P.

With such plane as the chosen surface and the Point P itself as the location to start from, I plotted a surface streamline in the forward and backward directions.

I found that the 3D stream line and the surface stream line thus generated are not the same.

Could some one please clarify upon this.

ghorrocks March 25, 2013 18:16

Of course they are different. Surface streamlines are constrained to stay on the surface, but 3D streamlines can go where ever they want. So surface streamlines are only influenced by the flow adjacent to the surface, but 3D streamlines are influenced by the entire domain.

oj.bulmer March 26, 2013 05:52

Unless the simulation is 2D, often the surface streamlines seem to be "cut" or broken. Hence, even if you want to see streamlines on (or near) a particular plane/surface, it makes sense just to create a line in plane/surface towards inlet side and start a 3D streamlines from there. Since they are 3D, they remain complete and not broken when they wander out of plane, at the same time; revealing the physics near the surface of interest - which is the objective.


saisanthoshm88 March 26, 2013 06:04

It is intended to be a 2D simulation , however with CFX it is always a 2.5D

oj.bulmer March 26, 2013 06:43

Indeed, it is represented as 2.5D given the limitation of software, but since the intention is 2D, the variations in third direction (third perpendicular axis or theta direction for axisymmetry) should be zero. Incidently, the streamlines on surface are likelier to remain on the surface, unlike actual 3D case, where they wander off the plane in third direction. Hence surface streamlines are usable here.

This of course may not be true in case of swirling axisymmetric case.


saisanthoshm88 March 29, 2013 16:58

Even though the thickness added in CFX for a 2D planar problem is (should be) of a negligible dimension does it not mean that the flow varies with in such negligible thickness.

Therefore I have the confusion in choosing between 3D and surface stream lines for a 2D simulation result (either from CFX or Fluent).

For the 3D and surface stream lines look different.

Though Fluent solves the 2D problem by a 2D solver, when I load the .cas , .dat files into CFDPost for post processing then a thickness gets automatically represented in CFDPost so I again have this confusion between 3D or surface stream lines to choose from.

ghorrocks April 1, 2013 18:43

If you have a 2D simulation you should draw surface streamlines on the symmetry surfaces to avoid problems with the small deviations which pull the streamline off the surface.

saisanthoshm88 April 1, 2013 19:31

Hi Glenn. Thanks for your response but doesn't the flow vary with in the negligible thickness that is extruded out to solve a 2d planar problem in cfx.

oj.bulmer April 2, 2013 06:05

The symmetry enforces any gradients in normal direction to it as zero, meaning, it doesn't allow any change in velocities perpendicular to the symmetry plane. This happens at both symmetry surfaces which are very close to each other and hence influence the flow between them to ensure the flow remains parallel to them. Hence, there is (ideally) no deviation in the direction perpendicular to symmetry.

If you observe change it may as well be a numerical artifact and a result of a bad mesh, extreme boundary conditions or number of factors, but it shouldn't be any strong to have any major bearing on the flow.

Logically, it doesn't make sense to have a 3D streamline in 2D simulation since if you see it is different than surface streamline, aren't we actually antithesizing our earlier assumption of 2D? :)


ghorrocks April 2, 2013 18:40

Yes, Saisanthosh you comment does not make sense. If you constrain the streamlines to the symmetry planes then you reduce the integration errors from noise in the third dimension. So 2D models should use surface streamlines on a symmetry plane to reduce this error.

elaf_2003 September 23, 2014 09:10

CFD post
Hi mates
I would appreciate if some body can help me to about how I can
starting CFD post[/B] in comment lines and export data.

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