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John Grimm February 28, 2002 13:27

Pipe flow
 
This problem may seem very trivial to many of you, but any help would be greatly appreciated. I am trying to model the movement of water in a straight pipe, but I am a little confused about the modelling of the near wall region. I have been forced to use wall functions against the advice of a previous posting due to lack of computer resources. In the Fluent Manual a y+ value of between 30 and 60 is recommended. For my case this has a non-trivial effect on the velocity profile close to the wall and I am unsure which flow field is 'most accurate'. Are there any more specific guidelines for the values of y+ for a pipe?

I am also interested in validation approaches. I have compared my velocity profiles with theoretical profiles and they match quite well. I was recommended to use the Hinze book to look at turbulence data, but I am trying to get my head around how to use the data and if it is appropriate to validate water flow with air flow measurements. Does anybody know if it is OK to do this (I have been told that when non-dimensionalised turbulence comparisons are the same for air and water) and if any data is available where turbulence has been measured in water flowing through a circular pipe.

Also can anyone recommend further validation approaches?

Thanks in advance for spending the time to answer my question, John

Asok March 1, 2002 02:23

Fluid mechanics forum
 
I am waiting to see resonses for the mail of Grimm.The query here is a numerical one.I will be very obliged if some one can say an on-line site for fluid mechanics like our cfd on-line forum,because on many occassions beginners like me run in to doubts on fluid mechanics(FM) fundamentals.Why not Mr.Jonas flash the web site of one such.I may say that on many occassions fundamental questions on FM go almost unanswered here.Regards

gita March 1, 2002 11:59

Re: Fluid mechanics forum
 
Hi There,

Your response tempted me to say a few things from general view point.

Nothing can replace a "good" university library which is the source of knowledge. Your suggession for opening a forum on FM is good. But one thing you should know is that CFD is approx. 4 decades old(I am counting from Godunov's famous paper using FD) whereas FM is more than 150 years old. It's like an ocean. You have enormous amount of literarure in theory and experiment.

When I look at some of the responses in the forum, I get a feeling that some people don't even make an effort to find out certain things(I am not talking about laminar,instability,turbulence). I am guessing that 99% have access to computers. Many things have become web-based. If you put a search on "TVD-Total Variation Diminishing" you get umpteen no of sites related to it. Go to those sites and see what the authors have got to say. I am damn sure that one would find the answer(atleast 50%). Somebody posted a question on this and my intention is not to offend the person.

"Research is 99% perspiration and 1% inspiration- quoted by Edison and requoted by Einstein"

Every research student should visit the library(I'm talking about those who have access to the lib)couple of times a week.

If a forum is opened on FM,that will be great. But people might be lazy enough to do search and may even post questions like what is a Reynolds no. and it's significance. He may say that he is a beginner in FM.

If one wants to have their fundas cleared then one needs to get the right direction(research is not spoonfeeding!). A beginner is supposed to read some good books starting from light to heavy. After making an effort if the student was still unclear and if he posted a question,then I can understand.

I just wanted to say what I felt. I am sorry if I'd sounded harsh.

Another big problem for Jonas now....

Regards gita

Jonas Larsson March 1, 2002 14:47

Re: Fluid mechanics forum
 
I certainly think that this forum is big enough to also host pure fluid dynamics questions. Creating another forum will not create more answers automatically.

The only other fluids-forum with any significant traffic is the sci.mech.fluids newsgroup. There is also a new forum on efluids, but it seems pretty dead so you are probably better of going to the usenet newsgroup.

ma March 2, 2002 06:47

Re: Fluid mechanics forum
 
excellent gita

Asok March 2, 2002 08:23

Re: Fluid mechanics forum
 
Hello!Thank you Gita for your views and I am not hurt.I think I may have been understood to have requested our present cfd-online forum themselves to start a new forum for fluid mechanics(FM)-not a bad idea either and thanks to Mr.Jonas.I was enquiring of the existence of any FM-online forum and I am just browsing the web site told by Mr.Jonas....

Though the N-S eqns. were developed some 200 years ago,the hard truth is that not all the readers/researchers have digested them.Every time I teach N-S eqns,I have to tell to my students that probably the next time I have to teach it in a better way.I need a teacher- a much better one than me and that is why I am here.

Most of the learners post questions after trying all means and ways for getting a answer.Good libraries can not be the outright solutions and all learners need not be fortunate enough to access them or identify and buy good books.Also reading books doesn't quarantee that the reader can understand everything.

While not taking efforts is unfair,it may not be unfair to look out for a web site where one can get his doubts cleared quickly,in a better way and with a personal touch.I myself may be tempted to post a question on the significance of Reynold's number since the possibilities of getting spectacular, hitherto unheard of explanations and openings to new vistas in the phenomenon of fluid flow are much more higher.

Most of the experts including Gita are so kind and forebearing/tolerating enough to spend their precious times in helping a variety of forum users.Today's learners are tomorrow's teachers and the credit goes with esteem to the forum promoters/organisers.

Any forum will be shared naturally by experts as well as learners and anybody with a query is a learner existing in some level which can differ from very preliminary to advanced.At present the cfd-online forum is doing a great job in this direction.Regards to all.

Michael Malin March 4, 2002 08:23

Re: Pipe flow
 
The recommendation to locate the near-wall grid point in the range y+ 30 to say 100 is because this is a region of approximately constant shear stress (see Hinze). The assumption of constant shear stress is used in the development of standard wall functions. The wall-function formulae should force the velocity to obey the logarithmic law of the wall at the near wall grid point, and so the actual predicted value of the "near-wall" velocity will change if the physical location of the grid point is changed. However, this value should still satisfy the law of the wall.

If the predicted turbulence profiles are normalised with the friction velocity and pipe diameter, then they can be compared directly with Hinze's data in the fully turbulent region. If the Reynolds number based on bulk velocity is sufficiently high, then it is does not matter whether the fluid is water or air.

I would have thought that a high-Re turbulence model plus equilibrium wall functions would give a good prediction of turbulent pipe flow. The exception is when the near-wall grid point is located too close to the wall (in the laminar sub-layer), because then the wall functions would become invalid and the pressure drop would be predicted incorrectly.

Asok March 4, 2002 08:58

Annular flow
 
Hello friends!I will be grateful to receive information on anlytical/numerical/experimental/bench mark results for pressure drop in concentric annular passages for laminar/turbulent flows.Well!I am not putting this without trying.I have just managed to get a two decade old paper.My search is still on and hope our forum users will be highly helpful.Regards and thanks.

John Grimm March 11, 2002 13:15

Re: Pipe flow
 
Thank you very much for your response. I have been off work for a while, but I am now working through the Hinze book once again. I would like to stress the importance of the y+ values for my simulation. If I use a y+ of 30 for the boundary layer the result is significantly different when compared with a y+ of 60. I have looked in numerous books, but cannot find any more detailed advice. Do you, or does anybody else, know if further advice exists for modelling the near wall condition in a pipe if wall functions are used (I cannot use other near wall treatment)?

Thanks once again for your help


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