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showing phase (+/- direction) with color maps for mode shapes
Posted 4 feb 2022, 21:40 GMT-5 Acoustics & Vibrations, Results & Visualization, Structural Mechanics Version 6.0 3 Replies
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It is possible to set the range or to use a color map to show negative versus positive displacments for mode shapes when doing an eigenfrequency analysis of a structure? For example, if I create model of a vibrating beam, and use a color map like WaveLight (Red-White-Blue) the result is that displacements are shown in Red regardless of which direction the displacement is going, and nodes are shown in Blue. This makes it very difficult to identify some vibrational modes when you cannot tell the direction or phase of the displacements for adjacent regions. It is possible to set the color map or the range so that positive displacements are Red, negative displacments are Blue, and nodes are White? This would make it MUCH easier to idenfity structural vibration mode shapes if different colors could be assigned to positive and negative displacements, so that regions of the structure which are vibrating with opposite phase would be displayed with different colors. Is this possible? Does anyone know how to do this in COMSOL?
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Related question . . . is it possible to display displacement (positive and negative) instead of displacement magnitude when doing a structural acoustics eigenfreuqency analysis to show mode shapes of a structure?
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In 6.0, you can make the color table's range symmetric by setting Scale to Linear symmetric in the Coloring and Style section. In earlier versions there's a Symmetrize color range check box.
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As you note, the problem here is that you are plotting the norm of the displacement, which is strictly positive. If the vibration is in one direction only, like for a flat plate, you can choose to plot a single component of the displacement instead. Then, you have the type of signed result that you are looking for.
For a general 3D structure, there is no unique way of doing this. For example, the x and y displacements can have the same sign at one location, and opposite at another. You can of course experiment with looking at one component at a time.
You can access the phase of any variable by using the arg() operator. But again: This is only meaningful for a variable that is a linear function of the displacements. It does not work for, for example, magnitudes.
In my experience, animations or vector plots are the best tools to really understand mode shapes.
-------------------Henrik Sönnerlind
COMSOL