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svchb October 13, 2011 08:46

discontinous pressure at the corners of a nozzle simulation (simpleFoam)
I had some problems with a much more complicated simulation so I started simplifying my mesh and setup until I ended up at a steady laminar incompressible flow...
Anyway this doesn't work and i am probably forgetting something stupid... but i am trying around for hours now...
So any help is appreciated! :)

So my problem is that i get extreme pressure values at the corners of my inlet... The velocity field looks okish for something with such a messed up pressure distribution...

So to my current setup:
solver: simpleFoam

RASModel laminar;
turbulence off;

rho 0.524;
nu 62.53;

internalField uniform 193368;
wall zeroGradient;
outlet fixedValue 193368;
inlet fixedValue 193368;

internalField uniform (0 0 0)
wall fixedValue (0 0 0)
outlet zeroGradient
inlet flowRateInletVelocity flowrate 0.615 value uniform (0 0 0)

nimasam October 13, 2011 14:49

change inlet pressure from fixedValue into zeroGradient, i guess it would solve ur problem

svchb October 14, 2011 02:52

i tried that... it's divergent...

and i know why :D... the flowRateInletVelocity bc forces a parallel flow with constant velocity... locally the pressure increase approaches infinity because the wall bc "no slip" condition forces the velocity to reduce to zero at the wall...
because of the geometry the fd becomes unstable and the system is generating energy... so to lose energy again the system is going into a negative pressure to get to the energy state that is enforced by the outlet bcs... the velocity is not affected as much because of mass conservation...

svchb October 15, 2011 02:20

oh well you are obviously right... :)
there needs to be some pressure increase between the inlet and the outlet :)
changed that to a fixed pressure after the convergence problems...
which were caused by the discontinuies and the zero gradient pressure bc at the inlet, which lets the pressure of the inlet approach infinity...

I switched to directMapped BCs, changed the mesh and the inlet to zero gradient pressure :) works as long as i don't get into the temperature range i actually require for my simulation... becomes unstable except if i refine the mesh two times... which then takes forever to converge so i need to tweak around even more... (works for Re<5000 unrefined...)

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