Modal Analysis using Ansys Workbench

 Modal Analysis using Ansys Workbench :

AIM:

  • What is modal analysis?
  • Purpose of modal analysis
  • Types 
  • How to do model analysis?
  • Conclusion

Analysis

  • What is modal analysis?

Modal analysis look for natural frequency and normal mode of  a structure.Natural frequency and modes are some of the important factors of a structure, they determine how the structure will respond .

 

  • Purpose of modal analysis

By knowing naturing frequencies of a component we can design them to avoid specific ranges to prevent resonance.In this way we can design from car components, buildings, blades and any other component to prevent faliure of the structure.

  • Types 

Here three types of model analysis will discuss three types of model analysis-

1. Free free model analysis(models which are free to move like drones , boats , aeroplanes etc)

2. Models which have fixed boundary condition

3. prestressed model analysis(models which have an initial stress condition)

  • How to do model analysis?

1. Free free model analysis:

First drag and drop modal

open designmodular by double click on geometry and we will design a rough structure for visualization.

Close designmodular and open model

Before going to model we can choose material of the model by clicking on engineering data.

Choose engineering data sources by right click

Then choose material

 

Then open project- geometry-solid and we can assign any material that we have added to the library

 

Then we can mesh the geometry and can assign sizing option

We can assign boundary conditions like fixed support, thermal condition , displacement etc by right click on model

We can see any solution from deformation, stress, strain etc by right click on solution

 

After assigning boundary conditions and preffered solutions right click on solution and click on solve

In tabular graph right click- select all - create mode shape results and in solution rigth click - evaluate all

Here we have choosen 16 modes from analytical settings.

What are modes?

Modes are the state of the model in a certain frequency by which we can find whether the model will be excited by force at this frequency in a certain direction.

Modes are the result of numbers of degrees of freedom in a certain model as there may be millions of DOF in a complex structure so there may be millions of modes.

so How we will find that how much mode we require to get satisfactory results.

As this is a free-free structure its first 6 freqency are zero or nrerer to zero.

In solution information we can see the participation factor and effective mass.

Participation factor and effective mass are both same in definaiton but different in mathematical notation resulting in to determine which frequency in what direction can lead to more excitation.

 

These are larger participation factor and effective masses - so in this frequencies and in that certain direction the body will be much more excited.

Now how will we find How to determine number of modes?

when ratio of effective mass and total mass is nearer to one that number of modes should be taken.

 

Video: Total deformation 16:https://www.youtube.com/watch?v=qJtjy9D1UHs

 

 

2. Models which have fixed boundary condition

We draw a simple beam.

After meshing we have taken fixed support here

 

After solving the model previously  described

 

Results:

We can see maximum participation factor in x direction 1071.7hz and in z direction 905.47, so that the excitation is more in that frequency in that directions.

 

video:https://www.youtube.com/watch?v=0a_f_px0hbM

 

3.prestressed model analysis

For this case first we have to calculate stress for the geometry first and then its calculated stifness will be added to the modular stifness during model anlysis.

our first calculation may be linear or non-linear but model analysis will be always linear.

 

We first taking stating structure and fading modal to its solution.

We have taken a simple rectangular beam

 

Boundary conditions :

 

zero temperature with environment temperature 22 degree

 

Then we calculated normal stress in first solution

Total deformation of the first mode in second solution

Now we take parametric of - first solution normal stress- average

Thermal condition and Total deformation frequency 

 

It had taken Thermal condition as input and normal stress and total deformation frequency as output.so varying the temperature we can get different outputs.

 

 

From this chart we can see that with increase in temperature normal stress decrease(compressive stress increase) and frequency increase.

Video :prestressed_model

 

Conclusion:

In this way we can conclude that modal analysis is a very important thing in structural analysis through which we can came to know in which direction and how much frequency the structure will have more excitation and it will also help us to safely build our products by eliminating the errors of being failed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


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