discontinous pressure at the corners of a nozzle simulation (simpleFoam)

svchb · October 13, 2011, 08:46

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

RASProperites
RASModel laminar;
turbulence off;

transportProperties
rho 0.524;
nu 62.53;

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

U
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

... 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...)

October 13, 2011, 08:46	discontinous pressure at the corners of a nozzle simulation (simpleFoam)	#1
svchb New Member Sven Join Date: Feb 2010 Posts: 4 Rep Power: 16	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 RASProperites RASModel laminar; turbulence off; transportProperties rho 0.524; nu 62.53; p internalField uniform 193368; wall zeroGradient; outlet fixedValue 193368; inlet fixedValue 193368; U internalField uniform (0 0 0) wall fixedValue (0 0 0) outlet zeroGradient inlet flowRateInletVelocity flowrate 0.615 value uniform (0 0 0)

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October 13, 2011, 14:49		#2
nimasam Senior Member Nima Samkhaniani Join Date: Sep 2009 Location: Tehran, Iran Posts: 1,266 Blog Entries: 1 Rep Power: 24	change inlet pressure from fixedValue into zeroGradient, i guess it would solve ur problem

October 14, 2011, 02:52		#3
svchb New Member Sven Join Date: Feb 2010 Posts: 4 Rep Power: 16	i tried that... it's divergent... and i know why ... 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...

October 15, 2011, 02:20		#4
svchb New Member Sven Join Date: Feb 2010 Posts: 4 Rep Power: 16	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...)