Turbulent Characteristic Length/Turbulent Length Scale

fluentnoob · July 2, 2009, 12:35

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)² 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.

July 2, 2009, 12:35	Turbulent Characteristic Length/Turbulent Length Scale	#1
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 16	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.

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July 3, 2009, 05:42		#2
pipin New Member pipin Join Date: Apr 2009 Posts: 20 Rep Power: 17	Hello, In the notice of FLUENT (chapter 7.2) they give the characteristics: I=0.16Re^(-1/8) l (length scale) = 0.007 L (L=diameter) k (Turb. kinetic energy) = 1.5(uI)² 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

July 3, 2009, 08:40		#3
fluentnoob Member DT Join Date: May 2009 Location: Lisbon Posts: 37 Rep Power: 16	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.