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Effect of tension on eigenfrequency

Hi,
I'm trying to model the effect of tension on the eigenfrequency, and I think I'm misunderstanding the use of boundary loads. I have a long and narrow bridge (fixed at both ends, free along the length). I would expect that adding a boundary load on either end of the bridge, in opposing directions, would essentially be like tensioning a string, which will cause the resonance frequency to increase. But I'm not seeing any change in resonance. Why not?

I've also tried anything else I could think of that might change the resonance - body loads, strain at either end, etc. Nothing effects the first fundamental resonance. Any ideas would be hugely appreciated!

Regards,
Jillian


1 Reply Last Post Jan 30, 2013, 5:36 PM EST
Josh Thomas Certified Consultant
Posted: 5 years ago Jan 30, 2013, 5:36 PM EST
Jillian,

Well, your fixed constraint conditions are overriding your boundary load condition at the end points. So, even in a stationary solution you will get a zero stress solution. You will want to consider only fixing one end.

There are a couple of ways to simulate the effect of tension (stress) on the natural frequency of your system. COMSOL has a very nice model in the model library called "Vibrating String" that I would advise you work through. I think that will help you a lot.

In my opinion, the most intuitive way to set up the problem is by solving in 2 steps. First, a stationary step that solves for the pre-stressed state of your system. Then, an eigenfrequency step that solves for the natural frequencies of the tensioned member(s). This method is covered in the Vibrating String example I mentioned.

Best regards,
Josh Thomas
AltaSim Technologies
Jillian, Well, your fixed constraint conditions are overriding your boundary load condition at the end points. So, even in a stationary solution you will get a zero stress solution. You will want to consider only fixing one end. There are a couple of ways to simulate the effect of tension (stress) on the natural frequency of your system. COMSOL has a very nice model in the model library called "Vibrating String" that I would advise you work through. I think that will help you a lot. In my opinion, the most intuitive way to set up the problem is by solving in 2 steps. First, a stationary step that solves for the pre-stressed state of your system. Then, an eigenfrequency step that solves for the natural frequencies of the tensioned member(s). This method is covered in the Vibrating String example I mentioned. Best regards, Josh Thomas AltaSim Technologies

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