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How to find rms vorticity values in Fluent?

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Old   March 26, 2020, 17:58
Default How to find rms vorticity values in Fluent?
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I need to make a comparison for each component in figure 5 from the following paper.


https://cfd.spbstu.ru/agarbaruk/doc/...-to-Re-590.pdf


Except I cannot find where to get rms vorticity values from my own simulation. Vorticity magnitude isn't what I need, and I cannot find anything else to do with vorticity in any of the XY plot options.


Can anyone help me out?
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Old   March 27, 2020, 03:20
Default RMS Values
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For getting RMS values, you need to enable Data Sampling in Fluent before starting the simulation. This appears at the Calculation Panel.
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Old   March 27, 2020, 05:08
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Data Sampling for Time Statistics is already enabled, with a sampling interval of 1 being used, and all 3 statistic collection options being enabled. Yet I still cannot find the rms vorticity in the XY plot menu.



If it makes any difference, I am currently using the Reynolds Stress Model.
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Old   March 27, 2020, 05:17
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Sampling is done only for primitive and few other variables. For vorticity, you have to define it as a CFF and then choose this CFF for sampling.
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Old   March 27, 2020, 05:23
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What should I put in the CFF to get each component (x,y,z) of rms vorticity?
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Old   March 27, 2020, 05:24
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CFF can contain only one value. So, you have to define three CFF, each containing one vorticity component.
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Old   March 27, 2020, 05:28
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Sorry, I meant what 3 different equations would I need to put into the 3 CFFs in order to get the components out of it. I don't really understand the maths behind vorticity and don't have the foggiest clue what I would need to put in myself.
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Old   March 27, 2020, 06:30
Default Equations for CFF
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If you read my previous post carefully, you will understand what you need to put as equation. Essentially, you just need to assign vorticity component 1 to CFF 1, and so on. So, the equation is

CFF1 = Voriticity X-Component
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Old   March 27, 2020, 06:37
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I don't have a "Voriticity X-Component" or any of the other components in the field functions list though. The only thing that I have in the field functions list that makes mention to vorticity is "Vorticity Magnitude" which is within the velocity list.
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Old   March 27, 2020, 07:44
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Then, you have to define it yourself. Vorticity is defined in terms of velocity gradients and you can access velocity gradients after issuing following command in Fluent

solve set expert

and then answering Yes to the question regarding keeping the memory from being freed. This will ensure that gradients are always available during calculation.
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Old   March 27, 2020, 10:01
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So, assuming that the X-component is ∂v/∂z−∂w/∂y, the Y-component is ∂w/∂x−∂u/∂z, and the Z-component is ∂v/∂x−∂u/∂y. Does that mean I am forced to use 3D analysis so that I can get ∂v/∂z, ∂w/∂y, ∂w/∂x, and ∂u/∂z? As none of the derivatives that involve the Z-direction are listed in the field functions list.
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Old   March 27, 2020, 10:09
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Two-Dimensional is the reason you do not have components of the vorticity available. In a 2D case, there is only one component of the vorticity, i.e., along z-direction. Vorticity is like turning a screw or a tap. You turn it in one plane, which in your case is x-y plane, but it acts in the direction normal to the plane, which is z-axis. So, you only have one component, i.e., z-component and that is equal to magnitude itself.
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Old   March 27, 2020, 10:23
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Thanks for everything so far, considering how long the new simulation will take, I will have to wait until late tomorrow to see what the results will be.
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Old   March 27, 2020, 14:55
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Think I'm gonna give up with this vorticity thing, spent 5 hours simulating for a bunch of results I have no idea how to use or to even make look normal.
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Old   March 27, 2020, 17:43
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A cute workaround is to take the mean velocity field and patch it onto the instantaneous field using the initialization. Now you the instantaneous vorticity is the mean vorticity. But it still involves custom field functions...


In Fluent you can't patch using a primitive field. The workaround was to create a custom field function and set its definition to be the primitive field. And then patch using the custom field. Maybe in new versions you can patch directly a primitive field and don't need to do the workaround, idk.
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