Hi there,

I want to calculate the inlet parameters for k and epsilon from my experimental results u', v' and w' or Turbulent intensity Tu.

How can this be done?

Ralf

I found some eq. in the forum, but does someone know, were they are from? And for what they are valid???

Mu_eff=20*Mu_laminar

k_inlet= (U_in*TU)**2

Tu=turbulent intensity.

U_in=sqrt(u*u+v*v+w*w)

e_inlet= (Cmu*rho_inlet*(k_inlet)**2)/Mu_eff

The non-general part on those formulas is the Mu_eff=20*Mu_laminar equation, that is just a wild guess. It might be a fairly okay for external aerodynamics where the turbulence level coming in is very low, but for internal flows with more turbulence Mu_eff will be much higher. Then it is more correct to try to estimate the turbulent lenght-scale and derive epsilon from that. You can find the formulas in any book about turbulence modeling. I'd recommend Wilcox's Turbulence Modeling for CFD. If I get some time tomorrow I'll try to add the relevant formulas to the Wiki here so that you can get them directly online. If someone has them in latex already feel free to help me.

I just wrote something about the turbulence intensity in CFD-Wiki here:

http://www.cfd-online.com/Wiki/Turbulence_intensity

Someone should add something about turbulent lengths scales, viscosity ratios etc. also. I'll see if I get some time later to do this also.

I also wrote something about the turbulent length-scale here:

http://www.cfd-online.com/Wiki/Turbulent_length-scale

hello... For a diffuser with outlet/inlet area ratio of about 5, what value would you suggest for turbulent length scale? Thanks.

That depends entirely on what is upstream of the diffusor.