Vol 69, No 2 (2023)

Diffraction of a high-frequency small-number whispering gallery mode running along a concave boundary, which turns into a flat one so that its curvature experiences a jump, is studied. The cases of rigid (Neumann) and soft (Dirichlet) boundary conditions are considered. Within the framework of the parabolic equation method, a mathematically correct scattering problem is obtained which is solved explicitly and investigated asymptotically in detail. Analytic expressions are found for all emerging wavefields. In particular, an edge wave diverging from the point of non-smoothness of the boundary is described. For the rigid condition, its amplitude is proportional to the magnitude of curvature jump, but not for the soft condition.

For the case of a realistic small-scale “double-stream nozzle–high-lift wing” configuration, a previously developed simplified semiempirical method for calculating the low-frequency noise of jet–wing interaction is tested. As input data for calculating the noise, we used the results of measuring the amplitude and convective velocity of perturbations in the near field of a turbulent jet near the trailing edge of an extended flap. The measurements were performed with a pair of pressure sensors flush-mounted on the lower surface of the wing model. It is demonstrated that the results of the calculated estimate for the spectral characteristics and directivity of the interaction noise agree well with direct noise measurements in the far field both under static conditions and in the presence of coflow simulating the effect of flight.

The noise of a modern regional aircraft was measured using the beamforming method during flight tests in the landing approach configuration with extended landing gear and deflected high-lift devices (slats and flaps). The localization and ranking of aircraft noise sources for full-scale Reynolds numbers has been carried out. Comparison of the flight test results with the results of acoustic measurements of airframe noise for a large-scale wing model obtained earlier at DNW-NWB showed good agreement between noise measurements of the large-scale model, scaled to full size, with the results for the airframe noise of an aircraft wing in flight tests.

A method has been developed for tuning the microphone array for optimal visualization of acoustic dipoles. Acoustic dipoles were generated under experimental conditions in an anechoic chamber using a subsonic air flow around a thin metal rod and thin rectangular plate. To confirm the operability of this approach, we compared the visualization maps obtained using the approach with the results obtained with a similar array class tuned in the traditional monopole method. The developed approach, which adapts the array to the dipole radiation model and modifies the transfer functions that take into account the influence of the incidence pattern of sound waves, demonstrates an increase in the array’s dynamic range with optimal microphone placement.

A method has been developed for determining the modal composition of correlated sound modes propagating in a cylindrical duct, the peculiarity of which is that the effect of sound reflection from the open end of the duct is taken into account by numerical or analytical calculation of the reflection coefficients. This method was validated in the TsAGI AC-2 anechoic chamber, in which the sound field was created in the duct of a small-scale air intake model using 12 speakers at fixed frequencies; the modal composition was determined using 48 microphones installed flush with the duct walls, and synchronous measurements of the directivity of sound emission from the open end of the duct in the far field were carried out. In this case, on based on the found amplitudes of sound modes in the duct, the radiation characteristics in the far field were calculated using an analytical solution for sound emission from a semi-infinite cylindrical duct, the results of which were compared with the measurement results. It is shown that the amplitudes of sound modes found according to the developed method make it possible to reconstruct the far sound field characteristics with high accuracy in cases where the number of generated propagating modes in the duct does not exceed the number of installed microphones in the cylindrical array.

The importance of jet noise for prospective supersonic transport (SST) in takeoff and climb modes dictates the need to develop methods for reducing jet noise and their subsequent validation in large- and full-scale tests. This paper presents the results of Russia’s first ground acoustic tests of a rectangular nozzle with a noise suppression system, installed on a subsonic aircraft engine to demonstrate the technology. The authors studied a rectangular beveled nozzle with an ejector and screens—elements of the SST airframe. The test results were compared for three nozzle geometries: a round nozzle corresponding to measurements for the original engine without modifications, a rectangular nozzle without screens, and a rectangular nozzle with screens. It was found that when the rectangular nozzle without screens is used, there two opposite effects occur: noise amplification compared to the round nozzle for angles of observation close to the jet axis, and noise attenuation at angles of observation close to the fan rotation plane. Installing screens increases noise up to 5 dB in the low-frequency region below 150 Hz and reduces noise by 2 dB for frequencies higher than 250 Hz. Estimation of community noise in the EPNL metric showed that this rectangular nozzle with the noise suppression system has the effect of reducing community noise in comparison with the baseline round nozzle.

The article presents the results of studying the fatigue characteristics of skin–stringer joints (T‑samples) made of a polymer composite material subjected to random vibroacoustic loading with a wide spectrum. Experimental data were analyzed to determine the damage initiation and growth mechanisms, as well as the criterion and failure mode for samples with and without impact damages. The effect of impact damage on the fatigue characteristics of the test samples at different impact energies and vibroacoustic loading levels is demonstrated. The main patterns of the influence of defects on the fatigue characteristics are revealed, which can be used in the future in creating computational models for damage development in structures made of the polymer composite material under vibroacoustic loading.