Questions tagged [modal-analysis]
32 questions
6
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1 answer
FEM modal analysis computing displacements
I have an FEM model that I am doing a modal analysis of. I wanted to check that how I am computing the physical displacement is the correct way, as I've read a lot of about normalizing modes, participation factors, effective masses, etc. and I'm not…
engstu95
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5
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2 answers
How do I conduct an experimental modal analysis with a three-axis accelerometer?
How can I compute the Frequency Response Function (FRF) if I use two three-axis MEMS accelerometer to measure the input excitation and output response?
I have read from some books that the FRF can be computed as the ratio of response to excitation,…
ice_lin
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5
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3 answers
How to know which modes are excited by a given input in modal analysis?
Consider a simple structure such as a beam, plate, ... Assume that you know how to determine the different modes of vibration of the structure. You are given an external input, such as a force or moment at a certain location. How can you then…
Karlo
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4
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What is the frequncy range of seismic vibrations?
I'm working on a structure which needs to sustain common seismic vibrations due to earthquakes. So I want to do some modal analysis to find natural frequencies of the structure and make sure they are out of the range of seismic vibrations. I'm being…
Foad
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4
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Which mode does get excited under action of dynamic loading?
I am working on the vibration of the continuous system, I have seen lots of books on vibrations which talks about the natural frequency and mode shapes of the continuous system. I am interested in finding when particular time varying load is applied…
Chirag Palan
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3
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0 answers
Getting the damping ratio from a frequency response function v.s. transfer function
I'm trying to find the damping values of a specimen, based on sweeping the forcing frequency from 0-10 kHz. As shown in the diagram, I measure the input force from the shaker, and the subsequent response using an accelerometer on top. (The steel…
hopper19
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3
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2 answers
Relation between natural frequency and the thickness
If I have a solid material, let's say two bricks, one of which is thicker and I do modal/vibration testing. What would be the relation between thickness and natural frequency of any of them, is there any equation that can explain it, or does it…
elluthfi
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3
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1 answer
Negative Eigenvalues in Ritz Solution of First-Order Shear Deformation Theory
I am trying to write a MATLAb code for determining the natural frequencies of a first-order shear deformation plate using Ritz method. I am using Legendre polynomials as trial functions in Ritz method, and my code works well for static analysis of…
Ali Baig
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2
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3 answers
How do I get the modes shapes (that I got from FEA of a system) in reality?
In FEA, I have a complex system onto which I have applied some boundary conditions and then conducted a modal analysis for it. In return, I got a few mode shapes (as much as I requested) for each of the natural frequency from the FEA solver. By…
Rameez Ul Haq
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2
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2 answers
Why and how are resonance frequencies of a system dependent upon the shape, mass and the way it is constrained?
I was reading a general overview on the frequencies before conducting a modal analysis for my structure using ANSYS, in hopes of obtaining the natural frequencies for my structure. I read in some article that if you change the geometrical shape of…
Rameez Ul Haq
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2
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1 answer
Are the modal participation factors bounded for shock response spectrum analysis?
Recently dug myself into the theory of shock response spectrum analysis, but one thing is not clear for me.
The theory says that the peak response of the structure can be calculated as the product of the participation factor and the pertaining point…
G.David
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2
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3 answers
Do internal / external stresses affect modal frequency?
Imagine a cantilevered beam, fixed on one end and with a tension line connecting the other end to ground (e.g. perpendicular to the length of the beam).
If I increase the tension, does the modal frequencies of the beam change?
On the one hand,…
user3533030
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2
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Plot zero-pole from frequency response matrix
I've represented with Matlab a system with three degrees of freedom. I'm trying to plot a pole-zero map from my Frequency Response Matrix H(w) in the case of Damped-Forced :
w=0:0.01:30;
for ii=1:length(w)
…
alb084
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2
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1 answer
Modal Testing and Frequency Response Interpretation
I'm studying vibrations in a gearhead, for which I have a general dynamic model of 12 degrees of freedom (only translational coordinates) and I have calculated its natural frequencies.
Now I want to try and correlate my model with a physical…
spe4ker
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2
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1 answer
What does multiplicity of a multiple natural frequency mean?
I'm doing some vibration tests on planetary gear sets and a lot of the information I´ve read mentions a lot about mode shapes and multiplicities of the natural frequencies. I understand how one can derive the natural frequencies from the equations…
spe4ker
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