[Nek5000-users] Specifying boundary condition from external text file

[nek5000-users at lists.mcs.anl.gov]

Hello again,

I want to provide more information, and more detail of my exact case to
hopefully make it easier to get assistance.

Using the 2D blasius example as a base, I have created a mesh file
"box.box" that allow me to modify the domain.
After renaming the necessary files (program is now called 'box') this works
fine with the an unmodified blasius.usr (now 'box.usr') code.

The next step I took was to change the boundary conditions in userbc() [and
useric()] in box.usr.
As I understand, the nodes on the inlet plane are called sequentially from
bottom to top, by element number ('e') and then by node number ('iy').
The elements are not numbered sequentially from bottom to top, but rather
left-to-right, then back to the start, then up, then left-to-right again.
Therefore for a domain of 410 elements in the streamwise (x) direction, the
element numbers along the inlet are, from bottom to top: 1, 411, 821, etc.
There are 12 elements in the wall normal (y) direction.

I have an array ('ArrangedInlet') in the subroutine containing the
horizontal velocity component, ux, value for each of the nodes along the
inlet according to their y coordinate  The original data lives in the file
called 'InletArranged.out'. This is output by a process in MATLAB so there
is freedom in what is included and how it is arranged and formatted.
The data points are currently entered manually. This is initially to test,
but it is intended that this is read automatically from the external file.
However in order to assign the correct velocity value to the correct
position along the inlet, I arranged the data in the array such that the
rows counted iy from 1 to 8 (lx1) and each is addressing a node: Row 1=Node
1; Row 2 = Node 2, etc. where the nodes go bottom-to-top (y=0 to y=10),
The columns contained the data for each element along the inlet - ordered
from bottom to top. At the moment each row .

Using the same domain as described above (NELX, NELY)=(410, 12):
column 1 contains data at each node ('iy') for element 1,
column 2 contains data at each node ('iy') for element 411,
column 3 contains data at each node ('iy') for element 821,
and so on.

I got around this by using the following transformation:
COL=((e-1)/NELX)+1
where:
COL is the column number of the array (1 to 12);
e  = gllel(eg) is the element number;
NELX = 410 is the number of elements in the streamwise (x) direction.

This is then read by the program by:
ux = ArrangedInlet(iy,COL)

And it works! ... but only for a serial case. Also, it currently only works
for the single set of data.

As soon as I run the case in parallel the addressing changes and this
method does not work.
The element addressing no longer appears to have a discernible pattern.

*So my questions are primarily:*
*1) How can I get the values onto the inlet and make it work in parallel? *
and
*2) Is there a more elegant/efficient method of doing this? IE. a way of
calling the inlet nodes only?*

I have attached my case "box_2D_Inlet.tar.gz".
I have tested it and it does work, however currently it bombs out after 9
iterations, but I'm really only testing the inlet parameters.

Looking forward to any assistance.

Thanks,

Tom.



On 17 July 2014 10:37, Thomas Yankos <s3286374 at student.rmit.edu.au> wrote:

> Hi All,
>
> I'm using NEK5000 for a university project. I've found it to be blissfully
> fast and relatively easy to use.
>
> The problem I'm doing is a blasius boundary layer case with perturbations
> (simulating Tollmien–Schlichting waves), initially in 2D as per the blasius
> example, then eventually moving to 3D.
> I've so far been successful in running the blasius example case, and
> modifying the mesh, polynomial order and other input parameters.
>
> Now however, I want to change the way the boundary conditions are defined.
> I have input files that are unique for each time step specifying velocity
> and pressures along a line (the inlet) in text format. It would be possible
> to re-format these as .csv or another file if necessary.
>
> *How can I get NEK to read these files as boundary conditions on the inlet
> for each time step?*
>
> I understand it will be a specification in the user userbc subroutine in
> blasius.usr however I'm not sure how to actually implement this.
> Doing it in 2D will be a precursor to a 3D simulation with the file
> specifying the inlet *plane *in a text file, rather than just a *line*.
>
> Looking forward to a response.
>
> Warm Regards,
> Tom.
>
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