Learning Forum

[Kastudent]

I have done the modal analysis, frequency table is shown below what are these number showing, my frequency range should be between 70-120. the first number shown is 160. how can I check the frequency between the number I want. can I enter it manually

[parkersheaffer]

Hi Kastudent,

You didn't include a table but I understand what you are referring to. When you run a modal analysis without changing any of the settings ANSYS will give you the first 6 natural frequencies(shown in the table) and a mode shape corresponding to one of the natural frequencies. 

I am not sure what you are modeling but if you have test data that shows for example your first natural frequency is 75hz while your modal analysis is showing 160hz for the first then this means your model as you have it setup is over constrained. Without any further information about your model the best advice i can give is to review your contacts/boundary conditions/mass. 

can I enter it manually

No a natural frequency depends on the mass matrix and stiffness matrix.

[Kastudent]

hi parkersheaffer

sorry im new to ansys haha, so lets say that my module is in a fork shape, it is fixed from the bottom and there is pressure applied on the top, the module will get vibration from the surrounding between 70-120 hz, so I have to know how it is going to behave around this frequencies, so whenever I start the modal analysis the first one shows is 160 hz so I want to know what this value actually shows, does it mean from this frequency and up on the module will get effected or what. 

thankyou

[parkersheaffer]

If you do not have an input of 160hz you should not expect a resonance at 160hz. If you were to strike your structure with an impact hammer you would see peaks in the frequency data at each one of the natural frequencies(for example 160hz,180,214...). the response magnitude will vary depending on where you strike which could then be used to determine the mode shapes. Knowing the natural frequencies of a structure and the mode shapes during design can save a significant amount of time and money. For example a bracket on a engine that has a first natural frequency that occurs during the first engine order.

I would suggest you do some of your own research on natural frequencies and become familiar with the frequency domain. Doing an analysis on something simple which you can either calculate by hand or have test data available is what I have found to be the best way to learn and build confidence in results.