# Evaluation of Noise-Reduction Techniques for Gas-Turbine Test Stands: A Preliminary Analysis
## 1 Introduction
Various internal configurations for silencers, including the effects of side-branch partitions, multi-chamber partitions, and their combined effects, were examined by [PERSON] and [PERSON] [6]. Acoustic pressure radiation from a perforated region was studied by [PERSON] et al. [7], who found that such areas can significantly reduce the pressures radiated from a duct outlet. The concept of perforated panels is used and numerically analyzed in this article. Numerical and experimental investigations were conducted by [PERSON] et al. [8], concluding that isotropic behavior was observed in perforated plates with circular holes under the square and triangular penetration pattern geometries also used in this article. Low-frequency tonal noise reduction was studied by [PERSON] et al. [9], using dissipative and reactive elements in silencers, thus enabling reactive elements to function effectively over an extended low to medium frequency range. [PERSON] [10] used a finite-element approach to evaluate sound transmission loss in perforated silencers, concluding that discrepancies in sound transmission-loss results may be attributed to the viscous effects of sound-wave propagation and geometric imperfections. [PERSON] and [PERSON] [11] performed an acoustic performance analysis of absorption materials used in sound attenuators, concluding that a type of glass wool (with a density of 24 kg/m\({}^{3}\) and flow resistivity of 14,000 Pa s/m\({}^{2}\)) inserted in a muffler yielded the best transmission loss compared to rock wool, polyester, or melamine. Experiments with parallel-baffled mufflers and perforated panels were conducted by [PERSON] [12], who created a testing stand and studied the transmission loss for various panel configurations. In his study, he did not use a staggered configuration; thus,it was necessary to analyze it to be more appropriate for noise attenuation in gas-turbine test stands. [PERSON] et al. [13] investigated noise reduction and the acoustic impedance of a ducted intake using a modified time-domain model instead of the classical transmission-loss model. Porous lamellas placed in a rectangular duct of an attenuator were evaluated numerically and experimentally by [PERSON] et al. [14], proving that acoustic attenuation is primarily due to viscous and structural dissipation. By contrast, in the present study, the viscous and structural dissipation through panels will be numerically investigated by applying impedance modification phenomena given by the perforation of a metal sheet. This investigation acoustically and aerodynamically evaluates attenuators with parallel acoustic baffles made from mineral wool in both linear and staggered topologies within a rectangular duct by considering simple-, double-, and triple-baffled configurations. These configurations represent the concept and a downscaled version of a future noise attenuator to be incorporated into a gas-turbine test stand. Specialized literature on baffled noise attenuators provides calculation models that yield varying results [15]. In this study, an analytical method for calculating the attenuation of a baffle attenuator is investigated alongside the finite-element method, and the results of both methods are compared with experimental data. The study assesses the acoustic attenuation in three flow conditions--without flow, indirect, and reverse flow--by taking into consideration the influences of the convective effect on attenuation and the self-noise of the air passing through the baffles for all studied configurations. The analyzed configurations are also evaluated from a gas dynamic perspective to determine pressure loss, using both an empirical method and the finite-element method. To ensure the accuracy of the 2D simulation models, a comparative analysis between 2D and 3D models is conducted to establish if the 2D simulations can offer a balance between computational efficiency and accuracy.