Thank you for all your efforts, Peter.
For the sake of illustrations, comparisons and evaluations w.r.t. the new solver, I am starting with the "old" KESolver as it is implemented (without FCT) in Elmer v8.4. The rest of the post concerns the results obtained from this version (v 8.4) without FCT.
Finding a steady state through transient simulations was the preferred option for complicated cases, I suppose, even with the old KESolver. This used to take more time (than running in steady state), but the convergence used to be more gradual and better in my experience. So we are not at all at a disadvantage when it comes to FCT based solvers.
I have done the Step_ke problem in both steady and transient modes and the results are attached.
The first set of pictures show the velocity profiles from both; the transient result (time 500) eventually approaches the steady solution.
The second set of pictures show the Residual behavior in both cases. Interesting to note that in the transient case, the residual fall (esp. for kepsilon) is softer and gradual as compared to the residual behavior in steady state. The convergence, although slower, is a bit more monotonic in this case. I have noted the same behavior in an enhanced way for more complex cases.
Pl. note that the residual in the plots refers to the quantity "..Solver: Relative Change :" printed in the output after the Result Norm.
Also note that I've employed a low Nonlinear system relaxation factor to make the convergence smoother.
Now I will do the same analysis with the new FCTbased KESolver and post the results.
Kumar
Turbulent Flow Simulations with Elmer  A Request

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 Posts: 47
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Re: Turbulent Flow Simulations with Elmer  A Request
Here are the second set of pictures that show the residual behavior in both cases.
Also attached is the case file.
Kumar
Also attached is the case file.
Kumar
 Attachments

 Step_transient.sif
 (2.81 KiB) Downloaded 33 times

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Re: Turbulent Flow Simulations with Elmer  A Request
Hi Peter,
Attached please find the results from the new KESolver of the RevisitBranch.
The only changes I made in the sif were: (1) just used 1 Nonlinear System Max Iterations per timestep for both solvers (2) Linear system and steady state convergence tolerances were retained as in the previous sif (used to test ver 8.4) (3) used ILU0 for linear system preconditioning in both cases (as this may be the practical option for larger cases) and (4) deactivated Mesh Levels = 2. So the parameters are more or less consistent in both sifs.
As you can see, the convergence with the new solver is vastly improved, reaching there fast and almost monotonous!!
I am testing another case with a larger mesh and will be back with further results. Thanks again,
Kumar
Attached please find the results from the new KESolver of the RevisitBranch.
The only changes I made in the sif were: (1) just used 1 Nonlinear System Max Iterations per timestep for both solvers (2) Linear system and steady state convergence tolerances were retained as in the previous sif (used to test ver 8.4) (3) used ILU0 for linear system preconditioning in both cases (as this may be the practical option for larger cases) and (4) deactivated Mesh Levels = 2. So the parameters are more or less consistent in both sifs.
As you can see, the convergence with the new solver is vastly improved, reaching there fast and almost monotonous!!
I am testing another case with a larger mesh and will be back with further results. Thanks again,
Kumar
 Attachments

 Step_keNew.sif
 (3.82 KiB) Downloaded 30 times
Re: Turbulent Flow Simulations with Elmer  A Request
Dear Mika,mika wrote: ↑09 Apr 2020, 13:46 If the mesh contains prisms or pyramids, at the moment the value of the keyword "Stabilization Method" must be defined to be some other string than "bubbles". If bubble functions were wanted for stabilization purposes, one could try to use for example
Stabilization Method = String "pbubbles"
Element = "p:1 tetra b:1 pyramid b:1 prism b:1 brick b:1"
A basic pelement definition without additional bubble functions could be
Stabilization Method = String "none"
Element = "p:2"
 Mika
Thank you for explaining about pelement and bubble functions. In addition to the Kepsilon solver and navierstokes equation, the understanding on the usage of
Code: Select all
Element = "p:2"
Yours Sincerely,
Anil Kunwar
Anil Kunwar
Department of Materials Engineering, KU Leuven, Belgium
Department of Materials Engineering, KU Leuven, Belgium
Re: Turbulent Flow Simulations with Elmer  A Request
Hi All,
With the following updates in the kepsilon solver,
it is good to know that the flow solution converges quite well.
Yours Sincerely,
Anil Kunwar
With the following updates in the kepsilon solver,
 Usage of Logical k/epsilon ratio
Code: Select all
Use KE ratio = Logical True
 Need for mentioning of transient convergence tolerance
Code: Select all
Transient Convergence Measure = String "solution"
Transient Convergence Tolerance = 5.0e4
it is good to know that the flow solution converges quite well.
Yours Sincerely,
Anil Kunwar
Anil Kunwar
Department of Materials Engineering, KU Leuven, Belgium
Department of Materials Engineering, KU Leuven, Belgium

 Posts: 47
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Re: Turbulent Flow Simulations with Elmer  A Request
Hi All,
I did a few more tests of the modified solver in more complicated geometries and have to report that the results have not been very promising. The tests worked well with the backward step as we saw before, but as the flow gets a bit more complex (including backflow and circulation), we run into serious convergence issues. To illustrate, I am referring back to a previous example that was discussed in these forums earlier:
http://www.elmerfem.org/forum/viewtopic ... lon#p18243
This one and some other cases I tested with the new solver (activating Use KE ratio = Logical True) show the same pattern of behavior :
(1) After the first few time steps, the number of iterations to convergence of the Navier Stokes solver progressively increases
(2) Eventually reaches the iteration limit and the solver stalls.
In the attached log file of the above test case we can see this behavior (Navier Stokes stalls at 14th time step).
The only way I have been able to bring the flow system to convergence is by:
(1) Activating Stabilize = True in the NavierStokes solver (i.e, remove Bubble Stabilization) and
(2) Deactivating Use KE ratio = Logical True
However, this is no real improvement as I think I am merely recovering the behavior of the old KESolver by this measure.
Kumar
I did a few more tests of the modified solver in more complicated geometries and have to report that the results have not been very promising. The tests worked well with the backward step as we saw before, but as the flow gets a bit more complex (including backflow and circulation), we run into serious convergence issues. To illustrate, I am referring back to a previous example that was discussed in these forums earlier:
http://www.elmerfem.org/forum/viewtopic ... lon#p18243
This one and some other cases I tested with the new solver (activating Use KE ratio = Logical True) show the same pattern of behavior :
(1) After the first few time steps, the number of iterations to convergence of the Navier Stokes solver progressively increases
(2) Eventually reaches the iteration limit and the solver stalls.
In the attached log file of the above test case we can see this behavior (Navier Stokes stalls at 14th time step).
The only way I have been able to bring the flow system to convergence is by:
(1) Activating Stabilize = True in the NavierStokes solver (i.e, remove Bubble Stabilization) and
(2) Deactivating Use KE ratio = Logical True
However, this is no real improvement as I think I am merely recovering the behavior of the old KESolver by this measure.
Kumar

 Posts: 47
 Joined: 17 Jun 2015, 10:04
 Antispam: Yes
Re: Turbulent Flow Simulations with Elmer  A Request
The attachment didn't work. So here they are.

 Posts: 47
 Joined: 17 Jun 2015, 10:04
 Antispam: Yes
Re: Turbulent Flow Simulations with Elmer  A Request
Sorry, the attachment is still not there.
 Attachments

 2DJetMixingTank.zip
 (154.35 KiB) Downloaded 10 times

 ElmerSolver_log.txt
 (133.53 KiB) Downloaded 12 times