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-   -   Can not add a new user defined visualization parameter in Cut plot! (http://www.cfd-online.com/Forums/floefd-floworks-flotherm/94046-can-not-add-new-user-defined-visualization-parameter-cut-plot.html)

george85 November 3, 2011 15:10

Can not add a new user defined visualization parameter in Cut plot!
 
I am trying to add a new user defined visualization parameter, that will alway substract the normal atmosferic pressure at sea level (101325 Pa) from the pressure I got in my analysis.
I suceeded in creating this new visualization parameter, but for some reason, I can not add it to my parameters list in the Cut plot parameters list.

Take a look what I did:

http://www.youtube.com/watch?v=Q5mWr...ature=youtu.be

Why I can not add the new parameter "substracted pressure" to my list? But I can add the default ones?

Thank you for the reply.

george85 November 6, 2011 07:25

It seems I did not have to add "Pa" in the forumula.

So instead:
{Pressure}-101325 Pa

I need:
{Pressure}-101325

Now it is working.


But, I want to divide this with 1000, so that I get kN/m2 instead of Pascals.
How can I do that?

This did not worked:
({Pressure}-101325)/1000

Any suggestions?

Boris_M November 6, 2011 15:29

Hi George,
no, you don't need to ad "Pa". FloEFD calculates with the values only.
If you go to the "Item Properties" tab in your formula definition you can set the unit in the last row of your user parameter.

Regards,
Boris

george85 November 6, 2011 18:39

Thank you for the reply Boris.
Is this what you meant:
http://img339.imageshack.us/img339/7560/63913352.th.jpg

But in that case I will only get the units in Pascals right?
And I want my units to be in kN/m2, and 1kN/m2 equals 1000 Pascals.

This is why I asked, how can I divide this formula:
{Pressure}-101325
with 1000.

I tried this, and it is not working:
({Pressure}-101325)/1000

Boris_M November 9, 2011 06:21

I don't have any problem with it. See the images below. The Parameter I created is the same as you did and it works. But you don't need to create this parameter, depending with which version you are working there should be already a parameter as predefined called "Relative Pressure" which basically is the same you did but referencing to the reference pressure and not a fixed value in the parameter calculation. This reference pressure can be set manually as it is also needed to calculate forces if not the whole body is in contact with the fluid.
You can see from the probes the pressure in the first line, my user defined parameter without units in the second and the relative pressure in the last line. You can see from my defined parameter that I devided by 1000 but for the others I created a custom unit for pressure (see the other image) where you can specify the relation to the SI unit and give the unit a name, in my case Kilopascal. You could also call it kPa and it would then be displayed as a Unit behind the value and automatically devide by 1000 as this is the relation to the SI unit, which is why I didn't use a unit for my parameter since I already devided by 1000, otherwise it would be devided by 1000 again.
Also please use a better mesh for your calculation this is way too rough as you can see by the result display from the non-interpolated values. First you wouldn't capture what really is happening in the flow and second you don't resolve the geometry accurate enough. Also I think your computational domain is too small as the flow enters and immediately hit's a building in your case. You should do it similar to the environment wind tunnels where they do these kind of measurements and visualization. Give it some space around your model to adhust to the obstacle and not force it within a fraction of a cell to change the direction from the inflow around the building.

I heavily suggest you do a training from your reseller as these are things they should teach you. I know we do.
FloEFD is easy to use so are CAD systems but it is not easy to do it from scratch without any training just like CAD systems.

Boris

The Parameter:
http://img444.imageshack.us/img444/5...rameter.th.png

The Unit system settings:
http://img809.imageshack.us/img809/1004/units.th.png


The Results:
http://img39.imageshack.us/img39/412...lt12zsn.th.png

http://img546.imageshack.us/img546/9...fault13.th.png

george85 November 9, 2011 14:33

You are right!
It worked for me too. All I had to do is restart my Solid works, and them the ne adition (...)/1000 works. Otherwise it shows me the same results as it was without the (...)/1000

Thank you for such a nice explanation.

I am trying to get a pressure impact of wind, on structures (like buildings) for example.
But I see that even in that case, I need to change the mesh of the contour plot, the way you did.


And there are no training at the local dealer. Believe it or not :(
Still if there are in my country, they are to far from me.
I am an architect, this is why you might find lame and strange, some of my questions.
But I am doing my best to learn from a lot of fields. Statics (structural design and analysis of structures), foundations calculation, light analysis...
Now I want to find out, how to get a wind load.

Thank you for the replies once again.


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