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How to get better convergence in transient simulation 

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April 17, 2013, 00:44 
How to get better convergence in transient simulation

#1 
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leo
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I'm doing a transient multiphase simulation with timedependent pressure inlet.
I found it was difficult to converge with my desired time step. If smaller time step was used ,it'll cost time. So what should i do to get better convergence with relatively bigger time step. 1.Will the initial value influence the transient convergence as i use the steadystate result as initial value? Should i start with a better converged steady result? 2. Should i start the simulation with smaller time step and then increase the time step as my simulation is periodic ？ Thank you in advance. 

April 17, 2013, 07:43 

#2 
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Glenn Horrocks
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Yes, the initial conditions will affect convergence. The closer the initial condition is to the real condition, and the smoother it is the better the convergence.
And yes, starting with a small time step and increasing it as the simulation progresses is standard practise. 

April 17, 2013, 08:34 

#3 
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leo
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Thank you for your reply.
I have a few more questions. 1)I have done the mesh sensitivity analysis when i was doing steady state simulation. Should i do it again when i do the transient one as i use the steady result as the initial value for the transient simulation. 2)I use adaptive time step when i was doing the transient simulation. In the first several iterations it didn't converge to my criteria.Will this affect the overall accuracy? My simulation is periodic ,i just want to get data of one cycle. Thank you. 

April 17, 2013, 18:45 

#4 
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Glenn Horrocks
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It is up to your judgement as to whether the mesh sensitivity done on steady state is applicable to a transient run. It probably is, but you need to think through what has changed and if it makes a difference to the mesh. Of course, if you want to be safe you can repeat the mesh sensitivity on the transient simulation.
If all you are interested in is the repeating periodic pattern then it does not matter if a few initial time steps do not fully converge. 

April 17, 2013, 20:49 

#5 
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leo
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If all you are interested in is the repeating periodic pattern then it does not matter if a few initial time steps do not fully converge.[/QUOTE]
So i just select the later cycles that are fully converged? 

April 18, 2013, 06:14 

#6 
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Glenn Horrocks
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Yes, that is correct.


April 18, 2013, 06:45 

#7 
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leo
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Thank you so much.


April 20, 2013, 03:26 

#8 
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leo
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Glenn,CFD is not easy, sometimes it exhausted me.
A few questions occur. As i previous said, i wanna to select the latter cycles that are fully converged as my data. But i found as the frequency of the inlet pressure increase(i set the inet total pressure as "Press = 1.0[MPa]+0.3[MPa]*sin(2*pi *f[Hz]*t" ),the simulation will go wrong at the latter cycles. I monitored massflow of the atomizer as the massflow is the physical parameter i care about. What causes to this problem? Will the timestep is the reason? I first set the timestep as "timestep=(1/f/8)[s]" and use the adaptive timestep to find the suitable timestep. As the f increase, few points are calculated in a cycle, as the timestep for the high frequency cycles is small. Can i just select the cycles which seems reasonable as my data ? 

April 20, 2013, 07:20 

#10 
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OJ
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The periodic plots of higher frequencies are dodgy. Either the flow is chaotic, or the timescales haven't been adequately resolved.
Some clarifications: Are you doing separate solution for every frequency? If yes, do you start with 1/8f as the timestep, with f being frequency? If both are yes, and if you are using adaptive timestep, then perhaps the smallest timestep value in adaptive settings need to be further reduced. How about keeping it 1e8 s with largest timestep being say 1000 s? If smallest and largest timesteps are not smal/large enough, the adpative timestepping can not home in on adequate timestep that lies outside these values. As long as you use adaptive timestepping, it doesn't matter what timestep you start with, as Solver quickly adjusts it to meet the given convergence criterai. OJ 

April 20, 2013, 07:26 

#11  
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Glenn Horrocks
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Quote:
Your graphs seem to show that the flow is in fact not periodic for a few configurations modelled. This may well be real. There is a transition from periodic to nonperiodic at some input value you have used. 

April 21, 2013, 03:19 

#12 
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leo
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Thanks i will try with a smaller starting timestep.


April 21, 2013, 07:37 

#13 
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Glenn Horrocks
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Rather than just arbitrarily reducing the tiem step, how about doing a time step sensitivity study so you work out what time step size you actually need. Or even better, use adaptive timestepping, homing in on 35 coeff loops per iteration and let the solver work it out for itself.


April 22, 2013, 09:41 

#14 
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leo
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Thanks,this simulation seems tricky. I am doing the sensitive analysis, and confused by a few results.
I will try to sort it out and then ask you for help. Thanks for help. 

April 27, 2013, 03:10 

#15 
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leo
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1)Hi,I come here to ask for your help again.
There some problems confuse me for some time, and i think i should explain it in detail. 1. Physical background I am doing a simulation of a pressureswirled atomizer where there is a aircone in the outlet region.So it is a multiphase problem.I want to study the dynamic response characteristic of the atomizer. I set the inlet total pressure as 1.0[Mpa]+0.3[Mpa]*sin*(2*pi*f*t),where f refer to the frequency. I care about the average mass flow rate of the atomizer in one cycle. How will it change with the pressure fluctuation frequency. Homogeneous and free surface model was applied. First i did the steay simulation, with the inlet total pressure as the 1.0Mpa. vf.jpg a slice of the water volume fraction Last edited by sjtusyc; April 27, 2013 at 03:59. 

April 27, 2013, 03:23 

#16 
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leo
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2) When i was doing the steady state simulation, I did the sensitivity analysis of the mesh and convergence criteria and other things.
Based on the steady simulation, I start to do the transient simulation. I used the adaptive time step to let the solver to find appropriate time step. But something weird happened. 1. surface tension To my understanding, i think the surface tension in this problem is not important, so i didn't take it in my consideration. I did confirmed it when i was doing steady state simulation. But has a big effect on the result in transient simulation. Take f=10Hz as an example. when the surface tension is considered, the transient mass flow rate is like QQ??20130427152223.png 

April 27, 2013, 03:27 

#17 
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leo
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3)As the picture tells, the averaged mass flow rate in the transient simulation is bigger than the steady simulation.
But when the surface tension is not considered it is not the case at all QQ??20130427152237.png The averaged mass flow rate is almost equals the steady simulation. 

April 27, 2013, 03:41 

#18 
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leo
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4)double precision
And i found out the double precision have an big effect on the result too. The previous data was got without double precision. When i turned on the double precision and take the surface tension into consideration, the reulsts are: QQ??20130427154027.png And the averaged mass flow is again equals the steady state. 

April 27, 2013, 03:50 

#19 
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leo
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5)As the averaged mass flow rate is what i care about, these results really confused me.
Will the surface tension lead to error and double precision will alleviate it ? And i check the pressure field. 1.When surface tension is considered with single precsion: t=0s.png t=0s t=0.1.png t=0.01s t=0.2s.png t=0.02s t=0.5.png t=0.05s t=0.1.png t=0.1s 

April 27, 2013, 03:56 

#20 
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leo
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6)
It seems that the pressure field is not good enough. When double precision is turned on the free surface tension is considered too. 0s.png t=0s 0.02s.png t=0.02s 0.05s.png t=0.05s It seems that the pressure field is better. 

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