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immortality May 17, 2013 07:02

the expected effects of turbulence on the flow
 
i have a doubt on what turbulence do on flow,i tested myself an internal problem with sst k-omega and expect that velocity should reduce after using a turbulence model due to dissipation.but it even increased!
I thought there are people know the aspects of turbulence and its influences on the flow generally by sure in this socitey from the world.
I don't think in this level of question the specif case i tested be important,i ask as a general rule,was my expectation true?
How does turbulence act on properties(specially velocity,pressure,temperature) in supersonic flows(regions) or subsonic flows(regions)?
Anyway if my expectation was true and the case was improperly set values i will send it and focus on it.
But now my question is general,if help.
Thanks.

immortality May 19, 2013 09:25

5 Attachment(s)
I should clarify a bit more.
I mean if we run a case in laminar regime and then change it to turbulent what expectations will be rational?
what changes are expected in the flow properties?
I attach one case U profiles of laminar and turbulent regimes.
in laminar case the velocity is less than when it runs as turbulent.
1)is the conclusion of reducing velocity due to dissipation in turbulent regime incorrect?
2)or maybe anything in case is incorrect?(I have a doubt on k value that is increasing constantly by time)
3)or nothing is incorrect at all and turbulence has act correctly:D
if scenario 3 is in case whats the reason of increasing velocity?
I appreciate the experiences submitted.
I'lm here if more information is needed.
thanks.

immortality May 19, 2013 10:06

A simple article related found
 
In my searches I found an article simply explains things I think I have to submit so that may help to last conclusion.
in the page 5 of this written there is a figure with some explanations.i think it says that velocity in turbulent cases is more than laminar. correct conclusion or not?
if so but i don't grasp where is dissipation role of turbulence then?:confused:
https://www.google.com/url?sa=t&rct=...46751780,d.ZWU
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dissipation effect
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also i read it now :http://www.cvphysiology.com/Hemodynamics/H007.htm
that has focused on dissipation and energy damping of system of fluid.
it says that turbulence decreases the pressure thus maybe it increases velocity somewhat like bernoulli flow(although totally different but has a clue).
I wait more for any other opinions in future days if someone interested in wants to add anything more or correct my conclusion so far.
thanks a lot.

FMDenaro May 19, 2013 10:56

I have a simple advice for you, try to perform a DNS in a channel flow to explore the real nature of turbulence ;)
If you look at real turbulence by means of "modelled" simulation, you will just see what you have modelled ...

immortality May 19, 2013 11:24

Thanks a lot Filippo,
but I think it will be a new challenge because i have never run a DNS and don't know a simple case related.
do you think turbulence has tendency to increase velocity?could say something generally?

FMDenaro May 19, 2013 11:40

Quote:

Originally Posted by immortality (Post 428559)
Thanks a lot Filippo,
but I think it will be a new challenge because i have never run a DNS and don't know a simple case related.
do you think turbulence has tendency to increase velocity?could say something generally?

just think to the channel flow case, in turbulence the statistically averaged velocity profile show an increasing of the velocity near the wall. Therein, the velocity gradient is higher than in the laminar case. The answer is that is possible but is not a general rule.

immortality May 19, 2013 13:16

is there any guidance or method on how should put k and omega values on wall for starting the first iteration? omega values increase a lot beyond 2000000!
what could be possibly wrong about these high values?(also k is high,goes beyond even 1000!)
I don't want model turbulence very accurate but only want a reasonable values.


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