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 mp121209 September 22, 2012 01:19

Transition modeling: Gamma-Theta model

Hello,

I have a small problem with transition model Gamma-Theta combined with SST to analyze the flow around a cylinder. Since the surface of cylinder is rough, it's needed to define "Sand Grain Roughness Height". But this is a straightforward problem cause I know the roughness on the surface.

When I give a value for "Sand Grain Roughness Height", it's recommended to specify a Roughness Correlation in the Gamma-Theta model. I wonder what is "Geometric Roughness Height" in "Roughness Correlation", and how we can calculate it based on "Sand Grain Roughness Height".

Does anyone give me some ideas?

 ghorrocks September 24, 2012 22:00

This is pretty specialised stuff - I would read the original references papers on this model to know how to approach this.

 mp121209 September 25, 2012 03:24

I've read documentation provided in CFX. The model is mentioned in the documentation, but nothing is more than that. Concept "Geometric Roughness Height" in "Roughness Correlation" is a specific concept of CFX, not of the original model.
I wonder to know what it is.

 brunoc September 25, 2012 05:02

The roughness height alone is not enough because you can have different types of morphologies making up the roughness, that is, roughness shape and density also play important roles in determining how wall roughness afects the boundary layer.

The documentation has a reference that shows how you correlate both types of roughness, but I think it it all comes down the nondimensional roughness you use on the Moody Chart for estimating pressure drops on pipes.

Cheers

 brunoc September 25, 2012 06:44

I just read your question again and realized I was answering a different question. But it still somewhat applies. The referenced paper shows how sand grain equivalent roughness is calculated from the original roughness' height, shape and density, so maybe you can revert the calculations to estimate your surface height.

Still, my guess is that, if you don't know roughness shape and density, retrieving its height won't be straightforward.