Why does pyDPF report negative harmonic indices

Jim Kosloski
Jim Kosloski Member, Employee Posts: 27
10 Comments Name Dropper First Anniversary Ansys Employee
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When listing model data pyDPF shows negative values for Harmonic Index?

Number of sets: 30
Cumulative Frequency (Hz) LoadStep Substep Harmonic index
1 670386.325235 1 1 0.000000
2 872361.424038 1 2 0.000000
3 1142526.525324 1 3 0.000000
4 1252446.741551 1 4 0.000000
5 1257379.552140 1 5 0.000000
6 1347919.358013 1 6 0.000000
7 679667.393214 2 1 1.000000
8 679667.393214 2 2 -1.000000
9 899321.218481 2 3 -1.000000
10 899321.218481 2 4 1.000000
11 1128387.049511 2 5 1.000000
12 1128387.049511 2 6 -1.000000
13 708505.071361 3 1 -2.000000
14 708505.071361 3 2 2.000000
15 966346.820117 3 3 2.000000
16 966346.820117 3 4 -2.000000
17 1031249.070606 3 5 -2.000000

It looks like for repeated modes it lists one as negative, but how does it determine this (it is not always the second repeated mode is negative).

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  • Andy M.
    Andy M. Member, Employee Posts: 1
    First Comment Name Dropper
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    @Jim Kosloski It is as Oscar said. I will add to it that there is a relationship between the base and duplicate sector modes when there are modes at the same frequency. That is, the base_sector_of_mode_1 = (sign) duplicate_sector_of_mode_2, and base_sector_of_mode_2 = (sign) duplicate_sector_of_mode_1. There is an indeterminancy here, so the signs are not predetermined and it depends on how the solver responds to the operating system, hardware, frequency range, matrices, etc. This is why the signs aren't always in the same order.