Damped Vibration 1

[southsea]

I have been tasked with a vibration analysis for some retrofit equipment to be fitted to a commercial aircraft. The actual derivation of the harmonics in each of the axis I have had no problem with using classical methods and software. However all the calculations I have used deal with responses in air. The equipment itself is submerged in avaition fuel, which will provide a certain amount of damping. My question is to what extent will the fuel affect the responses, i.e. Fn, Amplitude etc......

 

[tomirvine]

The apparent mass of a body in air differs from that of a mass submerged in a fluid. As a simplification, this effect is due to the buoyancy force acting on the body.

The effective mass of the body increases when it is submerged in fluid. Its fundamental frequency thus decreases.

Obviously, damping also increases when the body is submerged.

There are some handbook values showing the added mass effect for some simple submerged systems, but testing is required in most cases to characterize the change.

Tom Irvine

http://www.vibrationdata.com

 

[southsea]

Thanks Tom,
The system itself is quite simple in its elemental state, effectively a C shaped component formed from 1/8" aluminium sheet. Therefore if I apply Archimedes principle as a rule of thumb, this will give me the effective mass, therefore allowing the calculation of Fn at the decreased levels. Am I going in the correct direction?

 

[GregLocock]

No. Fluid/structure coupling is far more complex that that.



Cheers

Greg Locock

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

 

[tomirvine]

As a “first approximation,” the added mass is equal to the volume of the body multiplied by the density of the fluid. In other words, it is the water mass displaced by the body. This is Archimedes principle.

In reality, the added mass also depends on the body’s geometry which affects the acceleration of the fluid particles over the surface of the body. The added mass is the mass of the fluid that must be accelerated in order to allow the acceleration of the body.

Furthermore, the added mass value may depend on the Reynolds number and other variables.

Testing is thus required.

Tom Irvine

http://www.vibrationdata.com

 

[GregLocock]

Hmm. The only time I've used fluid /steel interactions is with steel in air, and the effect was rather greater than 1.22/7843 !

Cheers

Greg Locock

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