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In connection with subsonic jet noise production, this study investigates acoustic noise reduction in micro turbojet engines by comparing ejector and chevron nozzle configurations to a baseline. Through detailed statistical analysis, including assessments of stationarity and ergodicity, the current work validates that the noise signals from turbojet engines could be treated as wide-sense ergodic. This further allows to use time averages in acoustic measurements. Acoustic analysis reveals that the chevron nozzle reduces overall SPL by 1.28%, outperforming the ejector's 0.51% reduction. Despite the inherent challenges of Schlieren imaging, an in-house code enabled a more refined analysis. By examining the fine-scale turbulent structures, one concludes that chevrons promote higher mixing rates and smaller vortices, aligning with the statistical findings of noise reduction. Schlieren imaging provided visual insight into turbulence behavior across operational regimes, showing that chevrons generate smaller, controlled vortices near the nozzle, which improve mixing and reduce noise. At high speeds, chevrons maintain a confined, high-frequency turbulence that attenuated noise more effectively, while the ejector creates larger structures that contribute to low-frequency noise propagation. Comparison underscores the superior noise-reduction capabilities of chevrons with respect to the ejector, particularly at high-speed. The enhanced Schlieren analysis allowed for new frame-specific insights into turbulence patterns based on density gradients, providing a valuable tool for identifying turbulence features and understanding jet flow dynamics. [ABSTRACT FROM AUTHOR]

This study aims to highlight the noise reduction achieved through the integration of serrated blades on the leading and trailing edges within a small-scale cascade configuration relevant to turbomachinery contexts. Experiments were conducted using a newly developed 3D-printed test bench, enabling both acoustic and aerodynamic measurements. Turbulence was generated using a rectangular grid positioned at two axial locations. Non-dimensional spectra were computed and compared with experimental data, showing good agreement over a wide frequency range. Significant noise reduction was observed in the 1000–3000 Hz band, despite the lack of optimization of turbulence and serration parameters. Leading-edge serrations were found to be effective at lower frequencies in the axial direction and at higher frequencies laterally. In contrast, trailing-edge serrations had a minimal impact above 3500 Hz, performing worse than the reference condition across a large frequency range. Nevertheless, for this initial iteration at a small scale, overall sound pressure level reductions of up to 1 dB were achieved with trailing-edge serrations and up to 1.5 dB with leading-edge serrations, underscoring their potential for noise mitigation in relevant applications. [ABSTRACT FROM AUTHOR]

A combined method of large eddy simulations for non-premixed combustion in a turbulent boundary layer coupled with proper orthogonal decomposition of instantaneous velocity, pressure and temperature fields is developed in order to obtain a reduced order model. First, we investigate a channel turbulent reacting flow using Large Eddy Simulations (LES) technique. Polypropylene/O2 has been considered as fuel/oxidant pair. The turbulence-combustion interaction is based on a combination of finite rate/eddy dissipation model applied to a reduced chemical mechanism with four reactions. The LES numerical results are analyzed with respect to RANS simulations and with other reference data. The second part of the paper refers to the derivation of a Reduced Order Model (ROM) based on proper orthogonal decomposition (POD) technique for the unsteady flow field. In order to achieve that, the eigenmodes of the flow are computed from several snapshots of the instantaneous fields uniformly spaced and the most energetic ones are used to set up the Reduced Order Model. Constant regression rate of the fuel grain is considered. The flow and thermal fields obtained with ROMs are compared with the ones obtained from the full simulation and an analysis on the number of modes required to achieve the desired accuracy is presented.

The paper is a continuation of the efficiency and accuracy analysis of different high order numerical schemes previously published by the authors [1-3]. After the analysis of advection equation and the scalar conservation equation for different initial conditions and for different convex and nonconvex conservative fluxes, we further investigated the one-dimensional conservation equation. This paper focuses on a new comparison of the behavior of the most known Weighted Essentially Non-Oscillatory (WENO) type numerical schemes for shock tube problem. We consider nine different Riemann Problems. Analytical solutions are provided for almost all different initial Riemann conditions. A third-order TVD Runge-Kutta (TVDRK3) scheme was adopted for advancing the solution in time. The motivation of this assessment is related to filling a gap in the specialized literature characterized by the bias towards presenting only cases favorable to a certain method. Our purpose is to present objectively the capacity of each method for solving one-dimensional conservation law problem. Unfortunately, not all the schemes identify accurately the position of the shocks or rarefaction waves and some of them even do not converge to a solution for different Riemann initial conditions. [ABSTRACT FROM AUTHOR]

This paper focuses on a new comparison of the behavior of three Weighted Essentially Non-Oscillatory (WENO) type numerical schemes for three different nonlinear fluxes, in the case of scalar conservation law. The analytical solution is provided for various boundary conditions. For the time integration we adopt the 4-6 stage Low-Dispersion Low-Dissipation Runge-Kutta method (LDDRK 4-6). The schemes were tested on piecewise constant function for non-periodical conditions. The assessment was performed because the specialized literature mainly presents cases favorable illustrating only to a particular method while our purpose is to objectively present the performance and capacity of each method to simulate simple cases like scalar conservation law problems. All the schemes accurately identify the position of the shock and converge to the proper weak solution for the non-linear fluxes and different initial conditions. The paper is a continuation of the efficiency and accuracy analysis of high order numerical schemes previously published by the authors [1,2]. [ABSTRACT FROM AUTHOR]

Supersonic/hypersonic flows with strong shocks need special treatment in Computational Fluid Dynamics (CFD) in order to accurately capture the discontinuity location and his magnitude. To avoid numerical instabilities in the presence of discontinuities, the numerical schemes must generate low dissipation and low dispersion error. Consequently, the algorithms used to calculate the time and space-derivatives, should exhibit a low amplitude and phase error. This paper focuses on the comparison of the numerical results obtained by simulations with some high resolution numerical schemes applied on linear and non-linear one-dimensional conservation low. The analytical solutions are provided for all benchmark tests considering smooth periodical conditions. All the schemes converge to the proper weak solution for linear flux and smooth initial conditions. However, when the flux is non-linear, the discontinuities may develop from smooth initial conditions and the shock must be correctly captured. All the schemes accurately identify the shock position, with the price of the numerical oscillation in the vicinity of the sudden variation. We believe that the identification of this pure numerical behavior, without physical relevance, in 1D case is extremely useful to avoid problems related to the stability and convergence of the solution in the general 3D case. [ABSTRACT FROM AUTHOR]