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Old   September 28, 2005, 05:37
Default convergence in 3d
  #1
AMV
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in phoenics i am able to solve 2d problems with ease but for 3d problems i am not getting convergence. even laminar flows too ! whether any special care is required to get convergence in 3d problems in setting up parameters like endit, liter, dtfals etc ? Thanks in advance !
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Old   September 28, 2005, 11:41
Default Re: convergence in 3d
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mick
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Generally of course 3d convergence is alot more work then 2d.

Experimentation with relaxation settings can help and experience helps, it can be a bit of a black art, but thats numerics for you.

With v3.6 you might try the automatic convergence which will generally be on by default. (Menu-Numerics-Relaxation-Automatic convergence control).

If this does not help switch it off and try sensible dtfals values for the velocities, I usually gauge mine by (smallest cell dist in main direction of flow)/(fastest velocity) and then loosen them off some, i.e. make them a bit bigger often my first guess is 1 order of magnitude higher.

If PARSOL is on and the geometry is not so simple, I might put some linear relaxation on P1 (-0.75 to -0.5), but as a general rule you should never go < 0.3.

Tem1 I usually use falsdt and experiment with the value, 99 times out of 100 for me its always orders of magnitude greater (looser) then the value I have used for velocity. However linear relaxation with TEM1 can also work well and if the flow is air-ideal-gas-law then I may relax den1 as well. Also if using Immersol I go for linrlx and use the same value for TEM1 as T3, I generally start with values of (-0.5).

Hope this helps, but experience and experiment is the best aid. But also convergence is a relative thing I look for spot values to flatten and residuals to drop at least a couple orders of magnitude, mass balance from the result file, and normalised residuals for variables to be small.

For NORMALISED RESIDUALS see :

http://www.simuserve.com/support/converg/cona3.htm

or pasted below:

Question: - How do I convert the residuals obtained with SELREF=T to residuals normalised by reference inflow quantities?

Answer:

This question is best answered by means of an example, which in this case is taken to be 2d single-phase flow in the y-z plane with solution for mass continuity (P1), momentum (V1 and W1), enthalpy (H1) and mixture fraction (MIXF). Note that in this example the units of 'resref' and 'res sum' for P1 are kg/s rather than m^3/s because DENPCO=T in the Q1 input file.

The conversion procedure is as follows:

Near the bottom of the RESULT file you will find the following normalised residuals printout for the last sweep:

Whole-field residuals before solution with resref values determined by EARTH & resfac= 1.000E-03 variable resref (res sum)/resref

P1 3.718E-09 1.562E+00

V1 7.078E-11 1.819E+02

W1 1.880E-08 6.684E-01

H1 8.555E-03 4.698E-01

MIXF 1.317E-10 5.908E-01

In addition, the following nett source printout is given for all boundary conditions and source terms, including the reference inflow rates of each dependent variable:

Nett source of W1 at patch named: INLETF = 1.202E-06 Nett source of W1 at patch named: INLETA = 2.238E-04 Nett source of W1 at patch named: OUTLET =-5.453E-04 Nett source of W1 at patch named: WFNN =-2.016E-04 nett sum=-5.219E-04 pos. sum= 2.250E-04 neg. sum=-7.470E-04

Nett source of R1 at patch named: INLETF = 2.003E-06 Nett source of R1 at patch named: INLETA = 3.799E-05 Nett source of R1 at patch named: OUTLET =-3.999E-05 nett sum=-3.638E-12 pos. sum= 3.999E-05 neg. sum=-3.999E-05

Nett source of H1 at patch named: INLETF = 9.902E+01 Nett source of H1 at patch named: INLETA = 4.615E+01 Nett source of H1 at patch named: OUTLET =-1.452E+02 nett sum= 2.136E-04 pos. sum= 1.452E+02 neg. sum=-1.452E+02

Nett source of MIXF at patch named: INLETF = 2.003E-06 Nett source of MIXF at patch named: OUTLET =-2.003E-06 nett sum=-6.821E-13 pos. sum= 2.003E-06 neg. sum=-2.003E-06

The actual residuals can be recovered by multiplying the (resref) column by the (res sum)/resref column, as follows:

variable resref (res sum)/resref res sum

P1 3.718E-09 1.562E+00 5.8075E-09 kg/s

V1 7.078E-11 1.819E+02 1.2875E-08 N

W1 1.880E-08 6.684E-01 1.2566E-08 N

H1 8.555E-03 4.698E-01 4.0191e-03 W

MIXF 1.317E-10 5.908E-01 7.7808E-11 kg/s These 'res sum' values can now be normalised by dividing them by the appropriate reference inflow rates (taken from the Nett source printout), as follows:

variable ref value res sum %error

P1 3.999E-05 5.8075E-09 kg/s 0.0145

V1 2.250E-04 1.2875E-08 N 0.0057

W1 2.250E-04 1.2566E-08 N 0.0056

H1 1.452E+02 4.0191e-03 W 0.0028

MIXF 2.003E-06 7.7808E-11 kg/s 0.0039

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Old   October 5, 2005, 04:01
Default Re: convergence in 3d
  #3
amv
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hi mike regarding convergence in phoenics !

do u feel that it is a good idea to keep the con-wiz ON and put some further restrictions through MAXINC ? if at all DTFALS approach is so good, why CONWIZ uses linear relaxation for velocities ?
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Old   October 12, 2005, 09:22
Default Re: convergence in 3d
  #4
mick
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>>do u feel that it is a good idea to keep the con-wiz ON >>and put some further restrictions through MAXINC

Yes this is a very good idea try this with restricting MAXINC

I am not sure why CONWIZ uses linear relaxation for velocities as I am presently not sure what it doe's deep inside the code, this may be a roose.

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