# Analysis of Vorticity and Velocity Fields of Jets from Gas Injector Using PIV
## 2 Experimental Setup  
The schematization of the experimental setup and of three nozzle patterns are shown in Figure 1. A dual cavity Nd:YAG laser (Spectra Physics, Quanta Ray PIV-400, Milpitas, CA, USA) was used for the generation of the luminous background through the emission of two pulsed signals of light with a wavelength of 532 nm. The nominal energy of pulsed light was approximately of 60 mJ, having a duration of 6 ns and a time separation of 3 us. Since it was not possible to know a priori the correct time delay between the two pulses, the above value was found by linearly increasing it from 1 to 15 us during experiments at intermediate pressure ratio, i.e., PR10. The two laser pulses generated by the laser are then deviated by a series of mirrors and directed towards the CVC as shown in Figure 1. Before reaching the chamber the two signals pass through three cylindrical lenses; the first concave lens with a focal length of \(-\)50 mm and for the other two large lenses it was +500 mm. The two pulsed signals are thus converted onto sheets with a thickness of approximately 1 mm in correspondence of the injector axis. The air supply line reaches a maximum pressure of 8 bar and is connected to both CVC and particle seeder. The air input system in the chamber, as well as that for tracer particles, are designed to reduce their influence over the jet structure. Specifically, the air is supplied through a duct positioned on the upper wall away from the injector axis, while oil particles are conveyed through a perforated hollow toroidal tube located on the upper wall. Particular attention was paid to the calibration of the particle seeder to avoid two opposing conditions: lack or oversaturation of the tracer particles. This task was achieved by managing the inlet pressure of the particle seeder in relation to that of the pressure line. For the present study, the particle seeded was set to operate in order to produce tracers of 600 nm \(\pm\) 200 nm. The resulting images of scattered light were recorded using an interline transfer camera (TSI 630090, Shoreview, MN, USA) with a resolution of 2352 \(\times\) 1768 pixels and a 16-bit concentration per pixel. The helium was supplied from a tank at 200 bar and the gas injector mounted in the top wall of the chamber. The effective pressure of injection was set using a manual regulator and fluctuations monitored by a sensor. The pressure was observed to suffer a maximum reduction of less than 5% with reference to the nominal value, therefore influencing in a negligible way the flowrate characteristics. Optical access to the CVC was provided by four circular windows located on the side walls, each with an inner diameter of 130 mm. For the present study, two of these windows were obscured using laser beam stoppers or black screens. As for the three nozzle tip models (Figure 1, bottom), the geometric angle of divergence of the orifice paths from the injector axis was 15\({}^{\circ}\) for the first, 18\({}^{\circ}\) (external orifices)-12\({}^{\circ}\) (inner orifices) for the second and 20\({}^{\circ}\) for the third. The total area of the orifices on the nozzle tips is equal to 7.0, 4.3 and 4.0 mm\({}^{2}\) for the first, second and third geometries, respectively.