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-   -   Viscosity and accuracy (http://www.cfd-online.com/Forums/fluent/30538-viscosity-accuracy.html)

moiami November 19, 2002 02:47

Viscosity and accuracy
 
Hi,

I'm quite concerned with my process fluid viscosity. The viscosity that I have to input into Fluent may fluctuate in +/- 0.1Pa.s depending on most probably the temperature and how much I have processed/stirred it. It's a Newtonian fluid, by the way. Will the simulation results get affected so much or can I just ignore this viscosity change and just stick to one value? It's quite difficult to measure everytime I run the process. For your info, my velocity components I got from the experiment is in the range of 0.001 - 0.500 m/s.

Thanks

Jin-Wook LEE November 19, 2002 04:44

Re: Viscosity and accuracy
 
For your reference, for the TURBULENT FLOW, couple of tens % or hundreds % difference of molecular viscosity hardly affect to the flow pattern because turbulent effect is predominant. (In general, 100~1000 times larger than molecular visocosity effect),

Viscosity is very important for the LAMINAR FLOW. But couple of % difference in viscosity is not severe even for the laminar flow.

So most of all, you should give the information for the absolute value of the viscosity. That is to say, 0.1 Pa-s is big or small, compared to the average value ? Because, as you may know, relative comparison is important in the engineering as well as simulation.

And you should check that your flow system is turbulent and laminar.

As my personal experience, couple of % difference in viscosity(molecular viscosity, or laminar viscosity) is not so severe problem even for the laminar flow.

Sincerely, Jinwook


Alamgir November 20, 2002 00:21

Re: Viscosity and accuracy
 
or u can input that small defferences by the help of user defined function (UDF).

Alamgir

moiami November 20, 2002 21:04

Re: Viscosity and accuracy
 
Thank you both. I'll check back my viscosity value.

Another question: How to input the small differences by the help of user defined function (UDF)?

Is there an example in the tutorial or anywhere on this that I might have a look at?


Philipp Beierer November 21, 2002 07:16

Re: Viscosity and accuracy
 
yes, check the manuals, the subject about udf's. There you'll find an example about how viscosity is modified e.g. as a function of temperature

moiami November 22, 2002 22:43

Re: Viscosity and accuracy
 
Thank you. I'll check it out.


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