simpleFoam convergence problems and boundary conditions

nikz123 · August 23, 2019, 03:58

Hello ereryone,

I have a simple incompressible case of fan with three regions meshed separately (inlet, rotor, outlet) and coupled with intersecting patches. The aim of the problem is to calculate the volume flow rate that a fan can pull in.

The problem I am facing is with the turbulence model when I include turbulence model into the simulation the solver blows up. Things I did to come to this conclusion are the following

1) After coupling the meshes and checkMesh OK

2) Since I do not know the inlet velocity. I specified pressureInletOutletVelocity for inlet and outlet in 'U' and totalPressure for inlet and outlet in 'p'. Rest of it is standard noSlip and zeroGradient and cyclicAMI for corresponding wall patches and intersecting sections

3) With standard solver settings for fvSchemes and fvSolution adapted from pitzdaily tutorial I run the simulation with turbulence mode 'off' and I have the converged solution.

4) If I leave the simulation sufficiently to be in steadystate with laminar condition and switch on the turbulence mode later the solver suddenly blows up

5) The problem is since I do not know the inlet velocity or I want to calculate it from the simulation I have no idea how to provide boundary conditions in 'k' and 'omega' if I use komegaSST model for example. I have tried my luck with different values of 'k' and 'omega' (providing 'k' sufficiently small and 'omega' sufficiently large) nothing seems to work. The standard procedure to apply the rough estimate of 'k' at inlet from turbulence intensity does not apply in this case (or am I missing something here I don't know)

If anybody can give me some idea as to how one can specify the boundary conditions for turbulence models for such a case or any other approach to solve such simulation by providing meaningful boundary conditions would be very helpful. Thank you.

Regards

ARTisticCFD · August 28, 2019, 02:13

Hii Nikhil,

Looks like a complicated problem to deal with. I was trying to simulate a centrifugal pump couple of years back and I remember that I had to face the same problems. Due to some reasons, the solver blows off when turbulence model is included. I don't remember how I solved it exactly, but you can try the following things,

1) Try increasing the value of viscosity and lowering the value of density so as to affect the properties. Ideally, solution should become more and more stable.

2) Try increasing the nNonOrthogonalCorrectors from fvSolution file

3) Try changing the velocity BC at inlet to fixedValue or flow rate. If you increase the velocity value, the solver should become more stable.

Above steps had helped me to correctly identify the source of this problem. Hope this helps a little.

Good luck!!

August 23, 2019, 03:58	simpleFoam convergence problems and boundary conditions	#1
nikz123 New Member Nikhil Joshi Join Date: May 2013 Location: Germany Posts: 7 Rep Power: 13	Hello ereryone, I have a simple incompressible case of fan with three regions meshed separately (inlet, rotor, outlet) and coupled with intersecting patches. The aim of the problem is to calculate the volume flow rate that a fan can pull in. The problem I am facing is with the turbulence model when I include turbulence model into the simulation the solver blows up. Things I did to come to this conclusion are the following 1) After coupling the meshes and checkMesh OK 2) Since I do not know the inlet velocity. I specified pressureInletOutletVelocity for inlet and outlet in 'U' and totalPressure for inlet and outlet in 'p'. Rest of it is standard noSlip and zeroGradient and cyclicAMI for corresponding wall patches and intersecting sections 3) With standard solver settings for fvSchemes and fvSolution adapted from pitzdaily tutorial I run the simulation with turbulence mode 'off' and I have the converged solution. 4) If I leave the simulation sufficiently to be in steadystate with laminar condition and switch on the turbulence mode later the solver suddenly blows up 5) The problem is since I do not know the inlet velocity or I want to calculate it from the simulation I have no idea how to provide boundary conditions in 'k' and 'omega' if I use komegaSST model for example. I have tried my luck with different values of 'k' and 'omega' (providing 'k' sufficiently small and 'omega' sufficiently large) nothing seems to work. The standard procedure to apply the rough estimate of 'k' at inlet from turbulence intensity does not apply in this case (or am I missing something here I don't know) If anybody can give me some idea as to how one can specify the boundary conditions for turbulence models for such a case or any other approach to solve such simulation by providing meaningful boundary conditions would be very helpful. Thank you. Regards

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August 28, 2019, 02:13		#2
ARTisticCFD New Member Aashay Tinaikar Join Date: May 2019 Location: Boston Posts: 19 Rep Power: 7	Hii Nikhil, Looks like a complicated problem to deal with. I was trying to simulate a centrifugal pump couple of years back and I remember that I had to face the same problems. Due to some reasons, the solver blows off when turbulence model is included. I don't remember how I solved it exactly, but you can try the following things, 1) Try increasing the value of viscosity and lowering the value of density so as to affect the properties. Ideally, solution should become more and more stable. 2) Try increasing the nNonOrthogonalCorrectors from fvSolution file 3) Try changing the velocity BC at inlet to fixedValue or flow rate. If you increase the velocity value, the solver should become more stable. Above steps had helped me to correctly identify the source of this problem. Hope this helps a little. Good luck!!