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Old   August 5, 2013, 13:24
Question Help me by reviewing my boundary conditions
  #1
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Waqqas Ahmad
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Hi guys i am new at cfx and want to discuss my problem related to my boundary conditions.
i am working on labyrinth seal with super heated steam as working fluid.
this is my cfx-pre setup:
  1. total pressure at inlet :5bar absolute
  2. total temperature at inlet: 573.15K
  3. static pressure at outlet: 1 bar absolute
  4. Model: k-omega.
    Velocity at inlet is supposed to be 0 ms^-1
The problem arise when I simulate It gives me a warning that a wall has been placed at inlet at 100% of its area to stop fluid going out of the domain. What should I care for to remove this error.??
------------------------------------------------------------------+
| ****** Notice ****** |
| A wall has been placed at portion(s) of an INLET |
| boundary condition (at 100.0% of the faces, 100.0% of the area) |
| to prevent fluid from flowing out of the domain. |
| The boundary condition name is: inlet. |
| The fluid name is: Fluid 1. |
| If this situation persists, consider switching |
| to an Opening type boundary condition instead.
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Old   August 5, 2013, 15:02
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siw
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Reversed flow at the inlet so CFX is making a wall. Move the inlet further upstream away from this event. Another option (but not as recommended as moving the inlet) is to use an opening boundary condition.
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Old   August 5, 2013, 15:46
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Waqqas Ahmad
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i have used the first method but still i am getting same problem.
for a while i don't wanna use opening condition as in my case flow is into the domain and there should be no reversed flow at inlet.

while searching on net i found this on a web

"" 2.3.2. Recommended Configurations of Boundary Conditions

The following combinations of boundary conditions are all valid configurations commonly used in ANSYS CFX. They are listed from the most robust option to the least robust:

Most Robust: Velocity/mass flow at an inlet and static pressure at an outlet. The inlet total pressure is an implicit result of the prediction.

Robust: Total pressure at an inlet and velocity/mass flow at an outlet. The static pressure at the outlet and the velocity at the inlet are part of the solution.

Sensitive to Initial Guess: Total pressure at an inlet and static pressure at an outlet. The system mass flow is part of the solution.

Very Unreliable: Static pressure at an inlet and static pressure at an outlet. This combination is not recommended, as the inlet total pressure level and the mass flow are both an implicit result of the prediction (the boundary condition combination is a very weak constraint on the system).

Not Possible: Total pressure cannot be specified at an outlet. The total pressure boundary condition is unconditionally unstable when the fluid flows out of the domain where the total pressure is specified.

With more than two inflows or outflows, the other openings should be of the boundary condition type Opening. This is because the flow at these other boundaries could in general be in or out, and the direction will be part of the solution.""


what should be my initial guess?? is that mean for velocity/ mass flow rate?
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Old   August 5, 2013, 18:26
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Glenn Horrocks
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First of all - and most importantly - it is a warning, not an error. If the results from the simulation are OK then you can accept this warning and proceed regardless.

But if you think it is a problem then have a look at the suggestions you just quoted. You appear to know the inlet and outlet pressures so you have to use the Total pressure inlet and static pressure outlet approach. The initial guess for this should be the most accurate flow you can prescribe. Do that using CEL if possible, but otherwise something like a preliminary simulation using upwinding or something which converges easily but is not so accurate - this can be good to get an initial condition.
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Old   December 29, 2013, 02:26
Default same problem for me as well
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Rohit Adhav IIT Madras
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Hey friend,

same problem is coming in my simulation as well.. how did u solve it.?
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Old   December 29, 2013, 05:17
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Glenn Horrocks
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This thread describes what to do in this situation. It is also an FAQ (note the FAQ is for OUTLETs, the same also applies to INLETS): http://www.cfd-online.com/Wiki/Ansys...f_an_OUTLET.22
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