include mass of fluid in pump nat freq calcs?
include mass of fluid in pump nat freq calcs?
(OP)
We are attempting to do a quick simple estimation of change in natural frequency of a vertical pump as a result of adding mass along the stationary parts below the deck mounting level (to demonstrate no significant change in natural frequency from small fractional addition of mass).
The structure is treated as cantilevered about the deck mounting level. The envisioned modeshape is simple cantilever shape with natural frequency wn = 3.53*sqrt(E*I / [mu*L^4] where mu is mass per length (Den Hartog). Therefore the simplistic calculation is that the natural frequency will change inversely as the square root of mu.
i.e.: Ffinal / Finitial = ~ sqrt(mu_initial/mu_final).
The question: should the mass-per-length parameter mu include weight of the fluid, or only dry weight of the pump?
The structure is treated as cantilevered about the deck mounting level. The envisioned modeshape is simple cantilever shape with natural frequency wn = 3.53*sqrt(E*I / [mu*L^4] where mu is mass per length (Den Hartog). Therefore the simplistic calculation is that the natural frequency will change inversely as the square root of mu.
i.e.: Ffinal / Finitial = ~ sqrt(mu_initial/mu_final).
The question: should the mass-per-length parameter mu include weight of the fluid, or only dry weight of the pump?
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(2B)+(2B)' ?





RE: include mass of fluid in pump nat freq calcs?
Fluid structure interaction is a warmish topic in the dynamics community, mostly handled by multiphysics simulations at the moment.
Cheers
Greg Locock
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RE: include mass of fluid in pump nat freq calcs?
Just to check for understanding, in this case it is a relatively straightforward conclusion to simply include the mass of the water in the calc (lump it with the casing in our situation which does not distinguish rotor from casing).
What if we were attempting to model rotor, casing, bearings.... would we still include water mass with the casing?
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(2B)+(2B)' ?
RE: include mass of fluid in pump nat freq calcs?
Some people seem to like modeling pump bodies as completely rigid in a modal analysis of pump supporting brackets - that does not always give good results.
RE: include mass of fluid in pump nat freq calcs?
Some years back I was involved in a project of subsea air guns. Very much dependent on the correct modelling of FSI.
RE: include mass of fluid in pump nat freq calcs?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: include mass of fluid in pump nat freq calcs?
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(2B)+(2B)' ?
RE: include mass of fluid in pump nat freq calcs?
On the other hand, if you want to find the natual frequency of the rotor, bearing etc, that would require a FSI approach. (Vibration of structure entrapped in a fluid.)