[dradenkovic]

Hello.

I want to show root mean square of velocity fluctuation in cross section of a channel. I have been searching this forum a while in order to solve this problem but without success.

I know that I should obtain distribution similar to this




Any tip?

Regards,
Darko

[vkrastev]

Hi Darko,

in OpenFOAM you can calculate the rms of a velocity component (velocity along x, y or z axis) trhrough the Prime2Mean statistical function. You have to include a fieldAverage function in your controlDict, then run the simulation until the Mean and Prime2Mean quantities get sufficiently stable, then from the UPrime2Mean field you can extract the rms quantities like follows:

urms,x=sqrt(UPrime2Meanxx)

and the same for y and z. Here it is an example of fieldAverage function applied to the velocity field (this works for OpenFOAM-3.0.x, but should be ok for other versions too):

functions
{
fieldAverage
    {
        type            fieldAverage;
        functionObjectLibs ("libfieldFunctionObjects.so");
        enabled         true;
        outputControl   outputTime;
        fields
        (
            U
            {
                mean        on;
                prime2Mean  on;
                base        time;
            }
        );
    }
}

Note that the Mean and Prime2Mean fields are time-based statistical quantities, so at the end of the run you may want to add a space average operation to your Prime2Mean fields (e. g. tangential average in a cylindrical domain).

Regards

V.

[dradenkovic]

Thank you!

Regards,
Darko