What does hydralic diameter means? How it is related to length scale of turbulent phenonemena. prasat

I usually use geometry's hydraulic diameter for setting of turbulence. There must be no relationship to length scale of turbulent, but it could be a restriction to the largest length of turbulent. You may check out the FLUENT manual.

Hi!

Dear Prasat. I'm sorry to say but judging by this and your previous questions you didn't open a single book on fluid dynamics or computational fluid dynamics. If you want to solve problems with cfd, you must have some background in fluid mechanics and cfd theory. Otherwise you will never know if your result is any good.

Here are some links that you can look at:

fluent

http://sp81.msi.umn.edu:999/fluent/fluent6.0/help/html/ug/main_pre.htm

gambit

http://www.nd.edu/~sgi/sgi_65/opt/fl...elp/gambit.htm

http://www.udel.edu/topics/software/...docs/index.htm

fluid mechanics

http://paper.mech.ubc.ca/~mech280/m280/m280.html http://www.nag.co.uk/simulation/Fast...html/node2.htm

Turbulence

http://www.asme.org/education/prodev...s/course13.htm

I'm sorry if my post sounds rude or impolite, that was in no case my intention. I hope this will help you...

Short some examples on definition of hydroulic diameter:

For circural tube, hydroulic diameter is equal to diameter of the tube. if you have a rectangular tube you have to use this equation:

d_h=4*area/circumference; for circle you get d_h=d

good luck

MATEUS

Thanks mateus it is really good suggestion in genreal I know the meaning of hydralic diameter. But I am just wondering why it is being used here to specify turbulent length scale. As per my knowledge length scale can be 20% of diameter right. So what I want to know is what is the relation between hydralic diameter and length scale. Prasat

no offense, but you need to spend a few years in school learning the basics of fluid dynamics. Then, and only then, should you attempt to study CFD.

hi prasat,

The following link will explain:

http://chemeng1.kat.lth.se/staff/ulf_b/b13_heat.htm

Mark

In FLUENT manual you can find an equation which allows you to relate turbulence intensity I to the hydraulic diameter.

You should know that turbulence intensity is the ratio between the RMS velcity flutuations and the average velocity.

So, for a fully developed flow,


u'
I = -------- = 0.16 Re_Dh^(-1/8)
u_avg




where Re_dh is the Reynolds number calculated using the hydraulic diameter:

Dh = section/wetted perimeter

See the link given by Mark for specific application of this definition.

Look the index under turbulence and read the specification method for turbulence boundary conditions.

P.S.

Surely prasat has to learn more about fluid dynamics and CFD, but I don't see any problem if he's asking here "easy" questions.
I think this is a place where we exchange information to learn more, and if someone wants to use this forum as a support instrument also during his first steps in CFD he shouldn't be criticized.

But this is just my opinion.

Hi

ap

Hi Ap, prasat and everybody else!

Im not shure who exactlly Ap ment when he said that we shouldnt criticize someone who asks easy questions and that's not even important here. I have nothing against easy (or any) questions - we're all learning here, so any question is of course welcome.

What I wanted to say is that there are a lot (to many) people who work with cfd and don't have a clue about it. The problem is that they don't take the time to understand cfd - they rush to get results - any results, good, bad, as long that the colours look nice. Im exaggerating here but I'm affraid that's a sad fact. on the other hand you have to realize that only theory with no expirience is almost just as bad...

So study books, work on projects and try to have a lot of fun doing that!

regards

MATEUS

I perfectly agree with you when you say too many people use CFD without knowing what there is behind.

I believe that who works in CFD not only needs a good knowledge of fluid dynamics, but also a deep knowledge of math.

But honestly, I don't think we are here to tell other people to study: if they ask, they already know they have to learn more. We don't know each others and their cultural background.

Just an opinion. No offense to anyone.

Hi

ap