Helmholtz BC vs. Acoustics BC

[tilbury]

I am trying to do acoustic analysis in box-shaped air.
A pressure sine wave was excited on one side, and the impedance was set to ρ * c (density * propagation speed) on the other side to create a far-field boundary condition.

I created the boundary conditions as below in the Helmholtz solver and got the correct result that no reflected waves occur.

In the radiator boundary condition, the value was set in the imaginary part of flux.
Wave Flux 2 = Real 18.17

In the far-field boundary condition, the value was set in the real part of the specific impedance.
Wave impedance 1 = Real 343.0

I would like to perform this same analysis in the acoustics solver (The Linearized Navier–Stokes Equations in the Frequency Domain).

On the radiator boundary conditions
Im Surface Traction 1 = Real 18.17

On far-field boundary conditions
Re Specific Acoustic Impedance = Real 343.0

Although the boundary conditions were set like this, the acoustics solver produces incorrect results indicating that reflected waves exist.

I would like to know how to set the boundary conditions in the acoustics solver to obtain the same results as the Helmholtz solver.

More details are included in the previous question.
viewtopic.php?t=8272&sid=afa54d4bc6d54e ... a3e65b04d2

[mika]

Suitable BCs are explained in Section 13.2.2 of the documentation

https://www.nic.funet.fi/pub/sci/physic ... Manual.pdf

as "For example, outgoing waves may be approximated by setting Z = −rho_0 c and ZT = −iω/c on the outflow boundary". That is, one has to give also an impedance BC for the thermal fluctuations by using the keywords Re Specific Thermal Impedance and Im Specific Thermal Impedance. In addition, note that the value of Re Specific Acoustic Impedance has a reverse sign in comparison with the corresponding BC for the Helmholtz equation.

-- Mika

[tilbury]

Suitable BCs are explained in Section 13.2.2 of the documentation

https://www.nic.funet.fi/pub/sci/physic ... Manual.pdf

as "For example, outgoing waves may be approximated by setting Z = −rho_0 c and ZT = −iω/c on the outflow boundary". That is, one has to give also an impedance BC for the thermal fluctuations by using the keywords Re Specific Thermal Impedance and Im Specific Thermal Impedance. In addition, note that the value of Re Specific Acoustic Impedance has a reverse sign in comparison with the corresponding BC for the Helmholtz equation.

-- Mika

I really want to thank you for your help.