Learning Forum

[paola_1]

Hi, I have to get the weighted average value on a surface, so I use "Area-weighted average" and I select a plane that I have create with "Plane surface" and the option "Bounded".

The rendering of my variable is always negative, but the value that I get with "Area-weinghted average" is positive.

What could I have wrong? (I attach the rendering of the variable, the weighted average value is 0.011716292)

[Rob]

Please can you post a screen grab of the TUI showing this? n

[paola_1]

Yes. nnNote: the rendering is done with node values.n

[YasserSelima]

How did you define the plane? Does the above screenshot shows all the plane?n

[paola_1]

Yes, the above screenshot shows all the plane. I defined the plane with the coordinate x, y, z (it's a plane yz at x=24 m) in Plane Surface and I checked the option Bounded.n

[Rob]

The value looks wrong too, regardless of the sign. Check it's the same surface and can you plot something else that has to be positive? eg velocity magnitude. n

[paola_1]

Yes, this is the velocity magnitude.nn

[YasserSelima]

possible bug! n

[paola_1]

Thank you.n

[Rob]

What is the UDM2 definition? Can you check the result on a boundary surface too? n

[paola_1]

The definition of UDM2 is: -(dP/dx)/rho. Now I can't check the result on a boundary surface.n

[Amine Ben Hadj Ali]

Weird UDM definition: are you storing the total pressure loss of the domain? Can post the min/max/average of that UDM- in the domain from a volume integral report?n

[Rob]

I've just created the (dP/dx)/rho via a custom field function and displayed it on an outlet. How does that route compare to the UDM? n

[paola_1]

Weird UDM definition: are you storing the total pressure loss of the domain? Can post the min/max/average of that UDM- in the domain from a volume integral report?/forum/discussion/comment/105456#Comment_105456

It's a term of the momentum equation in x direction. I have to get the weighted average value of this term on a specific y-z plane. In the domain I have Min=-7.9515686, Max=6.2238784, Volume-Average=1.8835192e-05.n

[paola_1]

I've just created the (dP/dx)/rho via a custom field function and displayed it on an outlet. How does that route compare to the UDM?/forum/discussion/comment/105461#Comment_105461

I can't get this value because I would have to run for other time steps but I can't do this on my pc. nI've try with another UDF (-v*du/dy) and the weighted average value is the same of the weighted average value of a custom fiel function (=-v*du/dy).n

[Rob]

We use a lot of UDMs for various tasks, so I'd be surprised if there was a bug. You shouldn't need to do an iteration to get the CFF value, it's calculated in the solver when you do the plot/report. n

[paola_1]

We use a lot of UDMs for various tasks, so I'd be surprised if there was a bug. You shouldn't need to do an iteration to get the CFF value, it's calculated in the solver when you do the plot/report./forum/discussion/comment/105484#Comment_105484

Maybe I should have answered yes to 'Keep temporary solver memory from being freed?' before run the simulation? dP/dx is always 0 (like dP/dy and dP/dz).n

[Amine Ben Hadj Ali]

Disable node values.n

[paola_1]

Disable node values./forum/discussion/comment/105509#Comment_105509

Where? When I do the rendering of the variable? If i do that the values are always positive, but why is there such a big difference between node values and celle values? Could be because I did the last two time steps with a deltat of 1/10 comared to the initial one?n

[Rob]

Data in Fluent is stored on cell centres and facets for boundary conditions (that's in the manual somewhere). Normally this merely results in a pixelated image, which is also used to highlight regions of poor mesh resolution. When you use a plane this will take the cell centre value of the cell the plane cuts through, so it may be a bit messier than usual. nRetaining the gradients may be needed, CFF calculate on the fly so avoids this issue. Note, retaining the data does give a slight RAM overhead and may result in larger data files. nUsing 1/10 of dt shouldn't make a difference unless the earlier solution wasn't converged. Do you see any jumps in other variables?

[paola_1]

So, if the cell values on the plane are positive and the node values are negative it's ok?.o, I don't see any jumps in other variables.nI've tried to create -(dP/dx)/rho with CFF and the weighted average value is the same as that obtained with the UDM.

[Rob]

There are no node values. The node is the cell centre. Otherwise the contour is smoothed using the cell values to give a nicer image. n

[paola_1]

Ok, thank you.n