Description

The implementation of porous mediums within tubular structures has emerged as a pivotal strategy for sound absorption. This technique exploits the inherent properties of porous materials to dissipate sound energy, thereby mitigating noise pollution and enhancing the auditory experience within the confines of the tube.

In this problem, we aim to simulate the phenomenon of sound absorption inside a pipe using a porous medium as silencer. Thus, it plays a roll of a silencer. The primary objective of our study is to measure two key parameters over a wide range of frequencies:

Transmission Loss (TL): It represents the loss of sound power when sound travels through the pipe filled with a porous medium. It's a crucial factor in many engineering applications where sound reduction is desired.

Sound Pressure Level (SPL): It is the pressure deviation from the ambient atmospheric pressure caused by a sound wave. In our context, we are interested in understanding how the SPL changes over a wide range of frequencies when the sound propagates through the porous medium inside the pipe.

The geometry of the model is designed in ANSYS SpaceClaim software. Next, the computational domain is divided into separate cell zones in ANSYS Meshing software. Thus, 1,209,174 polyhedral cells are generated.

Methodology

To accomplish this, we are employing the FW-H acoustic model. This model is well-regarded for its ability to accurately predict the acoustical behavior of systems, making it a suitable choice for our simulation. Through our study, we hope to gain a deeper understanding of how sound behaves in these conditions, which could have significant implications for various fields, such as acoustical engineering, environmental noise control, and more.

The inlet and outlet are placed 200mm and 500mm far away from the silencer, respectively. The air enters the tube with a 5m/s velocity magnitude. It should be noted that the flow equations are solved in steady-state form. Then, the acoustic equations are also added and continued unsteadily.

Conclusion

As the air enters the pipe, it needs to penetrate through the porous medium which results in a remarkable pressure drop due to the complex porous shape. Indeed, a stagnation point forms on the porous wall and the velocity increases dramatically, based on the Bernoulli equation. Both can be seen in the figures below.

From an acoustic perspective, the same thing is happening. In order to fulfill our objectives, 3 receivers are implemented. One of them is placed before the porous medium, 100mm away from it. The two others are placed after with 200mm and 400mm distance from the porous silencer. Before investigating the results, keep in mind that the reference acoustic pressure is set to 2e-5 Pa. Thus, all the reported values are relative regarding the reference pressure level.