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Must exit velocity be always 1?

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Old   September 21, 2006, 20:30
Default Must exit velocity be always 1?
  #1
Kopa
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I tried to simulate flow past a cylinder. The exit boundary is 15 units away from the cylinder. I found that at the exit, the horizontal velocity is around 0.7.

Does it mean that my exit boundary is not far enough? Must it be 1? Btw, I'm using convective BC.

Thank you.
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Old   September 22, 2006, 10:35
Default common sense for posing questions
  #2
Mani
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if you want help you need to be more precise and complete in posing your questions.

is this external flow over a cylinder, or is the cylinder inside a channel (since you speak of "exit" and don't specify any particular point at the exit plane, I might assume it's a channel)

15 units away from the cylinder --- how are we supposed to know what that means if you don't say what 1 unit is? is 1 unit the cylinder diameter, channel height (in case of channel), or something else...?

likewise, 0.7 velocity means nothing, without knowing what the unit is. since you don't specify a unit, here, I would assume it's a nondimensional velocity, but how is it nondimensionalized? you seem to expect velocity 1, and I might guess that's your undisturbed freestream (or inlet) velocity, but you should really specify that.

is this steady or unsteady, compressible or incompressible flow?

consider this: you will find more people willing to answer your questions if you don't make them pull all this obviously necessary information out of your nose. you will get more accurate and appropriate answers if you offer all information necessary to avoid confusion and misunderstanding. most importantly, very often you'll find that thinking about a precise and definite way to pose a question actually helps you to answer it yourself!

some tips: don't assume that everyone already knows what you're doing. don't assume everyone uses the same tequniques and "standards" as you, and works on the exact same problem.
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Old   September 22, 2006, 20:55
Default Re: common sense for posing questions
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zxaar
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Well put.
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Old   September 23, 2006, 08:17
Default Re: common sense for posing questions
  #4
Kopa
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ok sorry my friend. I realise my mistake.

Here's my qn:

it's flow past cylinder at Re20 & 100. so it's steady & unsteady. input velocity is 1.0. the ns eqn has been non-dimensionized and if the flow is not disturb, exit velocity should be 1.0. however for both cases, the velocity along the horizontal center line (which cuts thru the cylinder since cylinder position is y=0) at the exit is around 0.75. away from the center line exit velocity is around 1.0.

my drag coeffcient/St no. is pretty close to the published result. exit is 15units away from cylinder.

exit bc is convective type ie du/dt+C*du/dx=0.

hope this is sufficent for you to assist me. thank you.
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Old   September 24, 2006, 16:49
Default Re: common sense for posing questions
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Mani
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you still haven't said if it's external flow or internal flow, compressible or incompressible (i.e. what is the Mach number)? I am assuming external incompressible flow.

also you seem reluctant to explain what 1 unit is, when you specify the boundary distance as 15 units. I am assuming that means 15 cylinder diameters, as that's the most commonly used unit (another one might be the cylinder radius).

based on these assumptions, the boundary is pretty close to the cylinder. if you don't use any freestream correction you should use about twice the distance (30 diameters away from the cylinder). at least you should try it and see if the solution is any different (your drag will be even more accurate). however, do not expect to see the velocity rebound to exactly 1 on the whole exit boundary. obviously, you will see a momentum deficit inside the wake (downstream of the cylinder) even far downstream. this momentum deficit is a measure for the drag (in fact, it is common practice in experiments to measure the momentum deficit in the wake profile and thus obtain the drag). it is surprising, though, that the centerline velocity has the same value for both Reynolds numbers. you should check on the width of the wake, i.e. compare the wake profiles for both cases. the drag at steady state (Re=20) should be significantly smaller than in the unsteady case (Re=100) and you should see that the momentum deficit in the former case is significantly smaller (in width and/or depth), than in the latter case.

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Old   September 25, 2006, 15:47
Default Re: common sense for posing questions
  #6
Adrin Gharakhani
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If it's external flow, two other parameters, which are ignored in this thread, are the "width" of the domain (normal to the freestream), and the location of the "inlet". Even if one uses slip BCs for the sides, there is a tendency for the solution to "squeeze" the flow to acount for the width effect (dictated by continuity). You want to make sure that the domain width is pretty far away as well. You also haven't mentioned how far upstream your "inlet" is, which should also be far enough to represent freestream correctly.

Lastly, are you assuming axisymmetry or accounting for the full domain? If the former, perhaps there is an error in the implementation of the symmetry condition (which should be checked along with the other recommendations made by Mani).

All this assumes that the value 0.75 you're getting is incorrect!! (Is it, if so by how much?)

Adrin Gharakhani
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Old   September 26, 2006, 06:54
Default Re: common sense for posing questions
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Mani
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That's right. I was assuming a full circular domain. At least you should now understand that if you have us make a lot of assumptions (to compensate for lack of info) we cannot effectively help you.
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Old   September 26, 2006, 20:10
Default Re: common sense for posing questions
  #8
zxaar
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Not picking on Kopa, but what just mani said is normally the problem here on this forum. Most of the help seekers they just write one line of question, without much giving details to others. It becomes very difficult to suggest anything to them then. the most common question asked will be of the form: My solution does not converge what shall I do. .

How to help if you can not know others things about the problem.
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Old   September 26, 2006, 21:40
Default Re: common sense for posing questions
  #9
Renato.
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In fact, all of you are indirectly giving a help for Kopa when you tell him that he must give more details about his problem.

The helpers of this forum should think that some of these help seekers are just student and not well familiarized with some CFD concepts and terms yet. Not to mention that some of them are not fluent in English -- which only make this problem worse -- It's easy for an American or British guy to describe his problem in deep details. The same observation is valid for helpers since some expressions only make the answer hard to understand. In this sense, even making a wrong or bad formulated question the helpers should try, patiently, to guide these guys in order to improve the description of his problem, or just skip the question. In this process of questions and answers, I'm sure that not only the interested will learn. We shouldn't forget that we are continuously learning something and sometime ago all of us hadn't had no clue about nothing that we've been talking about here.

The main aspect of this forum that I really love is that it's an open forum where we meet a zillion of different people from different areas trying to get some help in CFD (or not...). It's true that sometimes there are some bad intentioned people and some misundertandings, but, even so, it's a very good place to find good people to talk about CFD.

This is my humble opinion

respectfully

Renato.

p.s.: for Kopa, I'd say that if he's reproducing a problem he must replicate all that is described in his reference (even the wrong assumptions). In a first try, when you're validating something, you should try to match the same results shown in your reference using exactly the same parameters, mesh size and any other assumption. After that, you will be able to discuss about other things.

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