# Will the results of steady state solver and transient solver be same?

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 September 26, 2019, 04:01 Will the results of steady state solver and transient solver be same? #1 Member   Join Date: May 2013 Posts: 34 Rep Power: 13 Hi, foamers, I have this question quite a long time ago. I think the steady state solver will give the same results as the transient solver does. However, what will it be when a system won't have a steady state but a steady state solver is applied? will the results not converge? or it will converge but give a non-reasonable result? I simulated a flow using the steady state chtmultiregion solver, and all the initial residuals go below 1e-6 at last. I also simulated the same case use the transient chtmultiregion solver. However I found the flow pattern is quite different. here is the simulation domain, on pipe for inlet, one pipe for outlet. near the bottom surface I made a slice and vector glyph this is for steady state solver near where the outlet pipe is: the flow swirls and flows outside. however, for the result of transient solver, the flow won't swirl and flows out directly: the fluid will flow through the domain in about 10s, and I calculated for 25s using the transient solver. The k-e RAS model is applied. So, does anyone know why the results are different? Does this mean that this flow do not have a steady state? Thanks!

 December 3, 2019, 19:49 #2 Member   Join Date: May 2013 Posts: 34 Rep Power: 13 Anyone has any ideas?

 December 5, 2019, 02:37 #3 New Member   kai Join Date: Nov 2019 Posts: 6 Rep Power: 6 I suggest k-e model to be used only for steady state simulation without swirling motion. The model dissipates a large fraction of turbulent kinetic energy into turbulent viscosity, hence you will always observe non-physical behavior of swirling motion regardless whether you run it in steady (without dt term) or unsteady way (with dt term). It is correct that you could always observe convergence, the model you choose is easy to converge, but results are often non-physical. Kai.

 December 5, 2019, 03:10 #4 Member   Join Date: May 2013 Posts: 34 Rep Power: 13 Hi dear Kai, Thank you for the reply. And could you kindly give me more details about your reply? You think the k-e model should not be used while in the flow system the swirling motion exists? which means you think the transient results without the swirling motion in my simulation is correct? Am I understood you right? In my simulations, I don't have experiment results so I don't know the flow swirling flows out or not. The k-e model is applied in both the steady state and transient simulations, but steady state gives swirling flowing out and transient gives direct flowing out. Thank you.

 December 6, 2019, 07:03 #5 New Member   Fab Join Date: Apr 2019 Posts: 27 Rep Power: 7 Hi. This is interesting. I've found that sometimes (using simple and pimpleFoam. I don't know much about multiregionFoam), when the flow is physically unsteady, the steady-state solver shows the same results as the transient one (ie. varying solution with iterations and high residuals) and sometimes, the solution is "trapped into an attractor" and an unphysical steady solution is found. The solvers are highly sensitive to initial conditions and the results can change dramatically. I suggest you to try running a transient simulation starting from the steady state solution. It might just continue swirling. And you may wanna do the opposite, try to start a steady state calculation from the transient solution. You can also run your transient simulation for longer to see if there's any change.

December 9, 2019, 21:45
#7
Member

Join Date: May 2013
Posts: 34
Rep Power: 13
dear Rotidpor,

thank you for sharing your experience!
In fact I have tried to start the transient solver from the swirling results of the steady-state-solver, and the results was swirling and parameters such as velocities did not oscillate very much.
I will try another thing that you said , start the steady-state-solver from the no-swirling results of the transient solver to see what will happen.

best regards.

Quote:
 Originally Posted by Rotidpor Hi. This is interesting. I've found that sometimes (using simple and pimpleFoam. I don't know much about multiregionFoam), when the flow is physically unsteady, the steady-state solver shows the same results as the transient one (ie. varying solution with iterations and high residuals) and sometimes, the solution is "trapped into an attractor" and an unphysical steady solution is found. The solvers are highly sensitive to initial conditions and the results can change dramatically. I suggest you to try running a transient simulation starting from the steady state solution. It might just continue swirling. And you may wanna do the opposite, try to start a steady state calculation from the transient solution. You can also run your transient simulation for longer to see if there's any change.

