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Questions tagged [modal-analysis]

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3 votes
2 answers
76 views

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,...
0 votes
1 answer
85 views

Preliminary way to estimate damping quality factor (Q) from a SolidWorks FEM

I am analyzing the modal response of a structural FEM for a spacecraft structures course. I'm using Solidworks Simulation Premium. Need first mode to be above 40Hz and the damping Q-factor (quality ...
1 vote
1 answer
86 views

Why do Eigenmodes of a rigid system also diagonalize the Mass and the stiffness matrix seperately?

To set up my question, I will briefly recap the derivation that is usually given for eigenmodes of a system: If there is no force present, and a body is described by a vector $u$, then the equations ...
0 votes
0 answers
40 views

The Effect of Accelerometer Position on Modal Analysis of a Cantilever

We have obtained the first two vibration modal frequencies of a composite cantilever (33 gm). We placed the accelerometer (3.4 gm) near the fixed end and the two frequencies were F1=30 Hz and F2 = 256 ...
0 votes
0 answers
52 views

How to push apart the first two natural frequencies of a cantilever beam

We have first two natural frequencies of a composite Cantilever Beam obtained through experiment and analytical calculation. Experiment gives 31 and 250 Hz whereas formulae give 30 and 200 Hz. ANSYS ...
0 votes
0 answers
43 views

Mode shape directionality in Abaqus finite element analysis

For a coupled aero-structural analysis I'm working on, I have to extract the eigenvectors/mode shapes of my structure, so that I can run aerodynamic calculations on them. I have a parameter (in this ...
0 votes
1 answer
142 views

Why the mode frequency is approximate in modal analysis?

I have used the Ansys workbench to carry the modal analysis of one motor stator and under the free modal analysis, the first 6 mode frequency are all zero, it is ok because of the first 6 mode does ...
1 vote
2 answers
76 views

Effect of bracing on a music instrument top - is FEM modal analysis enough to predict this?

I'm currently building a music instrument (string instrument), and I am at the step where one applies certain braces to the soundboard. Since I actually don't know how the bracing will affect the ...
2 votes
3 answers
439 views

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 ...
0 votes
0 answers
566 views

How to increase Physical Memory for Modal Analysis in ANSYS

I am facing following error while attempting to run Modal Analysis: There is not enough memory for the Distributed Sparse Matrix Solver to proceed using the out-of-core memory mode. The total memory ...
0 votes
2 answers
432 views

Ansys Mechanical Modal analysis, Parametric materials, mode shape output for each parameter?

I am trying to match FEA results with an experiment, wherein I need to match mode shapes and frequencies. I am testing different models, and I would like to parameterize several material properties. ...
1 vote
2 answers
1k views

Why does the mesh size affect my modal analysis results?

The need for mesh refinement in a structural analysis module such as Static or Transient is quite clear; if we want the overall results such as displacement and stresses to be more accurate within the ...
1 vote
3 answers
808 views

FEM degrees of freedom

Consider a "beam". In order to perform modal analysis using the finite element method, the beam is meshed with 2nd order hexa elements. The total count of nodes is 1280. The same beam can be ...
1 vote
3 answers
64 views

What is Experimental and Operational MODAL ANAYLIS?

EMA and OMA are two methods used for determining the mode shapes of the natural frequencies of mechanical systems. I understand it until that point. I know that for EMA we need an artificial ...
1 vote
0 answers
191 views

Modelling acoustic instruments — predicting frequencies, decays, levels, and phases of modes of a circular membrane (drum head)

I have been working on some acoustic synthesis software for modelling several acoustic instruments. I am using the principle of modal synthesis, where every mode of vibration is individually ...
2 votes
2 answers
187 views

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 ...
2 votes
1 answer
129 views

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 ...
3 votes
2 answers
1k views

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 ...
3 votes
0 answers
147 views

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 ...
4 votes
1 answer
104 views

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 ...
2 votes
3 answers
379 views

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 ...
1 vote
0 answers
29 views

How to run a steady state dynamics, Modal step based optimisation using TOSCA?

What design responses must be given in the TOSCA gui. I chose volume reduction as an objective function with the constraints of displacement. Is this approach correct?
1 vote
2 answers
128 views

Analizing Vibrations Influence on Dynamic Weighing

I have a very sensitive scale used for dynamic weighing (by dynamic I mean that the scale and load cells are actually beneath a belt conveyor that actively weight product when it´s being transported ...
2 votes
0 answers
35 views

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-...
2 votes
1 answer
283 views

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 ...
2 votes
1 answer
205 views

FRF based material constant optimization

I need advice on how to optimize a finite element model of a transversely isotropic material. I have modeled the material with 5 nominal moduli in Abaqus and performed an eigenvalue extraction. ...
0 votes
1 answer
47 views

Analytical solution for wall eigenvalue analysis

I have a wall with the bottom end of it fixed to the ground. I want to derive from first principles the frequencies and mode shapes of the structure. I know that I will need to start with the ...
1 vote
0 answers
34 views

Why does Experimental Modal Analysis need a reference signal?

I need to conduct a vibration experiment with 3 accelerometers for a gear box. I'm reading the theory of mainly how to perform the vibration analysis but I don't understand why EMA needs a reference ...
2 votes
1 answer
415 views

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 ...
0 votes
1 answer
1k views

The distinction between lateral mass participation and vertical mass participation

Refer to the chapter 14 of the Wilson book "Dynamic Analysis of structures": calculation of stiffness and mass orthogonal vectors. The table 14.6 shows the lateral mass participation and vertical ...
2 votes
0 answers
5k views

How to get mass normalized mode shapes?

I have an MDOF system with 4 degrees of freedom with forced input at one of the dof (4th dof)(it is a sinusoidal wave with a particular frequency expected to be generated using a shaker); displacement ...
5 votes
2 answers
332 views

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 ...
5 votes
3 answers
1k views

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 ...
1 vote
1 answer
645 views

How to handle modal combination in response spectrum analysis for member forces for member with more than 1 element?

My textbook says for mode combination, the peak value $r$ of each response quantity is determined by combining the peak values $r_n$ of the modal contributions to the same response quantity. From ...
4 votes
1 answer
285 views

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 ...
6 votes
1 answer
1k views

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, ...