Segregated converges, but coupled diverges... why?

cfdblender · December 22, 2015, 09:12

Running an ground vehicle aerodynamic simulation and everything was fine until changed from SEGREGATED flow and energy to COUPLED flow and energy - then every simulation diverged, no matter what I tried (mesh improvements, different turbulence models, etc) ... until I switched back to segregated and it works again.

But why? Could it be the Mach number? (my simulation runs at M=0.175)

(I tried coupled flow & energy because I wanted to try a freestream boundary condition on the roof and walls of the domain to simulate an open windtunnel, but the freestream option is unavailable for segreated flow in STAR-CCM+)

me3840 · December 29, 2015, 17:44

The coupled solver is density-based in STAR-CCM+, so it's difficult to get running with low speed flows.

You need to utilize and tweak the expert driver and get a good grid sequencing solution in the beginning.

Also in my experience the coupled solver is more sensitive to poor meshes, so make sure your mesh is good quality.

December 22, 2015, 09:12	Segregated converges, but coupled diverges... why?	#1
cfdblender New Member Join Date: Jun 2015 Location: Great Britain Posts: 16 Rep Power: 10	Running an ground vehicle aerodynamic simulation and everything was fine until changed from SEGREGATED flow and energy to COUPLED flow and energy - then every simulation diverged, no matter what I tried (mesh improvements, different turbulence models, etc) ... until I switched back to segregated and it works again. But why? Could it be the Mach number? (my simulation runs at M=0.175) (I tried coupled flow & energy because I wanted to try a freestream boundary condition on the roof and walls of the domain to simulate an open windtunnel, but the freestream option is unavailable for segreated flow in STAR-CCM+)

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December 29, 2015, 17:44		#2
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 24	The coupled solver is density-based in STAR-CCM+, so it's difficult to get running with low speed flows. You need to utilize and tweak the expert driver and get a good grid sequencing solution in the beginning. Also in my experience the coupled solver is more sensitive to poor meshes, so make sure your mesh is good quality.