## 5 Conclusions  
To our knowledge, this study is the first to comprehensively compare ejector and chevron configurations for noise reduction in micro turbojet engines using an integrated analysis of statistical, acoustic and Schlieren imaging techniques. Due to the compact design of these engines, traditional noise-reduction methods, such as increasing mass flow and reducing jet velocity, are impractical. Instead, modifications to jet flow dynamics provide alternative solutions. This research highlights the distinct mechanisms and relative effectiveness of ejector and chevron configurations in mitigating noise. The first critical step was ensuring the suitability of the recorded signals for meaningful analysis. Statistical validation demonstrated that the signals representing flow properties were stationary and ergodic, allowing time averages to reliably represent ensemble averages. Figure 16: Ejector nozzle—Regime 3: 115k rpm, consecutive frames. (**a**–**j**)—ejector. This property ensured the robustness of the selected trimmed segments for noise analysis. Evaluations of mean, variance and autocorrelation confirmed consistency across segments, supporting the use of time-independent visualizations for analyzing jet dynamics. The chevron configuration proved to be more effective than the ejector in reducing noise across the frequency spectrum. Specifically, for the chevron configuration, the chevron nozzle reduced overall SPL(Z) by 1.28% and OASPL by 1.21%. Low-frequency noise (SPL(C)) decreased by 1.15%, with specific reductions in harmonics, such as 270 Hz and 1289 Hz, where SPL dropped by 4.1 dB and 3.8 dB, respectively. These reductions were attributed to the chevron's ability to disrupt large-scale vortices, promoting fine-scale turbulence and shifting energy to higher frequencies that dissipate more quickly. Conversely, the ejector configuration achieved modest noise reductions, with SPL(Z) decreasing by 0.51% and OASPL by 0.74%. In spite of the fact that it generated secondary airflow through ambient air entrainment, the resulting larger turbulence scales, clearly identified inside the frames, contributed to low-frequency noise. This noise propagates farther, and it is more difficult to be attenuated. This comparison highlights the effectiveness of the chevron configuration in mitigating jet noise by modifying flow dynamics and enhancing the dissipation of acoustic energy. Schlieren imaging supported these findings by providing visual evidence of jet flow behavior. Thus, for the baseline configuration, larger, uncontrolled vortices were identified as they detached quickly from the nozzle, contributing to chaotic, low-frequency turbulence. In contrast, the chevron configuration revealed smaller, tightly packed vortices that remained near the nozzle wall, enhancing mixing and noise control by promoting high-frequency turbulence. For the ejector nozzle, larger coherent structures were identified, with less effective mixing and greater contributions to low-frequency noise, particularly during high-speed operations. The analysis of the post-processed Schlieren images confirmed that chevrons produce finer turbulence scales, reducing noise more effectively than the ejector configuration, especially at higher regimes. These findings were consistent across the statistical and acoustic analyses, validating the chevron's ability to shift energy toward higher frequencies and attenuate low-frequency noise. Additionally, the study underscored the value of integrating advanced image processing with statistical and acoustic methods. The newly created code-based image-processing techniques provided enhanced insights into turbulence features, such as vortex formation, interaction and dissipation, enabling a more detailed understanding of jet flow behavior. This methodological integration demonstrates the potential of Schlieren imaging to complement acoustic measurements in characterizing flow dynamics.