|
[Sponsors] |
Setting Up and Monitoring Transient CFX Simulation |
|
LinkBack | Thread Tools | Search this Thread | Display Modes |
April 28, 2016, 11:04 |
Setting Up and Monitoring Transient CFX Simulation
|
#1 |
Member
Jack
Join Date: May 2015
Posts: 98
Rep Power: 11 |
Hello,
I have a steady RANS simulation of a case I want to run transient LES on. I tried running the transient simulation from scratch, but the solver crashes in the beginning. I have read that it is instead recommended to use the RANS simulation as an initial starting point for the LES. I am not sure how I go about doing this in CFX however! Does anyone have experience with this? Secondly I am wondering if it is acceptable to run my LES on a very very coarse grid and larger than desired time steps (so that I can actually get results on my desktop computer) before I attempt to run a proper simulation on a remote cluster. I am very new to this area so I just want someone with experience to chime in - is this an acceptable way to do it? Otherwise one would have to run the full LES simulation and it would take days/weeks/ even months before one gets a solution to verify the physics, and obviously that is not sustainable as there are bound to be mistakes! So i guess im wondering what is a good way to monitor the integrity of the LES simulation so that one doesnt have to wait ages before finding a mistake! Thanks! |
|
April 28, 2016, 22:07 |
|
#2 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,841
Rep Power: 144 |
It is not good that your LES model crashes when you start. That suggests something is fundamentally wrong and is likely to cause problems even for the run which starts with RANS. I would investigate this further as if you fix this it is likely all your runs will run a lot better.
You should always run simulations with coarse meshes first to check it is functioning as expected before you do a fine mesh run for accuracy. This is standard practice for CFD simulations. |
|
April 29, 2016, 14:01 |
|
#3 |
Member
Jack
Join Date: May 2015
Posts: 98
Rep Power: 11 |
Thanks for the help Glenn.
I did some corrections and now my case runs at least. However I am wondering: Is the reason to use a converged solution steady as starting point, that the system is not "shocked" to much by a non-physical initial guess, which could lead the unsteady solution to diverge? Currently my simulation runs for a 1 or two time steps (with max 10 inner iterations) but then encounters a c_fpx_handler: Floating point exception: Overflow Could this be because of what I mentioned? Thanks! |
|
April 30, 2016, 10:44 |
|
#4 |
Super Moderator
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,841
Rep Power: 144 |
In some, but not all circumstances a non-physical initial condition can cause problems. If the simulation is very stable it can handle it and quickly pulls it back to reality. If not stable - well then it will crash.
There is a FAQ on floating point error which discusses a few things to consider (http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F) but you are correct in that a non-physical initial condition may be causing the problem in your case. |
|
Tags |
cfx, les, transient |
|
|