COMSOL - Structural Mechanics - Membrane - vibration without input?

[pitchtwit]

I've just got a quick question that hopefully someone can answer.

My question is - why does the membrane in COMSOL vibrate without you telling it to?

Here are some specifics if required...

In COMSOL here are my setup details: -

Physics: (3D) Structural mechanics - Membrane
Study: Eigenfrequency

Geometry->Work plane->Square...Size->Side length = 27.4[mm]
Materials->Material 1...Geometric Entity Selection->Selection = All boundaries
Materials->Material 1...Material Contents: Density (rho) = 1200[kg/m^3], Young's modulus (E) = 3.6e9[Pa] & Poisson's ratio (nu) = 0.36.
Membrane...Boundary Selection->Selection = All boundaries;
Membrane...Thickness->d = 0.176[mm]
Membrane->Linear Elastic Material 1->Initial Stress and Strain 1...Initial Stress and Strain->Initial in-plane force->N_0 = 486.4[N/m], 0, 0, 486.4[N/m] (these go top left, top right, bottom left then bottom right)
Membrane->Fixed Constraint 1...Edge Selection->Selection: All edges
Study->Step 1: Eigenfrequency...Study Settings->Desired number of eigenfrequencies = 8
Study->Step 1: Eigenfrequency...Study Settings->Search for eigenfrequencies around = 50

When I compute this, I get a series of total displacement 2D plots for each eigenfrequency, showing the displacement and you can even view pressure too. But what is driving this?

I'd like to add an input later that I can define my self, rather than COMSOL just putting it in for me.

Many thanks.

https://dl.dropboxusercontent.com/u/11341635/Screen%20Shot%202014-08-08%20at%2008.08.15.png

 

[AlephZero]

The plots of the mode shapes just represent the possible vibration modes of the structure. The amplitudes of displacement, stress, etc, on the plots are arbitrary.

If you want to find the actual physical response to a given situation, you will have to supply the relevant forces, and do a different type of solution - e.g. steady state forced response.

 

[pitchtwit]

Thanks - that's good to know.

I've been struggling to set up a simple model using square membrane like the one above. I've tried loads of ways but nothing seems to work, and if you'd be willing to give me some advice I'd be really appreciative.

The model is simple - a small square isotropic elastic membrane with a small shallow cylindrical mass attached to the centre. The edges of the membrane are attached to a square thin rigid plastic frame.

I have the Young's modulus, Poisson's ratio & mass density for both the membrane and the mass along with the tension in the membrane.

I need to get a 1-D plot showing transmission loss on the y-axis and frequency on the x-axis - so basically as if a white noise/sine-sweep sound source is incident on the membrane from one side - with the transmission loss being the dB equivalent of the incident pressure minus the transmitted pressure.

What Physics would I use to do this, etc?

I'm a day or two away from pulling my hair out.

 

[Meysam]

I have made a simple model of a 3d-square membrane and used solid
mechanics and eigenfrequency analysis to get the eigenfrequencies. Now
that i have them, I want to take the fundamental mode. Doing the
Eigenfrequency analysis and finding the first 30 modes yielded close
to expected results for the modes that are out of plane. However, i
can't seem to get it to produce the eigenfrequencies that produce
in-plane modes, or breathing modes. Is there an option in the
eigenfrequency analysis to specify what direction to look in for the
peaks? AKA look in the XY plane instead of the Z direction for maximas
in deformation.

Many thanks.