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-   -   Turbulent Characteristic Length/Turbulent Length Scale (http://www.cfd-online.com/Forums/fluent/65993-turbulent-characteristic-length-turbulent-length-scale.html)

fluentnoob July 2, 2009 12:35

Turbulent Characteristic Length/Turbulent Length Scale
 
Hello everyone!

I wanted to know that while running cfd analysis of a flow in FLUENT, how does one decide on an initial guess for the Turbulent Characteristic Length?

For eg. I was trying to simulate flow over an obstacle and I took the length scale(l) as 1.5% of the inlet height. The obstacle is an ellipsoid and the inlet height is 1.4 m and the width is 1.5 m. The characteristic length of the obstacle is .8 m. Is 1.5%(of 1.4 m) too less an estimate for the Turbulence Length Scale?

How exactly is an initial guess for turbulence length scale made?

Thank you. :)

pipin July 3, 2009 05:42

Hello,
In the notice of FLUENT (chapter 7.2) they give the characteristics:
I=0.16*Re^(-1/8)
l (length scale) = 0.007 L (L=diameter)
k (Turb. kinetic energy) = 1.5*(u*I) u=velocity
epsilon (turb dissipation rate) = 0.09^(3/4)*k^(1.5)/l (length scale)
it's a first approximation, it's good for initialization.

Bye

fluentnoob July 3, 2009 08:40

Thanks. I'll look into it. :)
And also, on a separate topic, I've been reading about hydrodynamic derivatives and its written everywhere that Nr is large and negative while Nv is small and of uncertan sign. What I want to know is, are these values(large and small) with respect to each other? Or are they with respect to (Mr, Mv, Kr, Kv) or (Yv and Yr non-dimensionalized)?

If you can help me out with this, I'd be much obliged. Thanks. :)


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