# Must exit velocity be always 1?

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 September 21, 2006, 20:30 Must exit velocity be always 1? #1 Kopa Guest   Posts: n/a 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.

 September 22, 2006, 20:55 Re: common sense for posing questions #3 zxaar Guest   Posts: n/a Well put.

 September 23, 2006, 08:17 Re: common sense for posing questions #4 Kopa Guest   Posts: n/a 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.

 September 24, 2006, 16:49 Re: common sense for posing questions #5 Mani Guest   Posts: n/a 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.

 September 25, 2006, 15:47 Re: common sense for posing questions #6 Adrin Gharakhani Guest   Posts: n/a 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

 September 26, 2006, 06:54 Re: common sense for posing questions #7 Mani Guest   Posts: n/a 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.

 September 26, 2006, 20:10 Re: common sense for posing questions #8 zxaar Guest   Posts: n/a 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.