Comparison of ITHACA (projection based) and EZyRB (interpolation based) ROM's

[siedan17]

Hello,

I am experimenting with both of your ROM-Tools ITHACA-FV and EZyRB. I tried to compare results of both tools, because they use a mathematically different approach, namely an invasive projection based approach in most of the ITHACA tutorials and a non-invasive interpolation approach in the EZyRB package.

For example I tried to reconstruct the flow field of tutorial 03 for the same parameter values used in the offline stage - first in ITHACA and then also in EZyRB. In both cases I used 10 modes and the RBF Interpolation in the EZyRB case.
My experiences are that the reconstruction with the interpolation approach used in EZyRB works much better.
Furthermore I get differences, which look structurally quite different. (see pictures 1, 2 below)

I found similar results for the tutorial 06.

Is there an explanation why this may be? Do you get the same results?

I made one interesting observation when I compared the modes of both approaches. Firstly they differ in magnitude, but that would cause no big problem. Secondly, it seems that ITHACA doesn't use the first mode which appeared in EZyRB. The correspondence is:
EZyRB mode #2 <-> ITHACA mode #1
EZyRB mode #3 <-> ITHACA mode #2
and so on ...
(see pictures 3, 4 below)
Maybe that is the source of the difference, because the mode #1 of EZyRB seems to caption the flow field quite good and looks a lot like the solution except of the magnitude?

I would be thankful for a response!
Daniel