Assigning boundary value to every cell above it
 

Hello,

I'm stuck on something that seems rather trivial so I was hoping maybe somebody could give me a hand... :D

I would like to assign the value of friction velocity, utau, AT A WALL (for a given x-coordinate) to every cell above it (i.e: utau(x=i,y=allCells)=utau(x=i,y=0)).

My problem is that when I compute utau, I only get values at the boundaries, while the rest of the volScalarField is left as its initialized value (0). When I look at the output in the time step directories and ParaView, I can see that there are utau values for solid boundaries only, not for cell centers... which makes sense given how I defined utau (see below).

The question:
How can I tell OpenFOAM to assign the value of utau (at the wall) to all of the cells above it at each x-location?

Reason for doing this:
I am adding a scalar term to the dissipation equation that has utau in it. Since the solver loops through each cell, I need to assign the value of utau (at the wall) to each cell center in the corresponding column.
I'm assuming that utau will vary as you move in the x-direction since dUdy will be different.

Below are the definitions of utau and my implementation in OpenFOAM. I understand that the code below only gives me utau at the boundaries, I just don't know how to tell it to assign that value to all cells above.
I aplogize, for my coding skills aren't the best :(

utau = sqrt(tau_wall/rho) = sqrt(mu * dUdy / rho) = sqrt( nu * dUdy)

in OpenFOAM:

Thanks ahead of time!

Hi,

You can access the cells next to a patch using faceCells().

Hope this helps,
Fumiya

------------------------------------------
My slides about OpenFOAM:) (http://www.slideshare.net/fumiyanozaki96)

Hi Fumiya,

Thank you for replying!

I'm still a little confused about what you are suggesting... What do you mean by 'target patch'?
Also, how would I assign a single value (utauw at the boundary) to every cell above it so that OpenFOAM doesn't complain about assigning a Foam::Field<double> to a 'double' ?

I have looked over some existing code and I'm implementing the following:

The code above calculates utauw (at the wall) correctly, but it does not assign its value to utau (cells above boundary). As a matter of fact, utau still holds the value at which it was initialized at.

I need to somehow store the value produced by
at the boundary faces.

Hi CHARLES,

You might want to try

utau[faceCelli] = utauw.boundaryField()[patchi][facei];
//RHS: utauw value at each boundary face(label: facei) on the boundary(label: patchi)

instead of

utau[faceCelli] = utauw[patchi];

in your code.

Hope this helps,
Fumiya

Fumiya,

I'm sorry but after re-reading my post, I realized that I wasn't very clear.

What you suggested allowed the script to compile and run! It also gave the results you had expected: the utauw value (at the boundary) is being assigned to utau (at the cell centers in contact with the boundary). utau is now a nonuniform list, which also solved part of the problem that I was having before.

However, what I intended to do was to copy the value of utauw (from the boundary) to ALL cell centers above it, not just the boundary cell centers.

I tried to write another forAll loop that would loop through all of utau, which would compile but I get a segmentation fault in the log file on the first iteration. I think I'm trying to assign a boundary's value to a single cell, which may be the cause of the problem.

btw, thank you for explaining the boundary field indexing!

I believe I may have solved the segmentation problem...

However, I still have a few questions about the following part of the program:
1. Is the gradient, snGrad(), being calculated for the patch as a whole? or is it being done on a face-by-face basis?
The reason I ask is because the value of utauw that is output to the time directory doesn't change (for a flat plate) but one would expect that the gradient of the velocity would be a function of the streamwise-distance along the plate.
When I look at utauw for a different geometry, like a 2D bump, it varies in the time directory output.

2. Why is the following not correct
Error:
But it will work if this is done instead: