Learning Forum

[mujqureshi406]

Im having low natural frequencies from 14hz to 330 hz (120 modes) for a frame holding 80 kN load from turbomachinery. It is too low , can you please help me on grey areas and possible causes of this, however mesh convergence has been acheived

[peteroznewman]

I can help with this.  What is the minimum acceptable frequency for the first mode?

The first thing is to look at the shape of the structure to see if principles of rigid design were followed. If not, redesign the structure using those principles.

Can you show the shape of the structure so we can see if it is an inherently rigid structure?

After you have redesigned the structure to improve rigidity, the next phase is optimization of mass and stiffness.

The method to raise the first mode frequency is to identify regions of the structure that have high potential energy and high kinetic energy.

High potential energy means the structure is not stiff enough so you should change the design to increase stiffness.

High kinetic energy means the structure is too heavy so you should change the design to reduce mass.

Here are three videos that explain how to identify these regions.

https://youtu.be/sFNGXjlnIFQ

https://youtu.be/2xJWO3F6i7c

https://youtu.be/2xJWO3F6i7c