Understanding BCs/transported quantities

me3840 · January 25, 2017, 22:51

Hello folks,

Perhaps this question requires some knowledge of how OpenFOAM is coded, but I'm a little lost.

Take for example an LES case with the WALE subgrid model. The example I can find for it: /tutorials/incompressible/pimpleFoam/channel395 has several files in the 0 directory:
B, k, nut, nuTilda, p, U

Now being that I've done a fair few CFD cases and there are nearly-universal names for some of the transported quantities (p is pressure, U is velocity..), it's easy to "just know" what some of the quantities here are.

This still raises two scenarios which are frustrating:
1. I know I haven't inputted any BCs for a quantity I need, for example, I know somewhere I need to enter turbulent viscosity someplace. How am I just supposed to figure out that I need a "nut" file?
2. Let's say I'm a newcomer and still learning, and I copy the tutorial. "B" is not descriptive or particularly memorable, how do I know what this field is? What method can I use to track it down?

C-L · January 26, 2017, 04:14

There are a couple of ways you can check which variables you need:
- You could rename the turbulence variables to Btemp etc. and then run the solver and have a look at the error you receive - it will normally tell you which variables it needs.

-alternatively, if you know which turbulence model you want to use then you can find papers on it (normally referenced in the source code) and find out how they work and which variables they need. You could also look through the source code in '$FOAM_SRC' to find this out.

You do not need to specify all of these variables in general, only if you plan to swap to different turbulence models.

In terms of boundary conditions, sensible suggestions can be found in the tutorials for each of these variables. If more complex boundary conditions are required then I think you could come across the same issues as with commercial software! Again, if you wanted to see what boundary conditions are available you could also rename a boundary condition and see what errors come up when you try to run it. OF will usually tell you that it doesn't recognise something and then gives a list of options that it was expecting to find.

Hope that helps!

January 25, 2017, 22:51	Understanding BCs/transported quantities	#1
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	Hello folks, Perhaps this question requires some knowledge of how OpenFOAM is coded, but I'm a little lost. Take for example an LES case with the WALE subgrid model. The example I can find for it: /tutorials/incompressible/pimpleFoam/channel395 has several files in the 0 directory: B, k, nut, nuTilda, p, U Now being that I've done a fair few CFD cases and there are nearly-universal names for some of the transported quantities (p is pressure, U is velocity..), it's easy to "just know" what some of the quantities here are. This still raises two scenarios which are frustrating: 1. I know I haven't inputted any BCs for a quantity I need, for example, I know somewhere I need to enter turbulent viscosity someplace. How am I just supposed to figure out that I need a "nut" file? 2. Let's say I'm a newcomer and still learning, and I copy the tutorial. "B" is not descriptive or particularly memorable, how do I know what this field is? What method can I use to track it down?

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January 26, 2017, 04:14		#2
C-L Member Charlie Lloyd Join Date: Feb 2016 Posts: 57 Rep Power: 10	There are a couple of ways you can check which variables you need: - You could rename the turbulence variables to Btemp etc. and then run the solver and have a look at the error you receive - it will normally tell you which variables it needs. -alternatively, if you know which turbulence model you want to use then you can find papers on it (normally referenced in the source code) and find out how they work and which variables they need. You could also look through the source code in '$FOAM_SRC' to find this out. You do not need to specify all of these variables in general, only if you plan to swap to different turbulence models. In terms of boundary conditions, sensible suggestions can be found in the tutorials for each of these variables. If more complex boundary conditions are required then I think you could come across the same issues as with commercial software! Again, if you wanted to see what boundary conditions are available you could also rename a boundary condition and see what errors come up when you try to run it. OF will usually tell you that it doesn't recognise something and then gives a list of options that it was expecting to find. Hope that helps!