Role of the .usr file in NekRS

[JunaidAhmed2520]

Hi, I am trying to transition from Nek5000 and the usage of the .usr file is not clear to me having gone through the examples of conj_ht, channel and mv_cyl. The following are some of the questions that I would like to ask:

0. Should the .usr file always be defined?
1. Should the subroutines in the .usr files be called separately or is it done automatically?
2. What is the role of the useric subroutine while setting the initial conditions?
3. In the case of mv_cyl, I had observed that both the .usr and .oudf files contain the uservp function which sets the same variable properties but according to their own rules. Should both the .usr and .oudf files always define the variable properties in the same way? What happens if they differ from each other?
4. Lastly, what are the steps involved in calculating torque and vorticity data using NekRS?

I would appreciate any help in this matter. Thanks in advance.

[yslan]

.usr file is the main user file in Nek5000. NekRS uses it as an interface to Nek5000 so we can still read Nek5000's case and reuse the setup with minimal modification.

0. No. You can run the case without an usr file. See this example

1. At certain location in source code, those user subroutines (listed below) will be called.

      subroutine userchk   ! not called by default in repo/next
      subroutine useric (ix,iy,iz,ieg)
      subroutine usrdat3
      subroutine usrdat2
      subroutine usrdat0
      subroutine usrdat
      subroutine userservert
   

2. useric sets initial condition, but you can set it inside .udf as well. See tcf example.

3. uservp in .usr won't be called at all. The one in .udf is the main one.
   Same as userf and userq. I guess sometimes we keep them in usr as a dummy in case we would like to run the same case in Nek5000.
   The userbc is split into device functions in .oudf

4. For voracity, see this v23 example
   

   nekRS/examples/tgv/tgv.udf

   Lines 28 to 34 in 4f87e0e

   	nrs->curlKernel(mesh->Nelements,
   	0,
   	mesh->o_vgeo,
   	mesh->o_D,
   	nrs->fieldOffset,
   	nrs->o_U,
   	platform->o_mempool.slice0);

   
   For torque, v23 has drag computation in this example
   

   nekRS/examples/ktauChannel/ci.inc

   Line 65 in 4f87e0e

   const auto drag = postProcessing::viscousDrag(nrs, bidWall.size(), o_bidWall, o_Sij);

[JunaidAhmed2520]

Hi, thanks for the reply.
I realise that there are various other variables that the nrs structure contains apart from the ones mentioned in the read the docs website. As a user who is only aware of the information present on this website (it doesn't contain enough information about the platform structure), from where can I get to know more about the various available functions that nekRS provides. Could you please suggest some resources that are available so that I can get a complete idea about them. (including the postProcessing namespace as well)