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Sound wave reflection in ANSYS

Sound wave reflection in ANSYS

Sound wave reflection in ANSYS

(OP)
Hi

We are trying to moddel the following physical problem:
A plane pressure wave is traveling along a rigid duct in a fluid with some density rho1 and sound speed c1. The wave encounters a boundery to another fliud with some other density rho2 and sound speed c2. At this boundery should occur reflection (r) and transmission (t) of the incident wave according to

r = (rho2*c2-rho1*c1)/(rho2*c2+rho1*c1) and t = 1-r (*)

The transmitted wave travels towards infinty afterwards.

In ANSYS is the pressure wave is moddeled by assigning pressure DOF at the left boundery of the duct (amplitude=1). Left half of the duct is meshed with FLUID30 with acoustic properties; density rho1 and sound speed c1. The right side of the duct has been meshed also with FLUID30, but with density rho2 and sound speed c2. The right boundary has been meshed with the element FLUID130 that mimics an infinite boundary, that absorbs incoming waves. A harmonic analysis is made, such that the applied pressure becomes harmonic; p=1*cos(omega*t).

The problem is that the amplitudes of transmitted and relfected waves are not in accordance with formulas (*). It seems that the location of the boundary between the two fluids are of importance, but should be irrelevant. Also does the pressure wave amplitudes seem to amplify in some cases, thereby introducing more (errorneous) acoustic energy into the system.

We are using Ansys 14 Classic.

Can anyone spot an error in the modeling of our seemingly simple system? Or does anyone have an apdl-file of a similar problem?

Thanks,
Dubbelkex.

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