[amin.z]

Hi everybody

Here is a sentence that doesn't make any sense to me.

Streamlines drawn by fluent are averaged velocity streams since strictly a streamline is for laminar flow.

what do you think? Do you agree with this? I cannot figure out what's the reason limiting us in turbulent flows.

[LuckyTran]

I mostly agree. It helps to understand the differences between a streamline, streakline, and pathline

Streamlines are the family of curves defined by the tangent field to the instantaneous velocity field. In a turbulent flow field, the velocity field changes with space-time due to turbulence. The instantaneous tangent field therefore changes with time and hence the streamlines must be computed with this temporally changing velocity fields.



In RANS, you do not solve for the instantaneous velocity field but only the mean fields. The resulting "streamlines" that you calculate do not fit the original definition of streamlines above.


I agree that the streamlines you calculate with a RANS model are technically not streamlines, but the laminar-equivalent streamline.


The next part is whether it makes any sense to talk about streamlines in a turbulent field. Because of turbulence, the instantaneous velocity field is chaotic in space. Thus, it is highly improbable that you could assemble a bunch of smooth curves (lines) from a random tangent field. That is, given an instantaneous velocity field, you can always find the local tangents. However, you would not be able to draw lines connecting these tangents. So you do not satisfy the earlier definition the streamlines are the family of curves because they aren't smooth curves!

[amin.z]

Good point. Got it, that's thoroughly right for RANS models.
Thanks for your comment