# Jet Noise in Airframe Integration and Shielding
### Shielding Effects  
A series of tests [27, 28] were conducted at NASA Glenn Research Center in order to study the propulsion-airframe integration under jet-surface interaction tests (JSITs). [PERSON] [29], and [PERSON] et al. [30] focused on the noise generation mechanism in subsonic jets, and investigated the effects of the surface length, distance from the nozzle, lip to the trailing edge, and beveled nozzle configurations. [PERSON] [31] tested rectangular jets of various aspect ratios (ARs) in the proximity of a flat surface. Sample spectra are shown in Figure 5 for a Mach 0.97 unheated rectangular jet (\(AR=8\)) parallel to the plate at a distance of \(h~{}=~{}0\), and plate length of \(X_{TE}~{}=~{}12.0-in\). Measurements are shown at a polar angle of \(90^{\circ}\), which represents the microphone location directly above or below the surface, as well as for the isolated jet. The corresponding non-dimensional frequencies are calculated and the Strouhal number, \(St=fD/U_{j}\), is shown under the figure. Here, the length scale, \(D\), is chosen as the corresponding hydraulic diameter, \(D=30.22\), and the isentropic jet exit velocity of \(U_{j}=308.98~{}m/s\). Moreover, in an effort towards integration of the airframe with the nozzle design, [PERSON] [31] investigated the far-field acoustic measurements of a family of high aspect ratio rectangular nozzles in the high subsonic flow regime with various designs. These experiments reported that having an extended lip on one broad side produced up to 3 dB more noise in all directions while extending the lip on the narrow side produced up to 2 dB more noise, primarily on the side with the extension. Adding a non-intrusive chevron made no significant change to the noise while inverting the chevron produced up to a 2 dB increase in the noise. Regarding the supersonic jet-surface interactions, [PERSON] et al. [32] carried out experimental and numerical studies on a 1.5 Mach jet at various distances from a flat surface and observed that both scrubbing and trailing edge noise detected in low frequencies increased, as the distance between flat plate and the jet is reduced. [PERSON] et al. [33] and [PERSON] et al. [34] provided flow field and acoustic data for a supersonic round jet with a design Mach number of 1.5, operating in the over-expanded, ideally expanded, and under-expanded supersonic flow regimes. The plate was placed at a radial distance, \(h\), normalized by the jet exit diameter, \(D=~{}50.8\) mm. They tested a range of distances between \(0.5\leq h/D\leq 5\). For noise testing, the surface was assembled using multiple pieces of 12.7-mm-thick aluminum to allow six surface lengths, \(x_{TE}\), between \(~{}2\leq x_{TE}/D\leq 15\). For the narrowband spectra shown in Figure 6, the flat plate with a surface length of \(x_{TE}/D=10\) is located at \(h/D=0.75\), and the jet is at the ideally expended operating condition with the design Mach number of \(M=1.5\). A reduction in broadband  
Figure 4: General Hybrid Wing Body (HWB) configuration (Ref. [24]). shock-associated noise was observed both in the shielded direction, 60\({}^{\circ}\) and 90\({}^{\circ}\) microphone angles, when the flat surface was long enough to cover the shock cells in the potential core. [PERSON] et al. [35] tested a supersonic rectangular nozzle of a 2:1 aspect ratio and 1.5 Mach number with and without the plate for various nozzle expansion conditions and documented a range of plate positions where crackle levels were significantly intensified. In their study, the plate could be positioned at different stand-off distances, starting where the plate touches the inner wall of the nozzle exit at \(h/D=0\) and could be moved away from the jet up to \(h/D=4\). Figure 7 illustrates the Schlieren images of the free jet and the shielded jets for these studies, where the effect of the shielding plate on the jet is apparent in Figure 7b. [PERSON] et al. [35] reported that \(h/D=1\) and 3 have similar overall sound pressure level (OASPL) compared to the no-plate configuration.