Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.
Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.
Deflection of a simple cantilever
Posted Feb 24, 2015, 12:23 p.m. EST Version 4.4 3 Replies
Please login with a confirmed email address before reporting spam
currently I'm simulating a simple cantilever. The first thing i did was to determine its eigenfrequency. I got a value of about 504.5 Hz.
I'm interested in the deflection of the beam as a function of the (excitation) frequency. So I atteched (as recommended in the forum) an Edge Load to the free end of the cantilever.
When I don't use a damping i get a pretty high deflection. Furthermore, when the resonance frequency of the cantiliver is equal to the excitation source the simulation quits with an error (The relative error (0.0012) is greater than the relative tolerance.).
For me it is logical that the simulation stops. Because an undamped system would have a too huge displacement.
But when i introduce a Rayligh damping (mass damping parameter = 2/s and stiffness = 0) my cantilever has his lagest damping at his resonance frequency.
Has anyone an idea how I can fix this???
BR
Please login with a confirmed email address before reporting spam
My best guess is that you are looking at the real part (the default) of the displacement, which is zero at resonance (since there is a 90 degree phase shift there). Try plotting abs(whatever_quantity) and see if that looks more plausible. Also try explicit real() and imag().
Regards,
Henrik
Please login with a confirmed email address before reporting spam
thank you for your answer. I'm plotting the displacement (solid.disp) in a 1 D Plot. I tried to determine the absolute value and also the real and imaginary part but nothing changes. The imaginary part is zero.
I tried to attach my model, but it isn't possible to do this (nothing happens when i click the button).
Has anyone else an idea???
Please login with a confirmed email address before reporting spam
It will work better if you use a separate displacement component. Since the displacement components not necessarily are in phase with each other, the norm of the displacement solid.disp is not really a meaningful harmonic quantity (it a square root of squares). The expression used is real valued.
Alternatively you can use the real valued quantity solid.disp_rms, which is an RMS average of the total displacement.
There are also built-in variables solid.uAmpX = real(u), solid.uAmpY = real(v) etc.
Please also have a look at the tutorials in Model Library: 'bracket_frequency' and 'elbow_bracket' where frequency domain analyses are shown.
Regards,
Henrik