root mean square
Dear ALL
I am currently trying to model water flowing through a pipe in Fluent 5.5. To validate the model I have found a number of published papers on the subject. Many of the papers use a parameter called the 'root mean square velocity'. What does this mean and how can I get it from Fluent? Thanks in advance, John 
Re: root mean square
Urms is the root mean square of the turbulent fluctuations in the x direction. It can be recovered from the turbulent kinetic energy if the flow is isotropic (i.e. u'=v'=w') using k=1/2(u'^2+v'^2+w'^2). However the flow is unlikely to be isotropic and so if you are using an eddy viscosity model you will need to know the ratio of u':v':w'(which for example is 4:3:2 for equillibrium flat plate flow).
For eddy viscosity models the flow is assumed to be isotropic so it will be difficult. For secondorder closures you can recover Urms from the Reynold stresses. Steve 
Re: root mean square
Sorry 4:2:3 not 4:3:2

Re: root mean square
Hi Steve,
Thank you very much for your response. I am using the keps 2 equation model which if I understand correctly assumes the eddy viscosity is the same in all directions. Can I therefore assume that it will be impossible to get the rms of the turbulent fluctuations from Fluent directly (unless I change to the Reynolds stress model) and the only way I can get them indirectly is to know the ratio of u'v'w' for a pipe? Thanks once again, John 
Re: root mean square
Thats correct, however if the data you are comparing to has all three components of the turbulent fluctuations than you could combine them to give k and compare to k from FLUENT.

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