Segregated converges, but coupled diverges... why?

cfdblender · December 22, 2015, 10:12

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, 18:44

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, 10:12	Segregated converges, but coupled diverges... why?	#1
cfdblender New Member Join Date: Jun 2015 Location: Great Britain Posts: 16 Rep Power: 11	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+)

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Segregated or Coupled flow?	hamzamotiwala	STAR-CCM+	7	October 25, 2011 20:35
enthalpy in segregated and coupled	naomi	FLUENT	4	August 2, 2007 07:47
switching from coupled solver to segregated	Oz	FLUENT	2	November 8, 2006 17:02
Segregated or coupled solver?	Nori	FLUENT	0	September 29, 2005 15:24
Segregated or coupled?	Pierluigi Allegro	FLUENT	2	September 26, 2000 00:51

December 29, 2015, 18:44		#2
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	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.