[Hamda]

Hi,

As you see in the following picture, there are several spheres and I want to select one of them to investigate Temperature, velocity and pressure distributions. How can to select this specific sphere in CFD-post?


Thanks in advance

[ghorrocks]

The easiest way is to define that sphere as a mesh region (in meshing), and then you can simply refer to it as a locator.

If you have not done that and you have to post process an existing run you have to define a function which isolates the sphere. This is not too hard for a sphere as it can simply be all points less than the radius from the centre point of that sphere. I will leave the maths of defining that function up to you for your enjoyment

[Hamda]

Originally Posted by ghorrocks

The easiest way is to define that sphere as a mesh region (in meshing), and then you can simply refer to it as a locator.

If you have not done that and you have to post process an existing run you have to define a function which isolates the sphere. This is not too hard for a sphere as it can simply be all points less than the radius from the centre point of that sphere. I will leave the maths of defining that function up to you for your enjoyment

Thank you for your response. could you explain me how to define such a function? it means an expression? or I should insert volume ? I already inserted volume and got some contours.

[ghorrocks]

Define a variable in CFD-Post of the form x^2+y^2+z^2. Then draw contours of that function, and make an volume out of the contour which defines your sphere.

[Hamda]

Originally Posted by ghorrocks

Define a variable in CFD-Post of the form x^2+y^2+z^2. Then draw contours of that function, and make an volume out of the contour which defines your sphere.

I wrote an expression of the form of x^2+y^2+z^2 and then used it for a variable (called sphere) and then drew contour of that variable. I got the following pic.
But my problem is that when I use the defined variable I can't draw contours of Nu, temperature ....

[Gert-Jan]

Don't create a function. And especially not a function of a sphere around (0,0,0). That is what you did.

You should create a volume around the center of your sphere of interest. Go to pull down menu, Insert, Volume, select sphere, add the center of your sphere select the radius of your sphere and select below intersection, to obtain a volume containing all elements inside your sphere. Simple.

But this is all work afterwords. As Glenn mentioned, it is best to give names beforehand during meshing. Then it is easier to find them back in the postprocessing.

[Hamda]

Originally Posted by Gert-Jan

Don't create a function. And especially not a function of a sphere around (0,0,0). That is what you did.

You should create a volume around the center of your sphere of interest. Go to pull down menu, Insert, Volume, select sphere, add the center of your sphere select the radius of your sphere and select below intersection, to obtain a volume containing all elements inside your sphere. Simple.

But this is all work afterwords. As Glenn mentioned, it is best to give names beforehand during meshing. Then it is easier to find them back in the postprocessing.

Thank you for your response. As mentioned before I already did that:

Originally Posted by Hamda

Thank you for your response. could you explain me how to define such a function? it means an expression? or I should insert volume ? I already inserted volume and got some contours.

I inserted a volume and got some contours.

[Gert-Jan]

I don't know what you did because I can't see it.

- In the picture I can see that you defined 2 volumes, but they are not visible. So I can't see if you did the correct thing.............
- I see a contour of Variable 1, which probably is your expression. And you plot this on your interface which is part of your existing geometry. And the center is at the bottom, probably (0,0,0)

I repeat: You should create a volume around the center of your sphere of interest. Go to pull down menu, Insert, Volume, select sphere, add the center of your sphere select the radius of your sphere and select below intersection, to obtain a volume containing all elements inside your sphere. Simple.

[Hamda]

Originally Posted by Gert-Jan

I don't know what you did because I can't see it.

- In the picture I can see that you defined 2 volumes, but they are not visible. So I can't see if you did the correct thing.............
- I see a contour of Variable 1, which probably is your expression. And you plot this on your interface which is part of your existing geometry. And the center is at the bottom, probably (0,0,0)

I repeat: You should create a volume around the center of your sphere of interest. Go to pull down menu, Insert, Volume, select sphere, add the center of your sphere select the radius of your sphere and select below intersection, to obtain a volume containing all elements inside your sphere. Simple.

I already inserted a volume. and did what you said.Please look at the pic.

[Gert-Jan]

Originally Posted by Hamda

Hi,

As you see in the following picture, there are several spheres and I want to select one of them to investigate Temperature, velocity and pressure distributions. How can to select this specific sphere in CFD-post?


Thanks in advance

Briljant, than you initial question is answered, not?

Here you can investigate its volume average temperature and outisde pressure distribution.

[Gert-Jan]

If you want to evaluate heat transfer, then you need this volume average temperature.
But for outside variables you need a surface just outside each sphere, not?

Then, as Glenn already mentioned, take a step back and define unique names. The easiest way is to go back to your meshing, and give unique names to each sphere surface. Then in Post, create a user surface with "Offset From Surface" as input. This allows you to create a spherical surface around your each sphere at, as an example, 1mm from the surface.

[Hamda]

Originally Posted by Gert-Jan

Briljant, than you initial question is answered, not?

Here you can investigate its volume average temperature and outisde

pressure distribution.

yeah

But now I have another question. I wrote some expressions for Nu and htc and when I drew contours I got negative values for them. these are my expression

Myhtc=Wall Heat Flux/(T-780[K])
Nu=Myhtc*0.06[m]/0.31[W m^-1 K^-1]
But as you see in the pic. I got negative values for Nu. Is there any problem related to the expressions?

[Gert-Jan]

Nu must be positive indeed. But I cannot evaluate your equations exactly since I don't know what you are doing. Cooling, heating?
But isn't it just a matter of sign/direction/normal/definition?

[Hamda]

Originally Posted by Gert-Jan

Nu must be positive indeed. But I cannot evaluate your equations exactly since I don't know what you are doing. Cooling, heating?
But isn't it just a matter of sign/direction/normal/definition?

It is so weird to me too. Each sphere is fuel and generates heat(I already wrote an expression for energy source) and through the spheres , Helium moves and collects heat from fuel spheres.

Are my expressions correct?
I also checked wall heat flux on "domain interface 1 side 2" and it is negative too!!