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implmenting tempearture boundary condition

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Old   June 8, 2005, 18:56
Default implmenting tempearture boundary condition
  #1
rahel
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Hi all

I have to implement a constant temperature boundary condition on an immersed vertical tube in a fluidized bed. the boundaries of the immersed tube coincide with face of scalar control volume . so phase velocities and porosity on the boundaries of tube are zero. now how can I apply constant temparature boundary b'coz temperature is stored on the centre of the cell while the tube boundary is on the cell face.

regards

rahel

rahel
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Old   June 9, 2005, 00:29
Default Re: implmenting tempearture boundary condition
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agg
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what you should do is add a cell beyond the vertical boundary (ghost cell) and make temperature at the center of that cell negative of that just inside the boundary so that at the boundary temperature will be zero (by interpolation).

I hope this is clear, otherwise there are books/papers that explain this better with figures.

good luck.
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Old   June 9, 2005, 17:56
Default Re: implmenting tempearture boundary condition
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rahel
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could u cite some referneces plz.
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Old   June 10, 2005, 13:16
Default Re: implmenting tempearture boundary condition
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Mani
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>so that at the boundary temperature will be zero

zero absolute temperature?? are you sure?

The ghost-cell approach is one way to implement boundary conditions, and the approach agg describes works for any wall temperature (not necessarily zero), but is rather specific for cell-centered codes that use centered averages on the cell faces.

You would need to give us some details about your numerical scheme, in particular about the way the fluxes on the cell faces are described, in order for us to tell you how to implement the boundary condition.
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Old   June 10, 2005, 13:26
Default Re: implmenting tempearture boundary condition
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Jim_Park
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Set the ghost cell center temperature as follows:

T_ghost = 2*T_wall - T_firstcell

T_ghost is in the center of the ghost cell, T_firstcell is in the center of that cell.

This is for a constant mesh spacing in the direction normal to the wall.

If you have a varying mesh spacing, you need to adjust this formula (involves T_secondcell as well) so that the truncation error for the energy balance on that first cell is minimum.

This formula works for setting tangential velocity as well, substituting v for T.

So, if T_wall = 0 (an unusual boundary condition if T is in absolute units!), T_ghost is = - T_firstcell. A special case.
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Old   June 10, 2005, 14:43
Default Re: implmenting tempearture boundary condition
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agg
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I'm sorry, i gave zero as example because i was thinking velocity in my mind. Essentially i meant what Jim_Park has written up.
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