[mejahan]

Dear CFX Expert Community,
How can I quantitatively evaluate the recirculation zones in the flow domain in CFX-Post? I am not sure if the volume average of vorticity vector magnitude would help. I aim to promote the recirculation and stagnation zones in the flow by some geometry modifications, however, not sure what parameter/expression to look for or define.
Thanks.

[Gert-Jan]

I don't know it this helps, but what I usually do is to calculate how much fluid is flowing back, like in the example given in the picture.

There you see gas flowing from an inlet left to an outlet right (far away). Using the cross section, I can calculate how much gas is passing it. If this is 3, and only 1 is coming from the inlet, I know the amount that is flowing back equals 2.

Mostly, the goal is to reduce this recirculation ration as much as possible, depending on the application.

[ghorrocks]

This might be a bit academic, but it is an important principle: True, closed separations (with a separating streamline) only exist in 2D flows. 3D flows do not have separating streamlines (except in ideal cases). This means for 3D flows you need to resort to some form of arbitrary definition suitable for your case, such as Gert-Jan's recommendation. There are others, such as when the wall shear stress is zero in the flow direction.

So that means if your flow is 2D then you can work out the exact separation size by calculating the separating streamline and no approximations are required.

Also consider whether some other definition may be more useful for your application, such as the volume average of the standard deviation of the velocity in the flow direction.

[Gert-Jan]

in addition, my suggestion is also possible to do 3D. in Post, for evaluation in x-direction, just make 2 equations:

upos = max(0[m/s], Velocity u)
uneg = min(0[m/s], Velocity u)

Using the Calculator, evaluate these with areaInt(upos)@crosssection. Let your cross section cover your whole geometry. In this way, you can determine how much is going in positive x-direction and reverse in negative x-direction.

But as Glenn mentioned, be careful. Look what your cross section covers, where it is located, are many rericulations present, also in y and z-direction, etc.
Use it at your own risk. You need to determine yourself if these numbers make any sense. A skeptical attitude is desired. I just provide this approach as guideline.

Also, if you use DES or LES then this this method might troublesome because of all vortices. Then there will be better methods.

[mejahan]

Thank you Gert and Glenn for your comments and advice.
I should have provided more information about my problem. My research focuses on the design optimization of stents for the treatment of cerebral aneurysms. I am trying to promote recirculation and stagnation zones inside the aneurysm sac by the means of stents implanted at the neck of aneurysm (attached image). Since the flow is incompressible and steady, the net flow crossing the aneurysm neck is zero, therefore, I needed to define a condition to filter the reverse flow to calculate the mass flow into the aneurysm, as Gert suggested.
However, I would like to assess the effect of stent design modification in promoting the recirculation and stagnation zones inside the aneurysm sac. The flow is laminar 3D and there are multiple recirculation zones inside the aneurysm sac, however, I need to evaluate them quantitavely to show the hemodynamic improvements. I would appreciate to know your advice on this.
Glenn also mentioned about the Gert-Jan's recommendation, can you please provide me an article or reference to this method.
Thank you,

[Gert-Jan]

Use my approach. I think it is useful in your problem. Probably, there are multiple recirculations arond the wires of the stent, but these might be small. You have to judge for yourself.

It is possible to limit the cross section in a circular or rectangular way. This allows you to eliminate certain regions with recirculations that you do not want to include in your quantification. In this way, you might be able reduce the contribution of the small recirculations around the wires of the stent.

[ghorrocks]

Personally, I would recommend trying a whole bunch of different approaches on a few simulations and see which one is measuring what you think is important the best. I would not choose in advance and discount the rest.

So calculate Gert-Jan's approach, but also do some simple ones like volumeAve of the velocity standard deviation. Also have a look at some of high order velocity parameters like vorticity or curl - these might give you a measure of flow curvature or rotation and that might be useful as well.

So calculate them all, and any others which you think might be useful. Then you will be able to see what is useful and what is not.