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-   -   multiplication of scalar and volVectorField (https://www.cfd-online.com/Forums/openfoam/99115-multiplication-scalar-volvectorfield.html)

 mikeP March 26, 2012 11:29

multiplication of scalar and volVectorField

Hi,
I use following two methods in order to get the multiplication of Reynolds number and drag coefficient. (Drag coefficient, Cd, here is assumed constant scalar here for convenience)

Code:

```Re = (magUr*dp*rhoa)/mua; CdRe = 0.5*Re;```
Code:

```Re = (magUr*dp*rhoa)/mua; forAll (Re, celli) {     CdRe[celli] = 0.5*Re[celli]; }```
These should give the same volScalarField, CdRe.
However, when I use these two code parts, I get different velocity field results at the end.
Are these two code parts exactly the same? or can they give different results when I, let's say, interpolate them?

 marupio March 26, 2012 21:39

Are you sure you're dealing with a volVectorField? I didn't think you could access the internal field directly with operator[], I thought you had to use Re.field() or Re.dimensionedField() first.

You could be missing the operations on the boundaryField.

If you are dealing with vectorFields and vectors, the two pieces of code you show are equivalent, except that the indexed loop is about 10x slower. The operators are optimized with fancy pointer loops, register keywords, and special accommodations for vector machines.

 mikeP March 27, 2012 04:12

Dear David,

I am terribly sorry, now I noticed it. It is volScalarField, of course, not volVectorField.

Indeed, at the end, velocity field at the outlet is wrong if I use the second method. So, can it be due to the fact that I do not update the boundary field?

Let me explain it a little more: I am using a modified version of bubbleFoam, where I changed the definition of drag coefficient and relative reynolds number. When I use the first example above, my velocity field is reasonable. However, this option does not allow me to modify the Cd*Re cell by cell. But when I use the second example, although it should do the same effect, it does not. Velocity values are not correct at the outlet boundary. So, do you think it may be because I do not update the boundary field? How can I get the same results in both cases?

Thank you

 mikeP March 27, 2012 05:31

What I want to do is this:

Let's say I have a volScalarField Re:

Code:

```FoamFile {     version    2.0;     format      ascii;     class      volScalarField;     location    "1";     object      Re; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // dimensions      [0 0 0 0 0 0 0]; internalField  nonuniform List<scalar> 5 ( 298.831 300.729 302.117 303.149 303.901 ) ; boundaryField {     ground     {         type            zeroGradient;     }     top     {         type            zeroGradient;     }     outlet     {         type            zeroGradient;     }     inlet     {         type            zeroGradient;     }     defaultFaces     {         type            empty;     } }```
Suppose I have another volScalarField CdRe. (which is the multiplication of reynolds number and drag coefficient)

I want to modify CdRe cell by cell depending on the Re values, so that I can use it in my equations.

 mikeP March 27, 2012 11:49

I may have solved it.

I added this line after forall loop:

CdRe.correctBoundaryConditions();

Now it seems to be working fine.
Thank you!

 marupio March 27, 2012 12:34

You should try to stay away from indexed loops on the internal field whenever possible. They are slow. Try to use full-field operators.

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