# Evaluation of Noise-Reduction Techniques for Gas-Turbine Test Stands: A Preliminary Analysis
## 5 Pressure Loss and Self-Noise Evaluation  
Pressure loss, which refers to the reduction in air pressure as air flows through the duct system, was also evaluated for all configurations. This phenomenon occurs due to friction and turbulence created by internal surfaces or any acoustic treatments within the duct. Sound-absorptive materials, such as mineral wool and perforated steel sheets, increase the roughness of the duct's internal surface, thus causing more friction between the air and the duct walls. Variations in duct cross-sectional area, designed to enhance noise attenuation by mitigating the beaming effect, can cause pressure changes and contribute to overall pressure loss. Excessive pressure drops can lead to additional noise, and compensatory higher flow speeds may result from excessive pressure drops. The effect of sound on rectangular plates placed within the flow has been studied by [PERSON] and [PERSON] [25], who found that shear layers attach to the leading edge in the center of vorticity and close to the boundary layer. [PERSON] et al. [26] demonstrated the influence of rectangular geometries with different aspect ratios (ranging from 0.3 to 7) placed within the flow through extensive simulation and proving their impact on the flow field. The influence of different geometries of edges on the noise induced by flow was investigated by  
Figure 8: Panel arranged in various topologies: (**a**) Conf. I—single baffled_linear; (**b**) Conf. II—double baffled_linear; (**c**) Conf. III—triple baffled_linear. Figure 9: Attenuation correction in staggered baffles configuration. [PERSON] and [PERSON] [27], who concluded that rectangular edge geometries with 0.5 aspect ratios show a significant increase in flow-excited acoustic pressure. [PERSON] [28] investigated the sound produced by a low-Mach vortex flow on a two-dimensional splitter, concluding that the sharp edge in the flow does not affect acoustic scattering. Noise attenuation of a rectangular duct with a perforated panel liner under one-direction flow conditions was studied by [PERSON] and [PERSON] [29] using the transfer functions method and experimental validation. In our case, we will study the attenuation in direct-end reverse-flow conditions, similar to intake and exhaust application cases. The following Almgren formula [15] is used to calculate the pressure drop in a single-baffled parallel attenuator with different length \(L\) of the baffles:  
\[P=\frac{\xi\rho v_{b}^{2}}{2}+\frac{\lambda_{perf}L\rho v_{b}^{2}}{2d_{h}}+ \frac{\rho(v_{b}-v_{0})^{2}}{2} \tag{8}\]  
where the local loss coefficient \(\xi\approx 0.1\) and friction coefficient \(\lambda_{perf}\approx 0.05\) (0.05 for smooth walls and 0.1 for rough walls), and \(d_{h}\) is the hydraulic diameter. The first term from Equation (8) represents the pressure loss caused by the inlet and outlet effects of the flow through the baffles; the second term represents the pressure loss due to friction along the walls of the channel, and the last term is the pressure loss due to changes in air velocity. The pressure drop occurs at the entry and exit of the baffle zone. Subsequently, the pressure drop for all linear configurations was analyzed using Equation (8) and compared with the results obtained from the Ansys Fluent version 15.0 software. Additionally, the staggered configurations were analyzed only with FEM. The boundary conditions introduced were an airflow rate of 6.125 kg/s defined at the inlet, corresponding to a speed of 10 m/s. Atmospheric pressure was applied at the outlet, assuming the gas to be ideal to account for density variations, and the \(k-\varepsilon\) turbulence model was utilized. A steady-state flow analysis was conducted for these computations. The pressure loss was computed as the difference between the total pressure of the inlet and outlet. Based on the flow field, the noise power level inside the attenuator was computed using the Fluent acoustic software version 15.0 module, which is based on [PERSON]'s formula [30].