## 3 Results
The airflow (at a standard pressure of 0.56 MPa on the nozzle edge), employing EAS for metal atomizing and transporting, usually has a supersonic velocity of about 340 m/s. The structure of such flow comprises several cyclically repeated zones with the clearly seen boundaries (Figure 6): (1) the zone of dense airflow, created due to a change in the motion of outer gas layers (the so-called Mach disc [37]); (2) the zone of the initial gas-dynamic spike (\"initial\" section); (3) the transitional (\"second\") zone; (4) the main zone (the front part of the Mach disc). The Mach discs are formed inside the supersonic flow, the pressure of which at the edge of the nozzle is much bigger than atmospheric. Atmospheric pressure allows the gas stream to expand; it leads to a change in the direction of the external gas layers from axial to radial [37]. The alteration of a supersonic flow direction occurs on \"skew\" shock waves. The pressure, temperature, density, and velocity of gas vary when it passes the front of the shock wave. All these values are changed together to be described by  
Figure 4: Gas spectrograms of the airflow passing the crossed electrodes: (**a**) the “top view”, (**b**) the “side view” (1—depression area, 2—Mach disc). Figure 5: The metal particles stream being atomized in the arc discharge: (**a**) the “top view”, (**b**) the “side view”. the shockwave's common characteristic, which is the Mach number (meaning the number of Mach discs in the flow) [37]. In gas spectrograms of the airflow without pulsation (Figure 6a), it can be seen that at a distance of up to 1 mm (at the end of the second zone), the Mach disc is absent, the zone of dense flow in the inter-electrode gap was missing. A different flow character was revealed for the pulsating airflow (Figure 6b,c). With the pulsation, a depression flow appears near the nozzle edge that ends with a shock wave (having Mach configuration). A disc-shaped wave that crosses the flow's axis at a straight angle causes the centered depression wave. The gas flow on the nozzle's outlet (Figure 6c) forms a centered cone-shaped area, limited by depression waves. At 75 Hz frequency (Figure 6c), all flow areas are in vibration motion. Additionally, the presence of a \"barrel-like\" wave structure in initial and transitional zones with \"saw-like\" pressure distribution along the flow's axis is observed to be accompanied by shock waves. The zones of air flow's break are present between the waves. The length of breaking zones increases at 75 Hz frequency, enabling a possibility of accumulating molten metal on the edges of atomized electrodes. Analyzing the airflow and paying attention to pressure values lets us now analyze the flow characteristic. If pressure on the nozzle's edge is not equal to atmospheric pressure, then two modes of nozzle operation are possible, namely: the mode of \"under-expansion\" and the mode of \"over-expansion\" [38]. At the \"under-expansion\" mode, the pressure inside the gas flow at the nozzle's edge is bigger than the atmospheric value. As a result, the gas that leaves the nozzle is expanded in the atmosphere mixing with the environment. At that, the flow velocity outside the nozzle is not actually changed due to the formation of vortexes on the flow frontiers. In the case of \"over-expansion\", the flow pressure at the nozzle's edge is lower than atmospheric pressure [38]. Thus, it may be concluded that the structure of the atomizing flow at different pulsating frequencies will be similar to the structure of the \"under-expanded\" flow. It is known [37] that in \"under-expanded\" flow (the \"nozzle's edge pressure/atmospheric pressure\" ratio exceeds 10-15), the transitional zone begins virtually right after the first \"barrel\" because the Mach disc covers nearly the entire flow cross-section and the mixing layers are close to each other. It is important since the airflow is slowed down near the Mach disc, creating an increased pressure zone.