[Nek5000-users] Warning Message : L1/L2 DIV(V)

[nek5000-users at lists.mcs.anl.gov]

> 1.  In earlier conversation I was told that PN-PN formulation has resulted in better accuracy. As such, I am not sure how one would choose which basis (PN-PN / PN-PN-2) is appropriate for their problem. I am working in transitional flows. Would you recommend any particular choice of basis for this type of applications ?
In general I would recommend to use the PN/PN formulation. However there are some features (e.g. moving mesh, free surface flows, MHD, etc.) which only work for PN/PN-2 for now.
In addition we don't have the SEMG multigrid preconditioner in place for PN/PN. So depending on your mesh the cost of the pressure solve is higher (more iterations are needed for a given tolerance) although the cost is usually lower per iteration for PN/PN (no interpolation is needed).  

> 2. Yes, I believe I am running it on AMD 10h processors. I will compile with K10_MXM for my production run. Thanks.
For the moment the tuned version is only available for lx1=10 and lx1=8! 

> 3. As regarding the divergence : Though the warning pops up every step, nothing blows up in my simulation. The high values ( that are acceptable) are at the outflow where we add some divergence to the flow.
Sure you will have some pretty large values at the outflow. That's ok but I am confused by the large L2-norm of the difference in divergence.

> 4. I did observe one thing, If I lower the courant to very low value, the error drops down but not by a huge margin.
Yes there is a time step effect but usually your limited by spatial resolution.

Stefan