I have so far used Elmer in various physical disciplines sufficiently successful (time-dependent thermomechanics, coupled FSI, magnetics ...). Now I am on to assessing the thermal air flow by convection. For this, I firstly ran the GUI example for the vortex shedding phenomenon from the GUI tutorials. All works fine and quickly, so the example lends itself to playing around to find sensitivities. I realized, that the BDF order was set to 2, whereas for any other transient calculations for heat etc., I used 1 in the past (Nowadays, I assume this was correct, for reason given later). In my trials, I found for sure a dependence on the viscosity and density of the ideal fluid used, and as well and dramatically to the BDF order. I understand that the BDF order is the polynominal order of a function, that makes a new timestep inherit field conditions from the past timestep(s). On the vortex example, it decides upon if the solution shows a small vorticity (1, 2) or a highly turbulent field (4, 5). This is ok and does not surprise me, but I wonder now, how to figure out, what the correct BDF order would be to resemble something close to reality? Do I have to make a test and validate the setting by similarity of simulation with reality in each case, or would someone have a simpler or more effective idea? This all said with being a hands-on engineer with some physics background, but less mathematical background...
![Embarrassed :oops:](./images/smilies/icon_redface.gif)
I am looking forward to your thoughts,
Thanks, Thomas.
p.s. I think the BDF order of 1 for a transient heat simulation is just fine, as thermal processes are slow and highly damped by thermal capacities (much like a highly viscous fluid in the Vortex case).