# Pseudo Transient approach

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 January 28, 2005, 02:09 Pseudo Transient approach #1 Xobile Guest   Posts: n/a Hello, does anyone have any tutorial or simulation which uses pseudo transient approach as an initial guess for flow field initialization? I am running a high speed compressible flow and would really appreciate any help,

 January 28, 2005, 17:34 Re: Pseudo Transient approach #2 Piet Guest   Posts: n/a A tutorial what for? Just switch it on, set your time step (in your case 1.E-08) and run it! There's nothing special that could be mentioned in a tutorial!!! hope this helps Piet

 January 28, 2005, 23:41 Re: Pseudo Transient approach #3 Xobile Guest   Posts: n/a when i am running steady state option for high speed compressible flow... it is giving me a converged solution. But when i run the same case for pseudo-transient case, the simulation stops due to negative densities in more than 100 cells.. of course my time step was 0.1 will decreasing my time step to 1E-8 solve this issue??

 January 29, 2005, 03:13 Re: Pseudo Transient approach #4 sankar Guest   Posts: n/a try reducing residual tolerance for primary variables to avoid this problem

 January 29, 2005, 14:39 Re: Pseudo Transient approach #5 Piet Guest   Posts: n/a Yes, your are absolutely right. It can solve your problem. Maybe 1.E-8 is not small enough! Try to lower it further for several hundred steps and enhance it from time to time. Any hint for such cases is to start the calculation with a dependency of temperature. This means rho = f(T) and switch back after lets say 100 iterations. best regards Piet

 February 3, 2013, 05:03 #6 New Member   Join Date: Dec 2011 Posts: 29 Rep Power: 14 What is the difference between pseudo transient simulation and transient simulation?

 May 18, 2013, 14:22 #7 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54 Transient simulation is to solve the problem in time-accurate fashion, while pseudo transient is method to get the steady state solution zee294haider@gmail.com likes this.

June 30, 2014, 05:19
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Quote:
 Originally Posted by Far Transient simulation is to solve the problem in time-accurate fashion, while pseudo transient is method to get the steady state solution
Hi

In what problems, we are recommended to apply pseudo transient method?

Thanks

 February 16, 2018, 05:41 #9 Member   Hells Blade Join Date: Nov 2017 Posts: 61 Rep Power: 8 [QUOTE=itsme_kit;499259]Hi In what problems, we are recommended to apply pseudo transient method? Could someone having experience with pseudo transient method kindly comment when or in which cases does one implement in the soution methods in fluent

February 19, 2018, 15:38
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Lucky
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Quote:
 Originally Posted by itsme_kit Hi In what problems, we are recommended to apply pseudo transient method? Could someone having experience with pseudo transient method kindly comment when or in which cases does one implement in the soution methods in fluent
The pseudo transient method is an accelerated solver for getting the steady state solution. That means don't apply this for transient problems, only steady state problems. Generally you apply it when you want to get to the solution quicker. Usually this means you have tried SIMPLE and gotten a converged problem but it takes too long. So then maybe you have switch to the COUPLED solver, and here you manage to get a solution slightly quicker but still you want even faster. Then the pseudo-transient solver is a good idea. My recommendation is to start with SIMPLE/COUPLED solvers and only have you have a robust experience with these two solvers (for each problem you are dealing with) then to go to the pseudo transient solver. Accelerating the solution generally leads to oscillatory or unstable solutions so you want to make sure that you have a robust strategy in place first.

 June 1, 2019, 18:24 Pseudo Transient #11 New Member   Denis Join Date: May 2019 Posts: 3 Rep Power: 7 Hi, I am running a simulation using Pseudo Transient Flow (compressible, turbulent, internal flow). I saw that my residual the became flat very soon. Is that correct? When I run it normally my residuals go up and down like crazy. This is my first time running pseudo transient. Thank you for your help Denis

 February 26, 2020, 12:08 Pseudo Transient/Mixed Transient Steady State #12 New Member   Cole Join Date: Jul 2018 Posts: 3 Rep Power: 8 Hello, I was looking for advice on how to go about it or what is possible. I am using CHTmultiregionfoam (Actually a derivative of it) and would like to change something to improve calculational speed. Because the flow in the fluid region takes very short periods of time to reach a steady state, it would be desirable to model this region as a steady region with the solid regions remaining in transient. The desire is to resolve the steady flow profile at each time step with the new solid region boundary conditions applied until convergence. This would thus make the new flow solution independent of the prior time steps. And based on my fully transient runs this would not be a bad assumption and would help with CFL limitations caused by the fluid region. Any advice would be appreciated. I am working in openfoam but I'm mostly looking for advice or pointers to resources that have done similar. Cole

June 20, 2020, 13:57
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Vineet
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[QUOTE=LuckyTran;682142]
Quote:
 Originally Posted by adasgupta91 The pseudo transient method is an accelerated solver for getting the steady state solution. That means don't apply this for transient problems, only steady state problems. Generally you apply it when you want to get to the solution quicker. Usually this means you have tried SIMPLE and gotten a converged problem but it takes too long. So then maybe you have switch to the COUPLED solver, and here you manage to get a solution slightly quicker but still you want even faster. Then the pseudo-transient solver is a good idea. My recommendation is to start with SIMPLE/COUPLED solvers and only have you have a robust experience with these two solvers (for each problem you are dealing with) then to go to the pseudo transient solver. Accelerating the solution generally leads to oscillatory or unstable solutions so you want to make sure that you have a robust strategy in place first.

 June 20, 2020, 13:58 #14 New Member   Vineet Join Date: Jun 2020 Posts: 3 Rep Power: 6 Can we also obtain variation of some parameter with time by using pseudo transient method ?

 January 8, 2023, 01:10 #15 Senior Member   Mandeep Shetty Join Date: Apr 2016 Posts: 185 Rep Power: 10 /Deleted, as wrong message quoted. Last edited by granzer; January 9, 2023 at 00:57. Reason: Wrong message quoted.

January 9, 2023, 00:56
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Mandeep Shetty
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[QUOTE=LuckyTran;682142]
Quote:
 Originally Posted by adasgupta91 The pseudo transient method is an accelerated solver for getting the steady state solution. That means don't apply this for transient problems, only steady state problems. Generally you apply it when you want to get to the solution quicker. Usually this means you have tried SIMPLE and gotten a converged problem but it takes too long. So then maybe you have switch to the COUPLED solver, and here you manage to get a solution slightly quicker but still you want even faster. Then the pseudo-transient solver is a good idea. My recommendation is to start with SIMPLE/COUPLED solvers and only have you have a robust experience with these two solvers (for each problem you are dealing with) then to go to the pseudo transient solver. Accelerating the solution generally leads to oscillatory or unstable solutions so you want to make sure that you have a robust strategy in place first.
But doesn't Pseudo transient increase stability and accelerate the convergence? Some cases steady state Simple/coupled runs may become unstable but using Pseudo transient gives a converged solution. Then my question is when NOT to use pseudo transient when running a steady state problem?.