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Eigenproblem of acoustics
Posted May 3, 2022, 9:39 p.m. EDT Structural & Acoustics, Acoustics & Vibrations Version 5.3a 4 Replies
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Hi,
I currently practing a simulation that is an acoustic structure with two tubes and a cavity (like a Helmholtz resonator with two necks), and I used two scenario as follow:
One of the model is using background pressure field as input at one neck, and plane wave radiation at another neck.
The other nodel is using pressure boundary condition at one neck and plane wave radiation at anotehr neck.
I used eigenproblem study to find the eigenfrequency of the model, at the beginning I thougnt either the background pressure field or pressure boundary condition is a kind of forcing term, so it won't influence the eigenproblem. However, the eigenfrequencies of two model are different.
Did I misunderstanding the pressure condition? Maybe it is a kind of constraint?
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A pressure condition is a Dirichlet boundary condition for pressure acoustics. This means that the pressure eigenmode will always be zero where the pressure is prescribed (to any value).
-------------------Henrik Sönnerlind
COMSOL
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A pressure condition is a Dirichlet boundary condition for pressure acoustics. This means that the pressure eigenmode will always be zero where the pressure is prescribed (to any value).
Hi Henrik,
Thank you very much.
I also want to know what is the difference between constraints and boundary conditions in acoustics, I know that constraints affect eigenproblems and boundary conditions don't, but I don't know which are constraints and which are boundary conditions.
Thank you in advanced.
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To sort out the terminology here:
The two most common types of boundary conditions for PDE:s are Dirichlet conditions (I think this is what you call "constraint") and Neumann conditions (what I think you call "boundary condition").
Dirichlet conditions prescribe the variable itself, and Neumann conditions its gradient (or rather, the flux).
In pressure acoustics, the Dirichlet condition prescribes the pressure (as this is the dependent variable in the equation). This is, for example, the Sound Soft boundary condition.
The Neumann condition is a prescribed acceleration normal to the boundary (that is, the gradient of the pressure, divided by the density). This is, for example, the Sound Hard boundary condition.
Dirichlet conditions affect the eigenproblem, Neumann conditions do not. There is also a third common type of boundary condition: the Robin condition. In acoustics, it is an impedance. A Robin conditition also affects the eigenproblem.
At the beginning of this blog post https://www.comsol.com/blogs/how-to-make-boundary-conditions-conditional-in-your-simulation, you can find other examples of these types of boundary conditions.
-------------------Henrik Sönnerlind
COMSOL
Please login with a confirmed email address before reporting spam
To sort out the terminology here:
The two most common types of boundary conditions for PDE:s are Dirichlet conditions (I think this is what you call "constraint") and Neumann conditions (what I think you call "boundary condition").
Dirichlet conditions prescribe the variable itself, and Neumann conditions its gradient (or rather, the flux).
In pressure acoustics, the Dirichlet condition prescribes the pressure (as this is the dependent variable in the equation). This is, for example, the Sound Soft boundary condition.
The Neumann condition is a prescribed acceleration normal to the boundary (that is, the gradient of the pressure, divided by the density). This is, for example, the Sound Hard boundary condition.
Dirichlet conditions affect the eigenproblem, Neumann conditions do not. There is also a third common type of boundary condition: the Robin condition. In acoustics, it is an impedance. A Robin conditition also affects the eigenproblem.
At the beginning of this blog post , you can find other examples of these types of boundary conditions.
Thank you very much!! This is helpful.