My MSUP (Mode Superposition) harmonic response analysis gives different results than my full harmonic response analysis. Why?

The most likely problem is due to using an insufficient number of modes being used for the MSUP harmonic response analysis. Please see the attached example which compares displacement results for a given node for a model using a different number of modes in each MSUP harmonic response solution. The MSUP harmonic response analysis based on 100 modes has pretty good agreement with the full harmonic response solution, which is considered to be the true solution.

It is interesting to note that you need to use modes well past the frequency of interest for your harmonic response analysis. In the attached model, the 7th mode is above the requested frequency range (0 Hz to 100 Hz), but you need to extract modes above that frequency to get decent results. In the example, extracting 10 modes (i.e., up to 124 HZ) gives decent correlation to the true (full harmonic) results. Requesting 100 modes gives even better agreement. However, keep in mind that the MSUP approach is an approximation method, so you need to use sufficient modes to capture what is going on in the structure.

The next question will be "How many modes should I use, then?". The best thing might be to include modes from half the initial frequency of interest to twice the last frequency of interest. You can run increasingly more modes to see how the results change. At some point, the results will not change much and you will have reached the best solution for the CPU time required.

An ANSYS 11.0 enhancement involving residual vectors allows for better MSUP harmonic response analysis results with less modes extracted. For the model discussed above, the forces were moved to the modal analysis in order to create the force vector required by the RESVEC command. Then, the LVSCALE command was used to apply the forces in the MSUP harmonic response portion of the analysis (after deleting the forces to prevent duplicating the force loads). In the modal analysis, the RESVEC calculated the residual vectors and in the MSUP harmonic, they were included to give very good accuracy using only seven modes (see attached input file "resvec.inp").

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