Solution changing with dt.

qrie · July 1, 2013, 08:57

Hi,

I have a code in cylindrical coordinates that uses central differences. My problem is this: when I change the time step size, keeping everything else the same(grid included), I get vastly differing solutions.

Been breaking my head over this - any ideas??

Thanks in advance!

cfdnewbie · July 1, 2013, 11:21

Are you talking about a steasy state problem or an unsteady one?

qrie · July 2, 2013, 01:01

Its a steady state problem.

cfdnewbie · July 2, 2013, 03:18

Are you using an implicit or explicit solver for this?

qrie · July 2, 2013, 04:25

Advection terms are explicitly calculated. Viscous terms in the radial direction alone are implicitly calculated.

FMDenaro · July 2, 2013, 05:09

I think that the discussion of your problem must be better focused:
- what variable you see that strongly change with dt?
- what about the cell Re number? are you sure to fulfill the stability constraint?
- have you experienced the same behavior for refined h steps?

July 1, 2013, 08:57	Solution changing with dt.	#1
qrie New Member Join Date: Jun 2010 Posts: 20 Rep Power: 15	Hi, I have a code in cylindrical coordinates that uses central differences. My problem is this: when I change the time step size, keeping everything else the same(grid included), I get vastly differing solutions. Been breaking my head over this - any ideas?? Thanks in advance!

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July 1, 2013, 11:21		#2
cfdnewbie Senior Member cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20	Are you talking about a steasy state problem or an unsteady one?

July 2, 2013, 01:01		#3
qrie New Member Join Date: Jun 2010 Posts: 20 Rep Power: 15	Its a steady state problem.

July 2, 2013, 03:18		#4
cfdnewbie Senior Member cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20	Are you using an implicit or explicit solver for this?

July 2, 2013, 04:25		#5
qrie New Member Join Date: Jun 2010 Posts: 20 Rep Power: 15	Advection terms are explicitly calculated. Viscous terms in the radial direction alone are implicitly calculated.

July 2, 2013, 05:09		#6
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,764 Rep Power: 71	I think that the discussion of your problem must be better focused: - what variable you see that strongly change with dt? - what about the cell Re number? are you sure to fulfill the stability constraint? - have you experienced the same behavior for refined h steps?