Unconstrained Modal Analysis: Application and Considerations

[skylab2k8]

Hi all,

I am new to this forum. I have a doubt. I have a doubt regarding modal analysis. Where exactly does unconstrained modal analysis find its application?

Thanks in advance.

Regards,
Shankar

 

[brewnog]

The practical application I most commonly use would be to assess the natural frequency and mode shapes of a component, or an assembly; in order to best eliminate unwanted vibration from a system.

For example, modal analysis could be used to understand how a fuel pipe on a car vibrates, and therefore to illustrate where it could be best constrained or damped to reduce this vibration (which would otherwise lead to failure, fire, death etc!).

I hope this helps.

 

[skylab2k8]

Hi Brewnog,

Thank you for the reply. But in the case you described, we would be applying constraints on the model and then solve the problem. But if there are no constraints applied on the model what would be the case? Consider the following example. When a beam is treated as a cantilever, its natural frequency would be different to when it is treated as a fixed beam. Now if there are no constraints then the response would be different. So where exatly does a modal analysis on a body without constraints (or study of free free vibrations) find its applicaton?

Regards,
shankar

 

[brewnog]

Just how I described. You could test the component/assembly by dangling it from a string, or suspending it loosely, to identify its natural (ie unconstrained) mode shape. You would then use this to identify exactly where it should be constrained for maximum/minimum vibration.

So you would analyse your free-floating beam to determine its mode shape, and (depending on what you wanted to do) constrain it at its nodes or antinodes (or a combination thereof) to influence its behaviour. The constraints you could then apply to your cantilever beam could vary, depending on whether you wanted a stiff, rigid bracket, or a supple, bendy diving board.

As well as your mode shapes, your modal analysis would also identify the fundamental and resonant frequencies of that body. If your cantilever beam was found to have harmonics at (say) 50Hz, you'd avoid using it on an application with a 50Hz forcing frequency (an electric motor running at 3000rpm, for example), or realize that you'd need to redesign it to avoid excessive vibration.

 

[Mech_Engineer]

From what I understand, an unconstrained modal analysis would be able to find the most sensitive mode of a particular part. If you constrain the model, you might risk accidentally constraining out a very sensitive mode, so dropping the constraints altogether can give you some insight into the part's design.

Also, if you are uncommitted as to where a particular part is to be constrained, leaving the model unconstrained to start with may give you some insight into where the best constraint locations would be to negate the first one or two (or more) vibrational modes of the part.

EDIT: Basically what Brewnog said, ha.

 

[skylab2k8]

Hi,

Thank you guys. But, I am still unclear regarding the practical significance of the output of modal analysis of unconstrained body. Consider a bottle cooler on a ground. If a modal analysis is to be carried out on this, then the bottle cooler base is fixed and then modal analysis is carried out. What is the importance of modal analysis on unconstrained model here?

Regards,
Shankar

 

[Mech_Engineer]

Hi,

Thank you guys. But, I am still unclear regarding the practical significance of the output of modal analysis of unconstrained body. Consider a bottle cooler on a ground. If a modal analysis is to be carried out on this, then the bottle cooler base is fixed and then modal analysis is carried out. What is the importance of modal analysis on unconstrained model here?

Regards,
Shankar

I'm not sure what you mean by a "bottle cooler," but I think our explanations have been very clear as to what unconstrained modeal analysis is used for... no matter what the product applications might be.

 

[skylab2k8]

Hi Mech_Engineer,
By the term bottle cooler I ment commertial refrigerator.
Regards,
Shankar

 

[Mech_Engineer]

Without knowing more about the application, it's impossible to speculate why an unconstrained modal analysis would be done a a specific product. Are you trying to talk someone out of doing a modal analysis because you don't know why they are doing it in the first place?

 

[skylab2k8]

Hi Mech_Engineer,
I am actually trying to understand why an unconstrained modal analysis is to be performed in a case I described.
Regards,
Shankar

 

[Mech_Engineer]

I can't really think of any reason why you would do an unconstrained modal analysis (or a constrained one for that matter) on a refrigerator that is sitting on the ground...

Maybe to see if it has any natural modes near the operating frequency of the compressor motor?

 

[FredGarvin]

Hi,

Thank you guys. But, I am still unclear regarding the practical significance of the output of modal analysis of unconstrained body. Consider a bottle cooler on a ground. If a modal analysis is to be carried out on this, then the bottle cooler base is fixed and then modal analysis is carried out. What is the importance of modal analysis on unconstrained model here?

Regards,
Shankar

For your entire assembly, it may not be useful. However, perhaps it would be useful for the door or some other component like a shelf.

It could also serve as a first pass look at a certain geometry.

 

[flom]

How to define ANSYS commands in CDB file for printout eigenmodes in ASCII format?

sory

 

[joslopher]

Hi,
I have the same doubt about unconstrained modal analysis. I do not know exactly why it is useful for. I do not know how to get some insight into where are the best constraint locations from the unconstrained analysis.
Related to it, if I had to perform a modal analysis of a box-like structure with four wheels (one at each corner of the bottom surface), should it be constrained or unconstrained ? Indeed, there is a constrained but just in the negative z-direction, so that the structure is not completely bounded to the ground.
Please, could you help me with it?
Thank you very much.

 

[AlephZero]

Indeed, there is a constrained but just in the negative z-direction, so that the structure is not completely bounded to the ground.

You are forgetting about the weight of the structure. A linear vibration analysis tells you about small pertubations from the "static" state of the structure. If the reaction force to the weight of the structure is W and the dynamic forces vary between +w and -w, the combined reaction force varies between W-w and W+w, so the structure will remain in contact with the ground when w is small compared with W.

BTW the wheel will also be constrained against moving sideways, by the friction force between the wheel and the ground. The only unconstrained motion is rolling backwards and forwards, which will give a zero frequency mode for the complete structure.

 

[joslopher]

Ok! Thank you very much!

 

[joslopher]

I am not sure about the assumption that the wheels are "bounded" to the ground by the weight of the structure... Should I constraint also the positive z-direction on those nodes corresponding to the link to wheels because of the weight?
Weight affects to all nodes of the structure, so according to your explanaiton I should constraint the positive z-direction of all nodes...