## 4 Data Quality Checks and Experimental Corrections
### Shear Layer Correction  
If a sound source is placed in the wind tunnel core flow, and the measuring microphones are placed outside it, the sound waves have to pass through the shear-layer separating the open jet from the ambient air. This leads to a change in propagation direction (apparent radiation angle) and amplitude of the sound waves. In addition, the sound waves are convected downstream while propagating in the flow. To account for these effects, the data must be corrected. In this study, a widely used scheme from [PERSON] [23] to correct for shear-layer refraction is applied to the measurement data. The scheme allows for correction for both planar and circular shear layer, which correspond to the FL-17 CARDC and AC-2 TsAGI facilities, respectively. The shear-layer corrections depend on the measurement geometry (the incidence angle at the shear layer, in particular) and on flow velocity, but not on sound frequency. The geometry and notations of the shear layer correction are shown in Figure 11. Two-dimensional geometry (Figure 11a) corresponds to either the case of a cylindrical shear layer with the source located at the centerline or the case of a plane shear layer with the source and the microphone located in the x-z plane. The case of a plane shear layer with the source and the observer not in this plane corresponds to three-dimensional geometry (Figure 11b). Figure 11: Geometrical parameters for the shear layer correction: (**a**) 2D case, (**b**) 3D case (adopted from [24]). Figure 10: The parts of noise spectra from Figure 9 used for comparison of small-scale and full-scale data: (**a**) full-scale model in FL-17; (**b**) small-scale model in AC-2 TsAGI. The azimuthal and polar angles described in Section 2 are measured from the junction point of the landing gear with the support system. However, localization of the noise sources with the beamforming method [24] and numerical simulation [25] suggest that the major noise sources of the landing gears comprise wheels, torque link, and strut. Therefore, the upper point of the main leg will be considered as the position of the source, which should better correspond to the physics of the problem, although it has an obvious drawback that the polar angle of the microphones will be different for landing gears with different main leg heights. For instance, for the landing gear model with a high main leg in FL-17 CARDC polar angle \(\varphi=20^{\circ}\) becomes \(\varphi_{\text{M}}=-1.9^{\circ}\), while polar angle \(\varphi=40^{\circ}\) becomes \(\varphi_{\text{M}}=24.8^{\circ}\). A comparison of the results for radiation angles (\(\theta_{0}\), \(\varphi_{0}\)) obtained for the plane shear layer in FL-17 CARDC with 2D and 3D approaches is provided in Table 1. The polar angle \(\varphi_{0}\) for the 2D correction procedure is \(0^{\circ}\) by definition. It can be seen that both methods provide close values of azimuthal angles \(\theta_{0}\), and for \(\varphi=20^{\circ}\) the values of polar angles \(\varphi_{0}\) are also quite similar. Taking into account the uncertainties associated with the position of the noise source, it has been concluded that the 2D correction procedure can be used for shear layer refraction in FL-17 CARDC. Similarly, a comparison of the results obtained with the 2D correction procedure for radiation angle \(\theta_{0}\) in FL-17 (plane shear layer) and AC-2 (cylindrical shear layer) is provided in Table 2. It can be seen that although geometrical positions of the microphones in AC-2 TsAGI and FL-17 CARDC were chosen as close to each other as possible, the shear layer correction introduces some differences in radiation angles between the two facilities, which will be accounted for in the further analysis. In addition to the angle correction, the shear layer correction procedure also introduces amplitude corrections, which differ between the plane and cylindrical shear layer [23].