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-   -   Umean and U (http://www.cfd-online.com/Forums/openfoam-pre-processing/117696-umean-u.html)

xiyuqiu May 13, 2013 19:49

Umean and U
 
Hello All,

Does anyone know what is the difference between Umean and U from the flow field when running turbulent model? I first thought Umean is the mean velocity and U is the instantaneous velocity, but I do not see any random fluctuations with U. Can someone help? Thanks.

ngj May 15, 2013 15:22

Hi Yu,

You would need to tell, which solver you are using, and which OF version you are using in order to except answers to your questions.

You would also need to tell, whether you are using RANS, LES or laminar turbulence model in your simulations.

Kind regards

Niels

xiyuqiu May 16, 2013 13:09

Hello Niels,

Thanks for the reply.

I am using k-epsilon and k-omega model, simpleFoam solver, OpenFoam 2.0.1. Sorry about not being clear about it.

best,

yu

ngj May 17, 2013 03:34

Hi Yu,

It sounds a bit weird, because simpleFoam is a steady state solver, so I do not know, where you get the Umean field from.
I looked in simpleFoam and the incompressible RAS turbulence models, and I could not find the definition of the Umean field.

Secondly, a steady state solver does not have information on the turbulent fluctuations.

Kind regards

Niels

xiyuqiu May 17, 2013 13:05

Hello Niels,

Thank you for your input. When I finish my openFoam run, in the result folder 3000 (I let it run for 3000 iterations), there are Umean and U files in it. When I plot them in paraView, they are clearly different for some case, close for some other case. Also, when I run buoyantBoussinesqSimpleFoam, which suppose to be a steady solver as well, I see my Umean and U velocity fields clearly been different. I will try to post pictures in the next two messages

xiyuqiu May 17, 2013 13:16

2 Attachment(s)
Attachment 21858

Attachment 21859

ngj May 17, 2013 19:19

Do you perform any runTime processing in your controlDict? To me it sounds like an averaging procedure is activated.

Are your simulation converged?

Kind regards

Niels

xiyuqiu May 18, 2013 00:49

yes. i think there is a runTime processing in my controlDict. I thought it is some kind of sudo-steady simulation i.e. it is a steady simulation, but the run time is just a way to define the number of time steps needs to run. You are also correct about the residuals are not converging for the pipe case I showed here. However, what is shown here is the buoyantBoussinesqSimpleFoam heat transfer case. When I run simpleFoam solver, the case is converging and I got both Umean and U and they are different as well.

ngj May 18, 2013 04:11

Of course the two fields will be different, since you are averaging a lot of non-equal fields and comparing with the final time step.

In my perspective Umean does not make sense for a steady state simulation.

Kind regards

Niels

xiyuqiu May 19, 2013 18:55

So you think the Umean is the averaging between the time step? And the U velocity in the results is the mean turbulent flow velocity?


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