Setting Up and Monitoring Transient CFX Simulation
 

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!

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.

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!

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.