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.

Previous numerical studies hold inconsistent viewpoints of the porous coating on a circular cylinder reducing or increasing drag, and the present experimental study aims to clarify this argument. Additionally, a common viewpoint is that the porous coatings can suppress both the near-field wall pressure fluctuations and far-field acoustic pressure, but optimal parameters of the porous materials are still unclear. In this paper, the experimental studies reveal effects of three parameters, i.e., incoming flow velocity, pores per inch, and thickness of metal foams, on the pressure drag, wall pressure fluctuations, and associated noise level.

A static pressure probe is a crucial tool for measuring static pressure fluctuations, and its internal cavity structure can significantly affect the accuracy of the data obtained. This study investigates the impact of the static pressure probe cavity on the frequency response characteristics of fluctuating pressure using both experimental and numerical simulations. The results are validated by comparing them with the behavior of a second-order system. Our findings indicate that the internal cavity of the static pressure probe acts as a second-order underdamped system. This system amplifies fluctuating pressure signals at frequencies below the characteristic frequency while attenuating those above it. The frequency response characteristics of the probe’s cavity are similar to those of a Helmholtz resonator. Among various factors, the diameter of the pressure tap within the cavity has the most significant effect on the system’s characteristic frequency and amplification ratio. By optimizing the design of the static pressure probe’s cavity dimensions, the precision of fluctuating pressure data below the system’s characteristic frequency can be improved. In the research, based on the Helmholtz resonance equation, we provide a semi-empirical formula for predicting the characteristic frequency of fluctuating pressure in a static pressure probe. Furthermore, by leveraging the mechanism of the static pressure probe’s cavity as a second-order underdamped system, we propose a method for rapid and accurate calibration of the static pressure probe’s fluctuating pressure measurements.

The noise generated by the interaction between the wake of rods and airfoils is a universal phenomenon in the engineering field. The noise generation mechanism is mainly caused by the violent changes of the pulsating pressure on the airfoil surface, impacted by different vortex scales in the wake of rods. During this process, the energy is radiated outward by sound waves. To capture wakes and the sound generated by the interaction of rod-airfoil with relatively small computing resources is a challenge. In this paper, the sound pressure generated by the rod-airfoil interaction is calculated by solving the Reynolds average N-S equation and non-linear acoustic equations. This numerical method is verified by a rod-airfoil interaction benchmark. This paper focuses on the influence of offset distances between rods and airfoils. The results show that the peak of far-field sound decreases from 89 dB to 80.6 dB as the offset distance from the airfoil to the center of the cylinder increases (the maximum deviation is three times the cylinder diameter). The frequency of peak sound pressure also shifts.