# Internal field boundary condition confusion

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 September 4, 2016, 18:07 Internal field boundary condition confusion #1 Member   annn Join Date: Jun 2016 Posts: 40 Rep Power: 9 I am confused with the concept of the internal field and what value to assign it with in the dictionaries, I read that it is the everything but the border (boundary) of a face, but then if the inlet had one value and the outlet had another wouldn't both of their internal fields be the same since there is no way to distinguish the internal feild of an outlet to an inlet? Just as an example to see if I am getting this correct, in the velocity dictionary say the inlet was starting at a velocity of 5m/s and the outlet was 10m/s but I set the internal field as 0m/s what does this mean? the fluid is moving at 5m/s at the boundary of each cell of the inlet and at zero in the middle of the face?

 September 5, 2016, 06:12 #2 New Member   Michael Join Date: Aug 2016 Location: Australia Posts: 3 Rep Power: 9 From what I understand (I'm relatively new to OpenFOAM myself), an external patch such as an inlet or an outlet is simply considered to be the cells on the boundary or 'outside' of the geometry. In the 0 folder you specify the initial conditions and boundary conditions of your case, so in your example the initial timestep (0) would show all of the cells on the outside of the geometry belonging to the inlet patch having a velocity of 10m/s in whatever direction you specified, all of the cells on the boundary belonging to the outlet patch having a velocity of 5m/s and all the cells in between having a velocity of 0. Each cell can only have one value for each field (U, T, p, etc.) so it doesn't make sense to talk about the border of a cell having a different value to the middle of a cell. Of course once you run the simulation all of the internal field values will (hopefully) change, which you will see if you ever look at a computed timestep, it has a list of values for U rather than just one internal field value that applies to every cell. Also it's probably worth mentioning you usually shouldn't specify a velocity at the outlet as well as the inlet, as your case becomes overspecified and most likely won't run. -Michael

September 6, 2016, 06:25
#3
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larmes
Join Date: Aug 2016
Posts: 26
Rep Power: 9
Quote:
 Originally Posted by Michael_C Also it's probably worth mentioning you usually shouldn't specify a velocity at the outlet as well as the inlet, as your case becomes overspecified and most likely won't run.
but do you think that fixing a value for the pressures at the inlet and outlet and having te velocity be determined from the pressure might be over defining the situation?

 September 6, 2016, 07:58 #4 New Member   Michael Join Date: Aug 2016 Location: Australia Posts: 3 Rep Power: 9 It depends on your assumptions, i.e. whether you're assuming compressible or incompressible flow. Have a look at this thread: http://www.cfd-online.com/Forums/mai...condition.html

 Tags boundary condition u, internal field