internal pressure can influence natural frequencies in a steel pipe?

[mikycampo]

dear collegues,

i cannot understand if internal pressure can influence steel pipe natural frequencies.
i've modeled a steel pipe in ansys using PIPE16 elements and solid elements. I've fixed supported both the models in one end. For each model, i've made two simulation: with and without internal pressure.
In beam case, the natural frequencies are the same for the pressurized pipe and the other one. With solid modelling, th natural frequencies changs. why?

thanks a lot, michele

 

[manija]

mikycampo,

yes, the internal pressure has an effect in the natural frequencies of a pipe configuration, but the effect can be large or small depending of a lot of factors.
as you know the frequency depends on stiffness and mass, an
elbow is stiffened when subjected to internal pressure so the frequencyies are modified compared with an elbow without internal or external pressure.
in general if the axial loads produced by pressure are low
(compared with buckling loads) the frecuencies are not much
different, but a simple column with an axial load close to the elastic buckling load has almost zero frequency
hope this help and Happy New Year

 

[ApC2Kp]

mikycampo,
Tension on a string instrument (violin, guitar, harp, etc.) will change its natural frequency.
Pressure in a pipe acts on the end closures to create tension force in the pipe (F=PxA).
The pipe element (beam) model probably does not have end closures on the model. You might need to simulate the pressure end thrust by adding a discrete force at the end of the pipe. The force at the end of a deflected pipe would also tend to restore the pipe to straight shape.
Does the solids element model include end closure on pipe?
Check that increased pressure does result in increased frequencies. Might be interesting to see the effects of various pressures versus pipe diameters.

 

[mikycampo]

thanks a lot everybody!

There's someone that have used ansys pipe elements with internal pressure to do a modal analysis, experiencing a variation in first six frequency, increasing the internal pressure in a simple catilvered pipe?

thanks again

 

[jmw]

I think the most important point is why there is no effect revealed by PIPE16 (which I can't answer).

The effect itself is documented in the case of vibrating tube density meters. If you can access a manual for the 7845 from the Mobrey web site you will find that this particular instrument overcomes the worst effects by using an oval section tube. Thus as pressure increases the tube distorts rather than stiffen. But you would have access to some verified data to feed back into your calculations.

Incidentally, because these density meters "float" the virating tube between multi-ply bellows there are minimal end effects to worry about.

JMW

http://www.ViscoAnalyser.com