Dear All, I am encountering some trouble when performing stability analysis of a channel flow. I am trying to track an instability as I modify flow conditions and channel geometry. I managed to reproduce the problem I am having with my calculations with the ChannelStability test. I start by running the unmodified ChannelStability.xml, looking for 2 modes, using kdim=16 at Re=7500. In a matter of a couple iterations I get the expected solution: Magnitude Angle Growth Frequency Residual EV: 0 1.0003e+00 3.4978e-02 2.2341e-03 2.4984e-01 9.3618e-07 EV: 1 1.0003e+00 -3.4978e-02 2.2341e-03 -2.4984e-01 9.3618e-07 EV: 2 9.7225e-01 2.0326e+00 -2.0099e-01 1.4519e+01 2.7629e-01 EV: 3 9.7225e-01 -2.0326e+00 -2.0099e-01 -1.4519e+01 2.7629e-01 ... Now I want to get 4 modes. I know there are going to be modes with negative growth. As I run the calculations at some point there appears an eigenvalue with zero imaginary part (zero angle, frequency) and slowly grows, to the point that it becomes dominant, and produces some trashy result. As in the example below. Magnitude Angle Growth Frequency Residual EV: 0 1.0004e+00 4.9969e-02 2.2362e-03 2.4984e-01 7.9474e-08 EV: 1 1.0004e+00 -4.9969e-02 2.2362e-03 -2.4984e-01 7.9474e-08 EV: 2 1.0513e+00 0.0000e+00 2.5017e-01 0.0000e+00 2.2509e-06 EV: 3 9.8487e-01 2.3743e+00 -7.6211e-02 1.1871e+01 2.4000e-02 EV: 4 9.8487e-01 -2.3743e+00 -7.6211e-02 -1.1871e+01 2.4000e-02 ... I tried increasing the size of the vector space, but with the same result. Than I tried to decrease the time scale. This has an effect of impacting the convergence, and finally also produces zero frequency mode. Should I understand this is a bad idea to use time stepping approach to look for decaying modes? To test this I tried running the ChannelStability test lowering the Reynolds number to 2000 and recovering results available in literature [1]. I think, that since I used the ChannelStability.rst as a starting point the recovered values are close to those for the Orr-Sommerfeld (growth=-0.01979866, w=0.3121003). Still, at Re=2000 it is the Squire mode that is decaying slower (s=-0.016), but I suspect that due to my starting point I will not reach it. I also have a general question regarding the stability. What is the content of the .fld file produced at the end of stability calculations? Is it a linear combinations of the vectors currently locked in the vector space? Also, is there a way to have a "broader look" at the eigen spectrum? Cheers, Stan Gepner [1] Stability and Transition in Shear Flows. By P. J. Schmid & D. S. Henningson. p. 506, Springer, 2001
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