[Dani125]

Dear CFX users,

I am trying to visualize the vortex core structure through iso-surfaces of the q-criterion but I'm not able to visualize it properly. The goal is to observe the vortex core region inside a volume. I would like to visualize the core vortex like the picture in attachment (cf. Figure 1). What I was able to achieve is the vortex structure that you can see in Figure 2.

Can someone help me obtain a proper vortex core iso-surface that allows me to visualize the vortex core region inside the volume ?

Screenshot.png

Screenshot2.jpg

[ghorrocks]

What is wrong with what you have already done?

[Dani125]

I would like to evidence the direction of the vortex core from the center of the volume until the wall. It allows me to show that the vortex core does not follow a straight line inside the volume. As you can see on the following figure (attachment) the location of the vortex is well represented by the iso-surfaces but it should follow the yellow line. I would like to show that the vortex core follows the yellow line

[ghorrocks]

There are many ways of defining a vortex core - there is no universal definition. Have you had a look at the various options and read the literature to see how others have done it?

Alternately, do you know the mathematical function you want to plot?

[Dani125]

I have tried different methods available on CFD-Post and I have also looked in the literature and the most commonly used is the Q-criterion. In fact the first picture is extracted from a paper.

Here is the link : https://www.sciencedirect.com/scienc...21929015000998

I know the mathematical function of the Q-criterion but it is already implemented in CFD-Post. I'm not able to show it the way I want.

[ghorrocks]

Have you adjusted the isosurface value of the Q-criterion? What happens if you adjust it up and down?

[Dani125]

Yes I did. The first picture is level down and second picture is level up. The vortex core is still not well defined. When I decrease the value the curvature of the vortex center line is more visible but the volume covered by the isosurface is too big.
Capture2.jpg

Capture3.PNG

[ghorrocks]

And are you sure the vortex core actually is well defined? If your simulation has not given a well defined vortex core you cannot display one.

[Dani125]

Yes it should be well defined based on the first figure (velocity fields) and the second figure (streamlines and vortex core in black). On the second figure, based on the streamlines, you can see that the iso-surfaces can be reduced to a smaller vortex core.
Capture4.jpg

Capture5.jpg

[ghorrocks]

Have you looked at other ways of defining the vortex core, such as pressure or vorticity?

[Dani125]

I tried different mathematical expressions but the Q-criterion seems to be the more adequate. I even tried to identifies the vortex core by displaying iso-surfaces with different values of pressure and vorticity but it didn't get better than de Q-criterion.
By showing the vortex core my goal is to help the visualization of the flow inside the volume but it's not the main reason of my work. Therefore I won't dig too far in the mathematical expression of the vortex. I was just wondering if through Ansys it would have been possible in my case to quickly get a fairly accurate representation of the vortex core.
Thank you for your time.

[ghorrocks]

Looking at this more generally: There is no universal definition of a function to define a vortex core. Also, in 3D the exact definition of what is a vortex, a circulation, separation and so on because difficult. The result of this is than in 3D simulations there is no precise definition of a vortex. This means that if you want to visualise the vortices in a 3D flow you have to either compromise or use a specific definition which just happens to work in your case. Sometimes that could be pressure, vorticity, q, and there are others. It means it will take some time and experimentation to find what works best in your case.

It is starting to sound like the development work required to show what you ask for might not be worth the effort

[Dani125]

Thank you for your answer.
It was just a question of visualization but it was not the main purpose of my work so indeed it is not worth the time and effort.