Plane wave frequency domain analysis of resonance free pipes with moving medium

Abstract

Mufflers are inevitably a significant part of the exhaust systems of automobiles for bringing down the tail-pipe noise levels. Nevertheless, howsoever well-optimized a reasonably large muffler volume be (with respect to insertion loss, back pressure, breakout noise, flow noise, etc.), there are inevitably acoustic modes present, which lead to peaks in the noise levels and corresponding troughs in the insertion loss (IL) of an exhaust baseline. These acoustic modes are: (i) muffler internal modes, (ii) axial pipe modes and (iii) system modes. Of these, the axial pipe mode is the most detrimental one. Usage of microperforated (MPA) patches along long pipes at locations of sound pressure anti-nodality to dampen their resonances has proved to be an efficient acoustic solution. All the works reported so far on these resonance free pipes with such patches have been using 3D FEM or vehicle testing. This article discusses their acoustic performance employing linear plane wave theory. The predicted acoustic behavior comes in-line with those of published resources. Also, the flow leakage through the open patches and the corresponding attenuation impact have been demonstrated. It has been shown that the seemingly inconsequential leakage deteriorates the patch acoustics considerably. This result is verified with the published claims, thereby further validating the 1D analysis presented here. Also, the 1D approach has a scope for valuable benchmarking of resonance free pipes (RFPâ,,¢) in industry since it does not suffer any intrinsic modeling deficiency unlike some commercial simulation solvers. The article, thus, presents, validates and illustrates the potential of linear acoustic analysis toward the acoustic assessment and optimization of RFPs.