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February 22, 2002, 04:23 
Fluctuations of wall shear stress

#1 
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Hallo
I would like to know if anyone has experinces with the prediction of fluctation of wall shear stress?? Or if it can be done at all?? Regards Bo 

February 22, 2002, 06:11 
Re: Fluctuations of wall shear stress

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A DNS prediction by definition will predict everything.
For an LES code using wall functions (i.e. one that does not place the first grid point deep in the laminar sublayer) handling the fluctuations of wall shear stress about some form of average value is key and has been reasonably widely studied and tackled with a number of approachs. If the LES code places the first grid point deep in the laminar sublayer then the fluctuations simply get predicted without any need for direct modelling. A ke RANS code provides a model for the shear stress and hence has no sensible means of expressing a fluctuating of shear stress. A Reynold stress transport RANS code solves a transport equation for the shear stress and hence needs a model for the fluctuating component. There are a range of models and a large amount of research on the topic since it is one the weak points of an RST model. The above assumes you are referring to fluctations on the scale of the turbulent motion. If you have an unstable flow on a scale slower than the time scale of the energy containing eddies and it is these fluctuations you are interested in then any model which provides a value for the shear stress will do the job (but not necessarily well). 

February 22, 2002, 06:52 
Re: Fluctuations of wall shear stress

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Thanks for the thorough answer.
If I use Ke RANS and twolayer approach the problem is the same as I still only model the average turbulence parameters in each ti,estep  is this correct understood. Regards Bo 

February 22, 2002, 07:15 
Re: Fluctuations of wall shear stress

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Probably not. A Reynolds averaged quantity must be averaged over a time period significantly longer than that of the energy containing turbulent motion. Usually, but not always, this is substantially longer than one time step.
I am struggling to answer the question due to lack of information. Is your flow steady or unsteady? If unsteady, what is the time scale of the unsteadiness compared to the time scale of the turbulence? Why do you want the triple correlations when you are using a low order turbulence models? This may be a quicker route to an answer. 

February 22, 2002, 12:47 
Re: Fluctuations of wall shear stress

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Hallo Andy
*I am struggling to answer the question due to lack of information. Is your flow steady or unsteady? If unsteady, what is the time scale of the unsteadiness compared to the time scale of the turbulence? Answer: Steady and unsteady flow is observed. The timescale for the unsteady flow is larger than that for the turbulence *Why do you want the triple correlations when you are using a low order turbulence models? This may be a quicker route to an answer. Answer: As I have measurement demonstrating fluctuations of wall shear stress I would like to find a way to predict the areas of high fluctuation of the wall shear stress using CFD Regards Bo 

February 25, 2002, 13:21 
Re: Fluctuations of wall shear stress

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I do not seem to be much wiser. I can only add that if you want a reasonably accurate simulation of a physical phenomena (in this case the triple correlations) you will have to use a set of modelling assumptions that do not heavily approximate what you wish to study. This would tend to indicate the use of an LES model. However, there are a few cases where a RANS model may produce usable information depending on the type/source of your unsteadiness.


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