immersed object

[bnrg]

Hi,
I have successfully used urethane materials to pot electrical items subjected to vibration but now have a new problem, items that are immersed in oil. The parts are simple in design and everyone who has come by with their 2 cents has said that the oil will dampen the vibration, but I need to analyze this. The parts are high voltage in nature so bracing (ie making a nice short to ground) may not be a good option in initial design or as an add on later-I need to make them stiff enough upfront. All of my texts apparently assume that the parts are shaking in air and I have not found any literature to guide me here. The amount of air displaced in these cases is pretty low, but the resistance to moving the oil out of the way will likely be pretty high in comparison. Does any one have any references that will help me with this?
Thanks,
Bob

 

[electricpete]

How about you design them as if they were to be immersed in air... and then the unknown damping effect that you didn't take credit for is just a bonus that adds to your design margin?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[GregLocock]

That's a very complex field. If you look at the out of plane vibration for each surface you may be able to estimate the power transmitted to the fluid, using a sound intensity approach.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Tmoose]

For useful amounts of damping at machine tool type frequencies ISO VG 16-ish oil has to be squeezed out of a gap a few 1000ths of an inch, and then must also be forcibly replenished.

My suspicion is extra features like blades or close fitting stationary surfaces would have to be added to the components to pump enough oil to provide significant damping for small amplitude forced vibration control.

 

[cbrn]

Hi,
as GregLocock says, it's a very complex matter. Theoretically, each mode of vibration could be categorized (out-of-plane bending, for example), and depending on the order of the mode one could find equations which would lead to the "reduction factor" defined as the ratio between the natural frequency value "in air" and the natural frequency value "immersed in the medium", taking into account densities, viscosities, etc. A forced vibration analysis could then be done with the mode superposition method.
I said "theoretically" because this can become extremely involved: these factors can be calculated, as far as I know, only for very simple modes and for very simple shapes (pure-beam, pure-membrane,...).
What has been said here, that the reduction depends upon the participant mass of fluid, is totally true, so the idea of maximizing this mass, independently from knowing exactly which it is, is very good I think.
As a last resort, you could do (if you have the software), or have it done by a third-party, a FE vibration analysis using "fluid" elements surrounding the body of interest.

Regards