For a jet located near the wing, instability waves present in jet mixing layer scatter on the wing trailing edge, leading to additional noise generation. In this article, the possibility of reducing ...this noise by active cancellation of natural instability waves has been experimentally studied. The active control system consists of near-field microphones from which the feedback signal is taken, a data processing module for signal filtering, and a high-frequency dielectric barrier discharge plasma actuator. Decrease in pressure perturbations in the near field of a turbulent jet associated with axisymmetric instability waves in a narrow frequency band was demonstrated. It was shown that this reduction leads to a corresponding noise decrease in the far field.
Abstract
A study of the instantaneous and average velocity and vorticity fields in the flow created by the model helicopter rotor in the hover mode was carried out. The velocity fields of the flow ...generated by the model rotor were obtained by a two-dimensional TR PIV system, which provided two components of the velocity vector in the diagnostic light plane. The processing of the obtained raw images was carried out using a two-frame algorithm with adaptive interrogation windows. The experiments carried out have shown the possibility of using the PIV technique to visualize the tip vortex structure descending from the rotor blade. This possibility seems to be especially interesting as one of the means of validation of the numerical methods for calculating rotor aerodynamics and acoustics.
The possibility of controlling instability waves in the mixing layer of a subsonic unexcited jet is studied. These waves can be noise sources in both free jets and jets as parts of configurations. In ...the study the method of experimental diagnostics of the instability waves in the near field of a jet using an azimuthal multimicrophone array is realized. The data on the near field fluctuations are used for testing the control strategy proposed by the authors. The strategy consists in narrowband sliding filtration of the original signal and the formation of a narrowband controlling action on the basis of the linear principle of signal superposition. The results of the study represent the next step toward the realization of an active control system suppressing natural instability waves in turbulent jets.
Instability waves developing in the mixing layer of a turbulent jet are in many cases sources of noise generation (high-speed jets, jets near an aircraft wing, etc.). This study experimentally ...investigates the possibility of active control of instability waves in an unexcited subsonic turbulent jet. The feedback control system consists of near-field microphones providing the reference signal, a plasma actuator based on a high-frequency dielectric barrier discharge, and a data processing module. It is shown that, depending on the system settings, pressure perturbations in the near field of a turbulent jet associated with axisymmetric instability waves can be weakened or enhanced in a predefined narrow frequency band.
The results of experiments on the interaction of an argon plasma jet with a liquid are presented. The effect of a coaxial barrier discharge on the gas-dynamic regime of a plasma jet of helium, freely ...blown into ambient air, and also colliding with a solid surface, is studied. Three orientation helium jets were studied - the jet is directed up, down and horizontally. Schlieren images of a jet were compared with visible images of a plasma jet. It is concluded that the main effect of the discharge on the gasdynamic regime of the jet is associated with gas heating in the discharge zone.
Ga
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is a wide-bandgap material with a number of unique characteristics that make it a promising material for photonics: it is optically transparent to optical and near-ultraviolet radiation and ...has a high breakdown voltage and high radiation resistance. One of the shortcomings that currently prevent the use of this material in solar-blind photodetectors is the anomalously long rise and decay time of photoconductivity, which can reach hundreds of seconds. Such a slowdown of the photoconductivity significantly limits the application area of these materials. The nature of this effect is studied. The rise and decay times of the photoinduced current in α-Ga
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Schottky diodes grown by the HVPE method on sapphire are measured under LED illumination at 259 and 530 nm. Under exposure to ultraviolet radiation, the current through the photosensitive structure of two opposing diodes increases in three stages: a very fast increase with a characteristic time of 70 ms, a slow increase with a characteristic time of 40 s, and a prolonged decay with a characteristic time of ~300 s. Upon subsequent illumination with green radiation, the increase in current with a characteristic time of 130 and 40 s is superimposed on a slow decrease in the amplitude of the maximal current with a characteristic time of ~1500 s. The analysis of the current relaxation shows the presence of deep centers with an energy of
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= 0.17 eV. A significant slowdown in the relaxation of the photoinduced current can be associated with potential fluctuations near the Schottky barrier.
In this paper, we present the results of an acoustic flight experiment on the localization and range of sources of aerodynamic noise of a full-scale aircraft using a 108-microphone antenna. Based on ...this research, unique data on the spatial position of the main noise sources of a full-scale aircraft and their frequency composition were obtained and the relative contribution of each of the sources to the total aircraft noise for various flight modes was analyzed comparatively. The results presented will be used in the development of low-noise configurations of domestic passenger aircraft to ensure competitive community noise levels that meet current and future ICAO standards.
The operation of a unipolar pulse-periodic corona discharge is considered. A new method is proposed for analyzing its properties by considering the structure of discharge-emitted sound. Decomposition ...of the acoustic field into azimuthal components makes it possible to relate them to the force or thermal influence of a corona discharge on a medium and evaluate their relative contribution in this case.
The effect of high-frequency dielectric barrier discharge plasma actuators on the noise of a flow around a circular cylinder is experimentally studied. It is shown that the plasma actuators are able ...to reduce the vortex noise of a cylinder within the range of velocities typical for aeroacoustic applications.
The possibility of acoustic control of instability waves formed in the mixing layer of a jet is experimentally investigated. The feasibility of suppressing a hydrodynamic instability wave in a ...subsonic turbulent jet by an external acoustic action is demonstrated. This result can be used in designing active control systems for jet noise suppression.
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