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Old   May 14, 2005, 08:49
Default Symmetric BCs
  #1
mao
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Hello all,

I am analysing flow around a bldg and the flow domain is symmetrical about the longitudinal plane passing thru the center of the domain (of course, for a symmetrical bldg).

So, please, I would like any one to tell me what symmetric boundary conditions (pressure, velocity, k, epsilon) I should specify on the plane of symmetry.

Since I am considering a larger bldg, i couldn't run with the available computer memory.

plzzz, help me...and I thank you very much in advance for whatever u shall have to say.

regards,
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Old   May 17, 2005, 16:27
Default Re: Symmetric BCs
  #2
Mani
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In general, symmetric boundaries act like mirrors. A zero normal derivative is applied on all scalar quantities like pressure, k, epsilon. Vectors like velocity are mirrored, i.e. their normal components are zero at the symmetry plane, and their tangential components have a zero normal derivative. You will need to apply such boundary conditions on all independent variables.

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Old   May 18, 2005, 01:20
Default Re: Symmetric BCs
  #3
mao
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Hello Mani,

I really thank you so much for your excellent response. You said, "You will need to apply such boundary conditions on all independent variables." Did you mean on all "dependent variables?"

Also, I don't know how to apply Neuman BCs (derivative=0) in commercial packages, say ANSYS. Would you have known how?

Thanks again, mao
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Old   May 18, 2005, 02:59
Default Re: Symmetric BCs
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Rami
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Mao,

Let me add two notes here.

1. Symmetry BC is indeed represented by no-flux (or Neumann) BC. This is called a natural BC, meaning it is the default BC on any part of the boundary where no explicit BC is prescribed. You may look at any textbook where the weak form of the conservation equations (e.g., FVM, FEM) is formulated. Consequently, this is also true for ANSYS.

2. Having symmetric geometry and symmetric inflow does not guarantee asymmetric solutions may not exist physically. One famous example is the Von Karman vortex street. When you prescribe symmetry, you exclude such solutions, and it is up to you to decide if this makes sense in the problem at hand.

I hope this helps.
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Old   May 18, 2005, 12:37
Default Re: Symmetric BCs
  #5
Mani
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Mao,

I can't tell you how to apply it in ANSYS, maybe you should try an ANSYS forum or consult the manual or technical support. I am pretty sure there must be a way to specify such standard boundary conditions.

I did mean 'independent' variables. Once those boundary conditions are applied, the dependent variables are obtained from the independent ones. Anyway, I am not sure you'll have to worry about every detail of it, since you are using a commercial code.
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Old   May 19, 2005, 09:11
Default Re: Symmetric BCs
  #6
mao
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Hello Mani and Rami,

Thanks guys....your replies were enlightening. Yeah, I tried with the BCs Mani suggested and the results didn't seem to match, with the experimental ones.

As Rami said, I think it's due to the Von Karman vortices. They are not reproduced with the halved model. It sounds I can not go further...otherwise, I shall have to go for systems with higher configurations.

Thanks again.
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