A new analytical model is developed for the prediction of noise from serrated trailing edges. The model generalizes Amiet’s trailing-edge noise theory to sawtooth trailing edges, resulting in a complicated partial differential equation. The equation is then solved by means of a Fourier expansion technique combined with an iterative procedure. The solution is validated through comparison with the finite element method for a variety of serrations at different Mach numbers. The results obtained using the new model predict noise reduction of up to 10 dB at 90 $^{\circ }$ above the trailing edge, which is more realistic than predictions based on Howe’s model and also more consistent with experimental observations. A thorough analytical and numerical analysis of the physical mechanism is carried out and suggests that the noise reduction due to serration originates primarily from interference effects near the trailing edge. A closer inspection of the proposed mathematical model has led to the development of two criteria for the effectiveness of the trailing-edge serrations, consistent but more general than those proposed by Howe. While experimental investigations often focus on noise reduction at 90 $^{\circ }$ above the trailing edge, the new analytical model shows that the destructive interference scattering effects due to the serrations cause significant noise reduction at large polar angles, near the leading edge. It has also been observed that serrations can significantly change the directivity characteristics of the aerofoil at high frequencies and even lead to noise increase at high Mach numbers.

This paper is concerned with the application of porous treatments as a means of flow and aerodynamic noise reduction. An extensive experimental investigation is undertaken to study the effects of flow interaction with porous media, in particular in the context of the manipulation of flow over blunt trailing edges and attenuation of vortex shedding. Comprehensive boundary layer and wake measurements have been carried out for a long flat plate with solid and porous blunt trailing edges. Unsteady velocity and surface pressure measurements have also been performed to gain an in-depth understanding of the changes to the energy–frequency content and coherence of the boundary layer and wake structures as a result of the flow interaction with a porous treatment. Results have shown that permeable treatments can effectively delay the vortex shedding and stabilize the flow over the blunt edge via mechanisms involving flow penetration into the porous medium and discharge into the near-wake region. It has also been shown that the porous treatment can effectively destroy the spanwise coherence of the boundary layer structures and suppress the velocity and pressure coherence, particularly at the vortex shedding frequency. The flow–porous scrubbing and its effects on the near-wall and large coherent structures have also been studied. The emergence of a quasi-periodic recirculating flow field inside highly permeable surface treatments has also been investigated. Finally, the paper has identified several important mechanisms concerning the application of porous treatments for aerodynamic and aeroacoustic purposes, which can help more effective and tailored designs for specific applications.

An analytical model is developed for the prediction of noise radiated by an aerofoil with leading-edge serration in a subsonic turbulent stream. The model makes use of Fourier expansion and Schwarzschild techniques in order to solve a set of coupled differential equations iteratively and express the far-field sound power spectral density in terms of the statistics of incoming turbulent upwash velocity. The model has shown that the primary noise-reduction mechanism is due to the destructive interference of the scattered pressure induced by the leading-edge serrations. It has also shown that in order to achieve significant sound reduction, the serration must satisfy two geometrical criteria related to the serration sharpness and hydrodynamic properties of the turbulence. A parametric study has been carried out and it is shown that serrations can reduce the overall sound pressure level at most radiation angles, particularly at small aft angles. The sound directivity results have also shown that the use of leading-edge serration does not significantly change the dipolar pattern of the far-field noise at low frequencies, but it changes the cardioid directivity pattern associated with radiation from straight-edge scattering at high frequencies to a tilted dipolar pattern.

This paper presents a comprehensive experimental study on the unsteady pressure exerted on the surface of a round cylinder in smooth and turbulent flows. A highly instrumented cylinder with several static pressure taps and dynamic pressure transducers at different spanwise and peripheral locations was used, enabling extensive dynamic surface pressure, coherence, and turbulence length-scale analysis. The effects of the free-stream turbulence and turbulent length scale are investigated by placing the turbulent-generating grids within the wind tunnel duct. For both the laminar and turbulent incident flows, the surface pressure results show the emergence of the fundamental, first and second harmonics at most peripheral angles, while at the cylinder base, the surface pressure spectra are dominated by the first harmonic. It has also been observed that an increase in the level of the turbulence intensity results in an increase in the energy level of unsteady pressure acting on the cylinder. An increase in the length scale of the incoming flow structures is shown to result in an increase in the energy level of the tonal frequencies and the broadband content of the surface pressure spectra. The spanwise coherence results have also shown that an increase in the length scale of the flow structures increases the spanwise correlation length of the flow structures at the vortex shedding frequency at the stagnation point, while at the cylinder base, the spanwise correlation length decreases at the vortex shedding frequency.

The interaction of a zero pressure gradient turbulent boundary layer flow with a rough permeable surface has been investigated experimentally. The flow interaction characteristics have been examined using a long flat plate equipped with several surface pressure transducers and pressure taps. Three types of porous materials with different porosities and permeability constants were used in these investigations. To reveal the behavior of turbulent flows over porous surfaces, measurements were performed for the boundary layer growth, energy content of the turbulent structure within the boundary layer, and surface pressure fluctuations, before, over, and after the porous test-section. The interaction of the flow with the porous substrate was found to significantly alter the energy cascade within the boundary layer. Results have also shown that the boundary layer interaction with the rough porous surfaces leads to an increase in the pressure fluctuations exerted on the wall, particularly at low frequencies. The near-field investigations have shown that the penetration of the boundary layer flow into the porous medium can generate an internal hydrodynamic field within the porous medium. This, in turn, reduces the frequency-energy content of the large boundary layer coherent structures and their spanwise correlation length. This study paves the way for further investigation into the interaction of the porous media with different flow fields and development of tailored porous treatments for improving the aerodynamic and aeroacoustic performance of different aero- and hydro-components.

The use of streamwise finlets as a passive flow and aerodynamic noise-control technique is considered in this paper. A comprehensive experimental investigation is undertaken using a long flat plate, and results are presented for the boundary layer and surface pressure measurements for a variety of surface treatments. The pressure–velocity coherence results are also presented to gain a better understanding of the effects of the finlets on the boundary layer structures. The results show that the flow behaviour downstream of the finlets is strongly dependent on the finlet spacing. The use of finlets with coarse spacing leads to a reduction in pressure spectrum at mid- to high frequencies and an increase in spanwise length scale in the trailing-edge region due to flow channelling effects. For the finely distributed finlets, the flow is observed to behave similarly to that of a permeable backward-facing step, with significant suppression of the high-frequency pressure fluctuations but an elevation at low frequencies. Furthermore, the convection velocity is observed to reduce downstream of all finlet treatments. The trailing-edge surface pressure spectrum results have shown that, in order to obtain maximum unsteady pressure reduction, the finlet spacing should be of the order of the thickness of the inner layer of the boundary layer. A thorough study is provided for understanding of the underlying physics of both categories of finlets and their implications for controlling the flow and noise generation mechanism near the trailing edge.