# Most Appropriate Solver ??

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 October 15, 2012, 11:11 Most Appropriate Solver ?? #1 Member   Andrew Glassby Join Date: Sep 2009 Posts: 65 Rep Power: 9 Hi everyone, I am new to the CFD scene, although I have tried to dabble a little. I'm now trying to model a real world problem. This problem is the opening of a valve and discharge of a compressible fluid into a piping system and through a restriction orifice. I do not intend to model the movement of the valve, just the ramped increase of the gas flow through the inlet boundary. I have developed a 2D model of this domain which checkMesh seems happy with, showing no errors. Reading the documentation that comes with OpenFOAM has lead me to believe that I should be looking to use rhoSimpleFoam or rhoPimpleFoam for this model although this is probably as far as I have got with this. I have tried to understand how to use these solvers. I want to model this problem as compressible and turbulent. This is my first problem: The physical problem involves a very large pressure gradient across the orifice (or shock downstream). The valve inlet pressure will be 16.5 MPa, I have read that RANS K-e solvers would not be appropriate for such a large pressure gradient so I guess I should be utilising LES (but how?) I have made the domain quite small, the pipe has an ID of 52mm (orifice 20.3 mm ID) and I have constructed the mesh to show about 1000mm upstream and downstream of the orifice. Can I leave OpenFOAM to calculate the outlet boundary Pressure since I do not know the pressure to specify? Essentially this would be a function of the inlet pressure and velocity through the orifice. From a basic flow calculation across the orifice with 1kPa outlet pressure I would have a mass flow of approx. 10 kg/s. This creates some VERY large velocities downstream of the orifice, I have calculated the Reynolds number for this problem as 3.7e7. If I calculate the velocity in the low pressure side of the orifice I get about Mach 10! I guess I have rambled on a little so basically I need to understand the solver to use to model a high velocity compressible fluid through a pipe/orifice system with a large pressure gradient. I would also like to model some of the thermophysical and transport properties but I'm happy to go inviscid and ideal as a first step if you advise this! I hope someone can give me some pointers to get me moving with this! Best Regards Andrew

October 18, 2012, 06:46
#2
Member

Andrew Glassby
Join Date: Sep 2009
Posts: 65
Rep Power: 9
Quote:
OK.... That was probably a bit wordy and vague.

I've done some more experimenting with various solvers and I have settled on rhoPimpleFoam using k-epsilon as my RAS model rather than LES. I'm currently just running with a low velocity while I experiment.

I would like to understand, though how I should set my boundary conditions? I have set my inlet BC for U as a fixedValue as I plan to implement a ramp and this seems the most obvious BC to use for this method. I have set my outlet U as zeroGradient as I would like OpenFOAM to simply report what it calculates rather than stipulating this flow (over specifying??) my outlet P is specified as a fixedValue.

When I run this, however, I notice that the velocity (Magnitude) begins to build from both inlet AND outlet boundaries which I know is wrong since I'm modelling a compressible fluid.

Have I mistakenly used zeroGradient? is there a more appropriate BC to use on the outlet boundary?

btw I used the angledDuct example in the compressible/rhoPimpleFoam/ras directory as a template for this attempt.

Best Regards
Andrew

https://dl.dropbox.com/u/17191910/Op.../blockMeshDict
https://dl.dropbox.com/u/17191910/OpenFoam/controlDict
https://dl.dropbox.com/u/17191910/OpenFoam/fvSchemes
https://dl.dropbox.com/u/17191910/OpenFoam/fvSolution
https://dl.dropbox.com/u/17191910/OpenFoam/p
https://dl.dropbox.com/u/17191910/Op.../RASProperties
https://dl.dropbox.com/u/17191910/Op...icalProperties
https://dl.dropbox.com/u/17191910/OpenFoam/U

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