# Combined CFD-Stochastic Analysis of an Active Fluid Injection System for Jet Noise Reduction
## 3 Active Fluidic Injection System  
In this section, an experimental technology for reducing the jet noise based on a fluidic injection is assessed by means of the SNGR approach and applied to the jet analyzed in the Section 2.2. Several experimental studies, conducted over recent decades, have shown that the fluid injection technique effectively allows reduction of the noise from turbulence [22; 23; 24]. Therefore, the technological  
Figure 11: (**a**) Power spectra of far-field pressure signal for a few observer locations on the outer arc, 30\(D_{j}\). The PSD spectra have been staggered by multiplying the amplitude by a factor \(10^{2n}\), where \(n=\left(\frac{\theta-20}{40}\right)\) and \(\theta\) being the angle from the jet axis; (**b**) Overall Sound Pressure Level Directivity. Figure 12: Sound radiation from the jet. Real part of the acoustic pressure [Pa] at 700 Hz. solution proposed here consists of injecting a small jet of secondary gas into the main flow, which comes out from the nozzle of the jet engine exhausts (Figure 13). This secondary jet is derived from gas bleeding from the turbine located immediately upstream of the nozzle. The air extractions from the turbine and subsequent injections in the main stream of the exhaust gas at nozzle outlet favor a greater mixing in order to break the large turbulent structures at low frequencies, which are the main responsible for most of the radiated noise (Figure 14). Figure 15 shows the difference in the view of the exhaust plume emitted from a jet engine between the cases with and without fluid injection. Figure 14: Sketch of the low frequency turbulence region. Figure 13: Sketch of the active Fluidic Injection System. Figure 15: Sketch of the exhaust plumes emitted from a jet engine without (on the top) and with fluid injection (on the bottom). In this work, four experimental tests were carried out to evaluate the influence of the bled mass flow rate, the area of the injection section and the jet angle over the noise reduction. Two injection mass flow rate values are considered and corresponding about 5% and 10% of the total mass flow rate of the turbine. A maximum rate of about 10% has been used in order to preserve the nominal turbine power, whereas, pressure and temperature of the drained mass flow rate have been computed, assuming to be greater than the ambient conditions and according to the thermodynamic expansion. Two injection sections with a total area of around 4% and 10% of the nozzle outlet area have been considered and modelled in the computational domain and corresponding respectively to the 5% and 10% of the turbine mass flow rate. Finally, the spray conditions have been evaluated at three different injections angles equal to 45, 60 and 80 degrees. The following test matrix (Table 1) has been, thus, assembled for the RANS + SNGR analyses. Examples of the different turbulence levels obtained by varying the injection patterns are displayed in Figure 16.  
About the aerodynamic field results, Figure 17a shows the centerline profiles of the axial velocity obtained from four tests with injection. Increasing the jet angle, the predicted laminar core length decreases compared to the baseline case. In particular, focusing on the results of 3 rd and 4 th tests, the laminar core length of 4 th test is greater than the 3 rd one, because at the same jet angle, \(\gamma=80^{\circ}\), the injected jet has a higher speed, due to the lower injection section area. At the same time, increasing the jet angle, the intensity of turbulence profiles moves toward the nozzle exit, because the mixing is locally increasing (Figure 17b).