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Submerged cantilever beam modal analysis

Posted Dec 19, 2010, 3:41 p.m. EST 6 Replies

Hoypoy's Dad
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Since I am not so sure how to do modal analysis for sumberged beam, I solved the harmonic case using Acoustic Module based on fluid-structure analysis. My question is... comparing with the published data, my modal analysis and air harmonic analysis correspond with them correctly but my water harmonic results show more significant reduction in natural frequency due to fluid mass loading effect. Can any one tell me why or where to look?

For water harmonic/air harmonic cases, I applied the plane harmonic wave to the beam and applied the interacting boundary condition for each module(structure and acoutic) so they can communicate as feedback system since one output becomes an input to the other. But I only applied the boundary condition on the top portion of the beam.

Since it's not that meaningful to do 3 D analysis, I've been doing with 2 D.

Thanks in advance for any comment or suggestions you could give!!!
6 Replies Last Post Dec 24, 2010, 1:46 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 22, 2010, 4:48 p.m. EST
Hi

first of all I would refer to the excellent book: Vol3 Modelling of Mechanical Systems Fluid-Structure Interaction of F.Axisa and J. Antuns from B&H Elsevier isbn 978-0-750-66847-7 (vol 1 and 2 are as good)

Then following chapter 2 inertial coupling I would suggest to use only structural and load the boundaries with a load of then type damp*u*lambda

or in a more general term (for the u-x direction)

( stiffx+ lambda * damp -lambda^2 * rho ) * u

At least starting with physics equation as loads you do not need to fully solve all the NS tobegin with, thereafter one can add a true fluid around and see how that matches the previous results

--
Good luck
Ivar
Hi first of all I would refer to the excellent book: Vol3 Modelling of Mechanical Systems Fluid-Structure Interaction of F.Axisa and J. Antuns from B&H Elsevier isbn 978-0-750-66847-7 (vol 1 and 2 are as good) Then following chapter 2 inertial coupling I would suggest to use only structural and load the boundaries with a load of then type damp*u*lambda or in a more general term (for the u-x direction) ( stiffx+ lambda * damp -lambda^2 * rho ) * u At least starting with physics equation as loads you do not need to fully solve all the NS tobegin with, thereafter one can add a true fluid around and see how that matches the previous results -- Good luck Ivar
Hoypoy's Dad
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Posted: 1 decade ago Dec 22, 2010, 7:58 p.m. EST
Hi Ivar,

Thanks for the reply... what do you mean by "damp*u*lambda"? Should it be "rho*u*lambda"?

Does Acoustic module solve NS too? I thought they only solve wave equation or helmholtz.

Hi Ivar, Thanks for the reply... what do you mean by "damp*u*lambda"? Should it be "rho*u*lambda"? Does Acoustic module solve NS too? I thought they only solve wave equation or helmholtz.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 23, 2010, 2:59 a.m. EST
Hi

my formula was a general one to e adapted for your fase with a damping (related to lambda and to u) and a additional mass part (related to lambda^2 and to u), indeed check carefuly the units

--
Good luck
Ivar
Hi my formula was a general one to e adapted for your fase with a damping (related to lambda and to u) and a additional mass part (related to lambda^2 and to u), indeed check carefuly the units -- Good luck Ivar
Hoypoy's Dad
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Posted: 1 decade ago Dec 23, 2010, 2:47 p.m. EST
Hi Ivar,

Thanks for the quick reply again. Since I am only interested in natural frequency of the bar in the water, damping is not my concern. For the reason, I chose "no damping" to my model. Without relating additional damping term, can it be done only relating the additional mass due to fluid load?

BTW... Happy holidays to you, Ivar!

Jin
Hi Ivar, Thanks for the quick reply again. Since I am only interested in natural frequency of the bar in the water, damping is not my concern. For the reason, I chose "no damping" to my model. Without relating additional damping term, can it be done only relating the additional mass due to fluid load? BTW... Happy holidays to you, Ivar! Jin
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 24, 2010, 4:23 a.m. EST
Hi

yes you should be able to get along only with structural and some "physics" added mass boundary load for the fluide displaced. Get hand on a copy of "Modelling of Mechanical Systems Fluid-Structure Interaction" from your library you will find the physics and many examples therein. The volume should exist in French too, as it was normally written first in French I have heard, but that mght not help you ;)

--
Good luck
Ivar
Hi yes you should be able to get along only with structural and some "physics" added mass boundary load for the fluide displaced. Get hand on a copy of "Modelling of Mechanical Systems Fluid-Structure Interaction" from your library you will find the physics and many examples therein. The volume should exist in French too, as it was normally written first in French I have heard, but that mght not help you ;) -- Good luck Ivar
Hoypoy's Dad
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Posted: 1 decade ago Dec 24, 2010, 1:46 p.m. EST
Hi Ivar,

I got the book now... it seems very good book to start with. While I am at it, thanks for showing me a good direction. I really appreciate your feedback! Happy holidays!

Jin
Hi Ivar, I got the book now... it seems very good book to start with. While I am at it, thanks for showing me a good direction. I really appreciate your feedback! Happy holidays! Jin

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