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Cfx initial time choice for adaptive timestep

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Old   April 17, 2021, 07:48
Default Cfx initial time choice for adaptive timestep
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Hello Glenn, Iím PhD Student Naval Architect Mustafa from Istanbul/TURKEY. I have been using cfx for 10 years for calculating ressistance and self propulsion of our ship designs.

I read nearly all the posts of you about adaptive timestep abilities of CFX.

Now I m working in SVA Potsdamís VP 1302 propellerís 2015 year cavitation tunnel test experiment validation on cfx.

What I did is,

1) start the analysis with 6 million mesh with one phase (only water) with steady state till to the converge.

2) skip the steady state multiphase cavitation analysis (your suggestion)

3) then continue to the analysis With transient (3-5 loop with 1e-4 rms) adaptive timestep to the multiphase cavitation analysis.

ITTC (International Towing Tank Test Comitee)Ďs recommendation for the propulsion tests on cfd, turn the propeller between 0,5-2 degree per timestep.

When I start to the analysis I start with 40 degrees per timestep then this timestep decreases with 0,8. after 20 iteration turning speed decreases to 0,2 degrees from 40 degrees. Then 120 130 140 iteration still the program could not reach the converge.

I think my problem is choosing the optimum initial timestep between max and min wall timesteps.

What is the importance of initial timestep in the adaptive transient time step ? What is your recommendation for choosing initial timestep in the adaptive transient calculations.

Last 2 questions

1) what is the function of transient rotor stator in the solution part(not in the pre side) of cfx

2) when I pull the cfx steady groups solution to the duplicated transient group side with wire ( steady solution to transient solution) in the main workbench table for continuening the analysis with transient, in the solver menu the initial step was locked. Only I can choose default current solution (if possible) or initial solution. Which one do you suggest for choosing for continuing the transient step ? May be both selections can be same for the results.

Best.

Mustafa.
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Old   April 18, 2021, 01:19
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If you want an accurate time history then you need to choose an initial time step size which converges well. If it is too large then the initial results will be wrong.

If all you want is the final psuedo-staeady state result then it can be OK as long as it later converges well. But if the initial time steps are so bad that it causes instability in later time steps then you need to make it smaller.

It is generally a good idea to start with an adaptive time step size which is way too small and let adaptive time stepping increase it, rather than too big. If the time step is too small your simulation will still be converged and accurate, just taking a little while longer. If the initial time steps are too big the entire simulation can be destroyed.

Other questions:
1) TRS is the interface at a GGI. It means the solver will move the domains and match up nodes at the interface every time step.
2) This sounds like Workbench. As you have found, I think workbench makes using CFX more complicated and it can be hard to make it do what you want it to do. If you use CFX stand alone you have much more control over these things.

(In fact, I frequently use workbench to do the geometry, meshing and initial setup of my CFX simulations, but I then take it out of workbench and use it stand alone for debugging and the actual simulation. I have much more control over it that way.)
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Old   April 18, 2021, 01:46
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Thanks so much Glenn,

Itís no problem cavitation converging problem in the steady state side for me. But when I compare the results with experiments thrust coefficient results of the propeller are ok but the cavitation patterns are not same as with experiment. Because of the complexity of multiphase cavitation I Choosed transient solution for cavitation with your recommendations in the forum.

My question is,

1) If I continue with transient cavitation analysis, What is the difference between starting from steady state ďcavitation single phase or non cavitation single phase analysisĒ ? What is your recommendation for this ?

2) after I solve this problem I will use adaptive timestep single phase self propulsion ship calculations. First start with steady resistance analysis then continue with adaptive transient. My concern is here will the adaptive analysis Continue long time ? Because in self propulsion not only propeller but also ships hull and appendages. So I have to converge resistances of all those parameters in the transient part of the analysis.

Last thing may you remembered, Iím still continue in to the steady state rigid body free surface flow with 16.2 version. But the other non rigid body analysis are continuing with 21 version. I havenít solved the movement problem of the meshes after the 16.2 version of Ansysís. For free surface ship resistance continue with 16.2
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Old   April 18, 2021, 06:43
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Cavitation simulations are very difficult to get to converge in terms of residuals, so you should expect difficulties there. The documentation has some comments on convergence and the residuals for cavitation models, I trust you have read them.

For most simulations, if all you are interested in is the pseudo-steady state results then what happens in the early time steps does not matter, providing they converge to the correct solution. But for a reasonable amount of simulations you can end up with different final conditions depending on what the initial/early state is - these simulations are especially challenging.

For your full ship model, yes I would still use adaptive time stepping to find the time step size of your full ship model. It is the approach to use in most transient simulations, unless you have a very good reason not to.
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Old   April 19, 2021, 12:10
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For The propeller transient cavitation and nominal wake analysis side, initial timestep for the adaptive is very important. I tried both starting with the large time step (for instance 40 degree per time step) and small time step (for instance 1 degree per time step) both analysis could not converged well. Now Iím trying a mid size initial time step (for instance decreasing from 8-10 degrees per time step). I will share the results with you Glenn.
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Old   April 19, 2021, 18:16
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I would go smaller than 1 degree time step to start with, not bigger. Try 0.0001 so you can get the first time step converged, and let adaptive time stepping increase it from there.
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Old   April 21, 2021, 09:30
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Glenn what do you think about increasing number of coefficient for the cavitation analysis. You always suggest 3-5 loops for most transient cases. Normally ı an using that strategy. For example for the single phase analysis I perfect.But also you said we can try 6-10 loop inner iteration for the ďtricky multiphase casesĒ. What do you think about number of inner iterationís role in cavitation analysis ? Thanks so much *♂️
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Old   April 22, 2021, 19:05
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A cavitation model could be a case where 6-10 coeff loops per time step might be required. Give it a try and see if it works better than 3-5. Note that it will result in larger timesteps, but the solver will have more loops to get everything properly coupled. So it is not obvious whether it will help or not, so just try it and find out.
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