## 4 Results and Discussions
### Schlieren Imaging Analysis
Additionally, it examines chevron nozzles with and without ejectors, focusing on the maximum regime due to limited data availability for other cases. This refined approach strengthens the ability to validate and expand upon conclusions from previous research. The experiments were conducted using a micro turbojet in its baseline configuration, equipped with the standard nozzle, to capture Schlieren visualizations of the jet at operational conditions of 35k, 55k and 115k RPM. Subsequently, a nozzle with 16 chevrons replaced the baseline nozzle, and measurements were repeated under identical conditions. Finally, an ejector was mounted while retaining the baseline nozzle, and measurements were performed for this configuration exclusively at 115k RPM. The frames were captured at intervals of five consecutive frames, corresponding to 20.83 us apart, amounting to approximately 0.1 ms in total. Based on reference [26], presenting and analyzing consecutive frames is challenging due to the minimal visible differences between them. However, with the current image-processing code employed in this study, subtle density variations can now be identified and analyzed. Figure 12 shows the common characteristics that are present in the baseline configuration which remain valid for every regime. At the nozzle exit, located on the middle-left side in each processed frame, the flow first emerges and encounters the surrounding environment. The yellow triangle frame signifies the inner region where the potential core is located. In between the triangle and the wall of the central cone, one can see initial small-scale turbulent structures that begin to form as the high-speed jet exits the nozzle due to the initial shear layer. For regimes with higher velocities, the rate at which surrounding air mixes into the jet is lower. This is because the jet has more momentum to counteract the entrainment of surrounding air. As a result, the potential core is preserved over a longer distance, as the surrounding air does not mix in as quickly to decelerate or disrupt the high-speed jet flow. As the flow progresses downstream, these small-scale turbulent structures (eddies) grow in size, expanding and interacting more extensively with the surrounding air. This interaction leads to larger coherent flow structures that eventually break down into fully developed, chaotic turbulence. The lower-right region of the frame depicts this transition, where turbulent energy is dispersed across various scales, contributing to the overall chaotic motion. This visualization also captures the detachment of coherent eddies from the central cone as they reach its end. These detached eddies coalesce into larger vortical structures that move downstream and eventually dissipate. Such large-scale structures are prominent sources of low-frequency noise due to their organized, energy-containing motion. In the baseline nozzle configuration without modifications, the shear layer has more freedom to develop these large-scale vortices shortly after separating from the wall. Without additional elements like serrations or chevrons to disrupt this natural growth, the vortices roll up and combine into coherent structures that persist downstream. These large structures are less attenuated and propagate low-frequency noise over longer distances. It is worth noting that the acoustic signals generated by the flow are stationary and ergodic, which implies that the turbulent flow captured through imaging exhibits statistically consistent properties over time. This allows for the random selection of video frames for analysis, as the correlation of a stochastic function remains invariant under uniform time translations. This feature simplifies the imaging process while ensuring robust analysis. Accordingly, the instantaneous Schlieren visualization of the exhaust jet for both configurations, baseline and chevrons, for the three regimes are presented in Figures 13 and 14.