The research on helicopter rotor aerodynamic noise becomes imperative with the wide use of helicopters in civilian fields. In this study, a signal enhancement method based on cyclic Wiener filtering was proposed given the cyclostationarity of rotor aerodynamic noise. The noise was adaptively filtered out by performing a group of frequency shifts on the input signal. According to the characteristics of rotor aerodynamic noise, a detection function was constructed to realize the long-distance detection of helicopters. The flight data of the Robinson R44 helicopter was obtained through the field flight experiment and employed as the research object for analysis. The detection range of the Robinson R44 helicopter after cyclic Wiener filtering was increased from 4.114 km to 17.75 km, verifying the feasibility and effectiveness of the proposed method. The efficacy of the proposed detection method was demonstrated and compared in the far-field flight test measurements of the Robinson R44 helicopter.
Dielectric barrier discharge (DBD) plasma actuators, capable of generating quasi-steady wall jets, are well suited for flow control on various problems. Compared with the detail related to the induced velocity field, there are limited results available for the pressure field created by the plasma actuator. However, the profound merits of understanding the evolution of the pressure field are to reveal the controlling mechanism. Here, the time-averaged and the phase-averaged pressure field distributions are obtained by using a pressure reconstruction method based on the velocity field from PIV experiments. According to the discharge regimes, the formation mechanism of the pressure field is discussed. During the streamer discharge stage, the pressure close to the upper electrode is decreased under the influence of the induced heating caused by the high-frequency and high amplitude pulsed current, leading to the air above the plasma actuator being drawn towards the wall surface. During the glow discharge stage, under the effect of suction generated by the streamer discharge, the pressure near the wall is increased and the plasma actuator generates a favorable pressure gradient which provides advantageous conditions for the airflow acceleration. During the discharge quenching stage, the effect of the plasma actuator vanishes and the influence of viscous force is strengthened. Therefore, the adverse pressure gradient is gradually formed and the velocity of the wall jet is decreased compared to that of the glow discharge stage. The change of pressure field in a period can be summarized into three processes: pressurization, pressure release, and pressure recovery.
Rod–airfoil interaction noise is a major concern in several practical industrial and aeronautical applications. In this study, we constructed bio-inspired gradient distributed porous leading edges to reduce rod–airfoil interaction noise. Noise radiations by NACA 0012 airfoils with nonporous aluminum and porous leading edges were experimentally compared in an anechoic wind tunnel by changing the streamwise gap between the upstream rod and the downstream airfoil, as well as the angle of attack of the airfoil. The results of detailed acoustic tests showed that the proposed gradient distributed porous leading edges can significantly reduce noise radiation around and above the peak frequency of the baseline rod–airfoil interaction. Parametric studies on the piecewise porous characteristics showed that rod–airfoil interaction noise reduction is sensitive to the coverage percentage, position, and arrangement order of the porous materials. Porous leading edges with lower pores per inch, larger coverage, and gradually sparse distributed pores better reduced noise. Moreover, the position of the porous material affected the frequency band of noise reduction, and the noise reduction performance was better when it was located in the downstream strips of the porous leading edge.
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