December 9, 2019, 22:04
#8
Member

Join Date: May 2013
Posts: 34
Rep Power: 13
Hi, Randolph

it definitely helps me a lot!

can you help me to see if I understanding is right:

a) if we just use the laminar model to calculate a real turbulent flow, then actually we are doing a "very coarse DNS" and the results mean nothing because the grid is not fine enough to capture the real flow structure.

b) because the RANS equations are time-averaged equations, so if we use a steady-state-solver, apply the RANS model and if we got a converged result, then this result represents the time-averaged turbulent flow field.

c) however, in b), if the result did not converge, then what does this result mean? or is it just meaningless?

d) if the RANS model is applied to a transient solver in OF, what will it mean? because the RANS are time-averaged equations, in the transient solver what will the "time" in "time-averaged" mean? does it mean the flow field is time-averaged from the 0 to now? if not, does the variation of the results represents the flow developing during time?

I really learned a lot from your reply.
In fact I'm always doing DNS in very small scales in the past and never touched the RANS and LES, so it is a little difficult for me to understand the applications of these models.
it is also the first time I heard URANS, I will search the internet to learn it.

Thank you for taking your time

best regards.

Quote:
 Originally Posted by randolph Hi, This problem might be slightly complicated. Let's consider a classic problem of flow over a cylinder. ...... I really wish I am helping in some sense. Rdf

 December 19, 2019, 11:03 #9 Senior Member   Reviewer #2 Join Date: Jul 2015 Location: Knoxville, TN Posts: 141 Rep Power: 11 Hi, a) I think so. There is also something associated with the energy-preserving property of the solver/scheme, someone on this forum told me that native OF solver is not energy-preserving. b) Correct. RANS results are the representation of time-averaging/ensemble averaging for a statistically stationary turbulent flow. c) if the result did not converge, I think the results are meaningless. d) the transient solver for RANS is referred as URANS (unsteady RANS). I do not read enough material to justify the physical meaning behind URANS. In fact, in my own experiment, I often struggle with the interpolation of the URANS. one of my professors told me that you can think URANS as "filtering in time/frequency". But generally, I only use URANS for 2d simulation. if I need 3d simulation, I always go for RANS and LES. __________________ I went the other way around. I learn the RANS and then DNS/LES. Personally, I found it is much easier for me to understand the RANS model if I had experience with DNS first. Thanks, Rdf

 December 28, 2019, 05:21 #10 New Member   kai Join Date: Nov 2019 Posts: 6 Rep Power: 6 Hey, Sorry I was too busy to reply to this thread after my 1st post. I'd like to answer your last post now based on my experience (haven't read the rest): a) I think yes, a laminar model gives you DNS result if your grid is fine enough. (considering high order numerical scheme) b) Your steady-state RANS (say k-e) solution presents you mean flow field + clipped fluctuation. Often this fluctuation is incorrect if k-e is used for swirling flow. (try backward step simulation with k-e and other RANS model, backflow occurs at different positions) c) My feelings is unless your mesh is terrible, k-e will always give you a non-physical converged result. d) Time averaging in URANS is equivalent to steady-state RANS result if you have done averaging for long sufficient time. (at least 20 several swirling time or flow through time). Any difference you observe only indicate insufficient averaging time. Kai.

 Thread Tools Search this Thread Search this Thread: Advanced Search Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post spelletier Main CFD Forum 1 June 26, 2019 04:08 AdidaKK CFX 75 August 20, 2018 05:37 Martin Hegedus OpenFOAM Running, Solving & CFD 22 December 16, 2015 04:59 cuteapathy CFX 14 March 20, 2012 06:45 jing113cn FLUENT 2 January 15, 2010 03:18

All times are GMT -4. The time now is 18:00.

 Contact Us - CFD Online - Privacy Statement - Top