Question about active solution field

[Filipe Brandao]

Hi, from the tutorial we have that each field has  a given ifield assigned to it:

velocity --> ifield=1
temperature --> ifield=2

scalar 1 --> ifield=3
...

and so on. What happens for a case we just have momentum and passive scalar eqn (no temperature involved)? Will the passive scalar still be ifield=3 or it drops to ifield=2?

Also, no ifield is assigned to pressure?

Thanks
Filipe

[YuHsiang Lan]

Hi Filipe,

ifield = 2 is preserved for temperature.

Even though it's off, the temperature still occupies t(1,1,1,1,1). 

Therefore, the code sometimes use ifield = 2 for "temperature mesh", which is typically shared by all passive scalar.

You can then setup BC for scalars via temperature.

Have a look at this RANS example.

usr file still keeps the settings for temperature (ifield = 2) but the par file only has scalars starting from ifield = 3

https://github.com/Nek5000/NekExamples/tree/master/RANSChannel

(You will need to add this if using the latest Nek5000

[MESH]
numberOfBCFields = 1

)

pressure is attached to the fluid solver, so it shares ifield = 1.

For fun, here is a response from ChatGPT agent, Nek5000 Assistant (https://chatgpt.com/g/g-68a731e21f7c8191bcc684cc265a1d79-nek5000-assistant):

https://chatgpt.com/share/6a2d8993-113c-83ea-a7bb-f63eaf291fb0

Hope this helps,

Yu-Hsiang

--

[Filipe Brandao]

So, when assigning BCs, I can just do it like it below skipping ifield=2?

cbc(ifc,iel,1) = 'W  '
cbc(ifc,iel,3) = 'I  '

Also, would this work on usrchk? 

if (ifield.eq.3) call gradm1(dphidx,dphidy,dphidz,t) ... or do I need to assign t(lx1,ly1,lz1,lelt,3) into a temporary variable and then pass it to other subroutines?

Thanks

[YuHsiang Lan]

Dear Filipe,

> skipping ifield=2?

Yes, that looks right.

You can also check the logfile to confirm whether it's effective:

  Found the following boundary conditions

    for velocity:

            "P  "           14
            "SYM"            3
            "W  "            3

    for scalar 1:

            "I  "            3
            "P  "           14
            "t  "            3

    for scalar 2:

            "I  "            3
            "P  "           14
            "t  "            3

Inside userchk, you can do this to compute the gradient of t(1,1,1,1,3)

  call gradm1(dphidx,dphidy,dphidz,t(1,1,1,1,3))

gradm1 itself doesn't use ifield. It always use "T" mesh with nelt.

Hope this helps,

Yu-Hsiang

--

[Filipe Brandao]

Got it. Thanks

[Filipe Brandao]

Hey Yu-Hsiang,

I actually have a follow-up question on this topic. Since the temperature field is still present even though .par file does not specify it, do I need to restart the temperature field as well? Do I need to set XUPTS1 or just XUPS1 works for the 'startFrom' flag in the .par file?

Thanks

[YuHsiang Lan]

Hi Filipe,

"XUPS1" should work.

However, Nek5000 will read all available variables from the restart file by default. 

Specifying the restart option is not required unless you want to only read a selected list of fields.

https://nek5000.github.io/NekDoc/problem_setup/features.html?highlight=startfrom#restart-options

Hope this helps,

Yu-Hsiang

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