Modified pisoFoam for pitzDaily - solver blowing up !!

suman91 · October 12, 2016, 13:23

Hi there,

Am having some problem in solving flow over backward facing step in OpenFOAM. I am using LES and Smagorinsky Model. At start it all goes well but after some time utau values at the wall starts increasing and the deltaT of simulation goes on decreasing (till if falls to 10^-12 or so order). I cannot detect what is the problem. I have modified the pisoFoam solver so impose stress boundary condition using the 'fixedGradient' feature of OpenFOAM.

Am attaching my required files here. Please have a look and suggest where am I going wrong. Will be waiting for a reply. Thanks in advance.

solver.tar.gz

TestCase.tar.gz

suman91 · October 13, 2016, 02:43

Hi there,

I think the problems are coming with the pressure solver settings. At t=0.01 pressure values are about -120 at the inlet and then increases linearly to close to zero at the outlet. But at T=0.05, it falls down to -4000 approx. Obviously in an incompressible flow, the absolute value of pressure does not matter, it is the gradient that matters. But what I am not understanding are such low values. Considering inlet velocity has a magnitude of 10 in the x direction (which is max amongst all 3 directions), a value of -50 to -70 may still be understandable but -4000, am not getting at all. Not understanding why the pressure field is behaving such. Maybe that is triggering the problem.

suman91 · October 26, 2016, 09:32

Hi,

Back again with an update on this same issue. So to start with, setting the gradient at the wall was not so much of a good idea it seems. Somehow the wall normal velocity was being calculated to be a non zero value at the wall. This might be one of the reasons for the solver to blow up in that manner. The modified code I am using now, uses fixedValue (0 0 0) as a boundary condition at the wall and in the modified solver, I calculate the velocity at the wall using the gradient and set the velocity. The solver now runs properly and does not blow up.

The only issue am facing with this is, the reynolds stress when calculated comes out to be very less; flow is not becoming turbulent only. Am wondering if it's an issue with the gradient schemes being used or not. If anyone has any views on this, it will be really helpful.

So the question is, what is the best gradient scheme to be used for solving incompressible turbulent flow (using LES) in openFOAM?

October 12, 2016, 13:23	Modified pisoFoam for pitzDaily - solver blowing up !!	#1
suman91 Member Suman Chakraborty Join Date: Sep 2014 Location: Mumbai, India Posts: 42 Rep Power: 11	Hi there, Am having some problem in solving flow over backward facing step in OpenFOAM. I am using LES and Smagorinsky Model. At start it all goes well but after some time utau values at the wall starts increasing and the deltaT of simulation goes on decreasing (till if falls to 10^-12 or so order). I cannot detect what is the problem. I have modified the pisoFoam solver so impose stress boundary condition using the 'fixedGradient' feature of OpenFOAM. Am attaching my required files here. Please have a look and suggest where am I going wrong. Will be waiting for a reply. Thanks in advance. solver.tar.gz TestCase.tar.gz

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October 13, 2016, 02:43		#2
suman91 Member Suman Chakraborty Join Date: Sep 2014 Location: Mumbai, India Posts: 42 Rep Power: 11	Hi there, I think the problems are coming with the pressure solver settings. At t=0.01 pressure values are about -120 at the inlet and then increases linearly to close to zero at the outlet. But at T=0.05, it falls down to -4000 approx. Obviously in an incompressible flow, the absolute value of pressure does not matter, it is the gradient that matters. But what I am not understanding are such low values. Considering inlet velocity has a magnitude of 10 in the x direction (which is max amongst all 3 directions), a value of -50 to -70 may still be understandable but -4000, am not getting at all. Not understanding why the pressure field is behaving such. Maybe that is triggering the problem.

October 26, 2016, 09:32		#3
suman91 Member Suman Chakraborty Join Date: Sep 2014 Location: Mumbai, India Posts: 42 Rep Power: 11	Hi, Back again with an update on this same issue. So to start with, setting the gradient at the wall was not so much of a good idea it seems. Somehow the wall normal velocity was being calculated to be a non zero value at the wall. This might be one of the reasons for the solver to blow up in that manner. The modified code I am using now, uses fixedValue (0 0 0) as a boundary condition at the wall and in the modified solver, I calculate the velocity at the wall using the gradient and set the velocity. The solver now runs properly and does not blow up. The only issue am facing with this is, the reynolds stress when calculated comes out to be very less; flow is not becoming turbulent only. Am wondering if it's an issue with the gradient schemes being used or not. If anyone has any views on this, it will be really helpful. So the question is, what is the best gradient scheme to be used for solving incompressible turbulent flow (using LES) in openFOAM?