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Bizarre behavior of passing a scalarField to another scalarField

Hi,

I run into some bizarre behavior of passing a scalarField to another scalarField and I can not explain.

I was try to simply pass a scalarField of Y_pre to scalarField of Y. Initially, I just use

Code:

Y = Y_pre

But this does not correctly pass the value to Y. I end up with this following solution.

Code:

forAll(Y,celli)

{

    Y[celli] = Y_pre[celli];

}

My question is what is the difference between these two method? For additional information, there is a YEqn following right after these two different operation.

Thank advance for any insight.

Rdf