## 3 Results and Discussion
### Discharge of Inert Gases: Effect of Generated Noise  
The diagrams included in Figure 7 show the effect of the noise generated by standard and silent nozzles with various orifice diameters on both HDDs and SDDs, throughout a full discharge of nitrogen IG100. Notably, selected experiments using nozzles of 5, 10, 14, 17 and 23 mm diameter are presented; the results from the tests with the silent nozzles for low ceiling applications are omitted, as an approximately similar behavior was found between them and the other silent nozzles with the same orifice diameter. Overall, the experimental results demonstrate the ability of silent nozzles to bring SPL down to relatively harmless values through the discharge: standard nozzles present SPLs greater than 110 dB--the lower limit of the damaging threshold (Section 1)--over most of the gas release, and in most cases (i.e., for orifice diameter greater than 5 mm), peaks exceeding 120 dB occur; on the other hand, the SPL of silent nozzles exceeds the threshold only for about 25 s even with the larger orifice diameter (larger than 10 mm), also never reaching 120 dB. As a finding of general validity for both types of nozzle, SPL exhibits a sudden increase up to a local maximum upon starting the discharge, which is consistent with the theory associated with noise emitted by gaseous jets [5; 6; 7], reviewed in Section 1: since noise is largely due to turbulence effects, it may be interpreted as function of Reynolds number (\(Re=w_{m,CR}D_{m}/v\), where \(v\) is kinematic viscosity of the involved substance); as \(Re\) increases, turbulence develops until critical \(Re\) is reached (i.e., fully developed turbulence) [29], to which SPL local peaks arguably correspond. Instances of additional maxima occurring after the first peak appear mostly for the discharge through the standard nozzle (Figure 7a,c,d,e), and hint at some slight pressure variability imposed by the constant flow valve: constant flow valves are developed and employed to keep pressure and, consequently, flow rate as constant as possible throughout the discharge, which would virtually result in constant Re and SPL until pressure decays in the storage tank towards the end of the process. This practically translates into a plateau-like behavior, as apparent in Figure 7f,g,h, with the potential singularities previously mentioned occurring in some cases. The proposed discussion also substantiates another observation holding for both standard and silent nozzles (Figure 7): the larger orifice diameter, the higher the SPL maxima and overall SPL values throughout its trend. Assuming constant pressure (i.e., 70 bar, as mentioned in Section 2.1), at least throughout the controlled phase of the discharge, results in almost constant average velocity, even with a Bernoulli inviscid model [30]; as orifice diameter increases, both \(Re\) and released mass flow rate increase. Therefore, it can be inferred that the discharge time obviously decreases, while SPL increases, as higher flow rate is issued, providing that supply pressure is kept constant. An insight into the effect of the sound frequency stems from maps presenting SPL as a function of time and frequency throughout the discharge; Figure 8 shows experiments conducted for both standard and silent nozzles with 10, 17 and 23 mm orifice diameter as the most representative. It is interesting to note that SPL reaches higher values (i.e., greater than 110 dB) within the 1-8 kHz range, which is somewhat lower than the findings from Koushik et al. [15] on standard nozzles (i.e., more in the 10-20 kHz range). This suggests that the employed nozzles emit noise at lower frequency, hence higher wavelength, which ultimately yields more reliable applicability of the point source model (Section 2.3) as a result of the less remarkable directionality found with respect to the noise emitted by the nozzle used in [15]. Moreover, SPL appears higher than 100 dB over periods of time when frequency is greater than 4 kHz.