# Discussion Forum

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## Plane wave propagation inside a duct (Vibroacoustic)

Hello everybody,

I me dealing with an interesting vibroacoustic problem and wanna model it WIth COMSOL.

The subject is : A duct (Air inside)of one meter length and cross sectional 0.2m*0.2m (width=1m)
Boundary conditions : unit velocity at one end
zero velocity Boundary condition is applied at the other end (rigid wall)
The 4 lateral sides are assigned rigid wall.

Frequency = 100hz.
Material (duct) = Any (wood, alu,etc...)

My questions are :
- COMSOL doesn't contain Velocity boundary ? (Drive me mad)
- How to handle that with COMSOL ?

Your help would be very appreciated .......

5 Replies Last Post 08/08/2012, 20:45 GMT-04:00
Posted: 5 years ago
Hi

I do not have the acoustic module, but I see you have Normal Velocity BCs from the doc and in structural prescribed velocities BC exist. what do you need more ?

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Good luck
Ivar

Posted: 5 years ago
Thanks for the tip.
Great idea.

But as i m dealing with Helmholtz equation, i find aonther way, buy taking Partial diffrential equation.

My second question :

Don't know how to model interaction between A fluid AIr and solid vibrating (any) close.
I mean how to define Material in that case ?

I have tried linear elastic for the fluid and solid, but doesn't work !

What do you think ...Your tips are welcome SIr. Any !

Thanks again.

Posted: 5 years ago
Hi

A fluid is no linear elastic material, OK you can cheat and say Poisson =0 Young = 1/3 of absolute pressure. But i do not believe you will get very far. You can also model the air as an added mass to the solid surface (for eigenmode or frequency domain sweep), but I'm not sure how to model approximately the viscosity and damping in this way.

The Acoustic module proposes much more, but I do not have it, unfortunately, so I cannot help much here

--
Good luck
Ivar

Posted: 5 years ago

Sounds right.
I'm still working on it.

Thanks again.

Posted: 5 years ago
Hi Griffin,

Have you been able to solve this issue. If yes, can you provide us with some guidelines as to how to go about solving these problems.

Thanks
Sankha

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