Multigrid convergence problem

Young G · September 9, 2014, 22:40

Hi guys,

Recently I am computing a centrifigual conpressor stage connected with a volute using NUMECA.

To make it converge, I decrease CFL to 2 and use the converged result on the coarse grid as the initial condition. Though the computation converges well on the second layer coarse grid (global tolerance reaches 10-6), on the fine grid it blows up and negative pressure and density turn up after several iterations.

In terms of boundary condition, it is the same as previous stage calculation (without volute) which converges well (In: Velocity direction, out: mass flow). I also check the grid quality and no huge mistake appears.

Note that I uses FNMB to connect the volute and the stage and the first block which contains negative pressure grid is just adjacent to this face.

If anyone has faced the same problem, please help. THX.

Hamidzoka · September 15, 2014, 11:59

which turbulence model do you use?
Fine Turbo has some difficulties with convergence In case of sophisticated models like SST-K omega.
Try Spalart-Allmaras or Bladwin-Lomax models.
Does it help?

colinda1 · September 22, 2014, 02:49

Are you using FINE/Turbo solver or FINE/Open solver?
Such case should easily run with SST-K-omega model.
It may help to see a picture of the flow field in a cut through the wheel/volute combination at the rotor-stator interface showing the direction of the velocity through the Rotor-stator interface.

Young G · November 19, 2014, 00:42

Quote:

Originally Posted by Hamidzoka

which turbulence model do you use?
Fine Turbo has some difficulties with convergence In case of sophisticated models like SST-K omega.
Try Spalart-Allmaras or Bladwin-Lomax models.
Does it help?

Finally, I found the reason that the diffuser I used didnot match the volute. Increasing the solidity works.
THX.

Young G · November 19, 2014, 00:43

Quote:

Originally Posted by colinda1

Are you using FINE/Turbo solver or FINE/Open solver?
Such case should easily run with SST-K-omega model.
It may help to see a picture of the flow field in a cut through the wheel/volute combination at the rotor-stator interface showing the direction of the velocity through the Rotor-stator interface.

It turned out to be a physical problem, not a numerical one as I expected.
Thanks anyway.

September 9, 2014, 22:40	Multigrid convergence problem	#1
Young G New Member Join Date: Apr 2014 Posts: 13 Rep Power: 12	Hi guys, Recently I am computing a centrifigual conpressor stage connected with a volute using NUMECA. To make it converge, I decrease CFL to 2 and use the converged result on the coarse grid as the initial condition. Though the computation converges well on the second layer coarse grid (global tolerance reaches 10-6), on the fine grid it blows up and negative pressure and density turn up after several iterations. In terms of boundary condition, it is the same as previous stage calculation (without volute) which converges well (In: Velocity direction, out: mass flow). I also check the grid quality and no huge mistake appears. Note that I uses FNMB to connect the volute and the stage and the first block which contains negative pressure grid is just adjacent to this face. If anyone has faced the same problem, please help. THX.

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September 15, 2014, 11:59		#2
Hamidzoka Senior Member Hamid Zoka Join Date: Nov 2009 Posts: 282 Rep Power: 18	which turbulence model do you use? Fine Turbo has some difficulties with convergence In case of sophisticated models like SST-K omega. Try Spalart-Allmaras or Bladwin-Lomax models. Does it help?

September 22, 2014, 02:49		#3
colinda1 Senior Member Colinda Join Date: May 2012 Location: Brussels Posts: 151 Rep Power: 13	Are you using FINE/Turbo solver or FINE/Open solver? Such case should easily run with SST-K-omega model. It may help to see a picture of the flow field in a cut through the wheel/volute combination at the rotor-stator interface showing the direction of the velocity through the Rotor-stator interface.