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.

This paper provides an overview of the design and performance of the new aeroacoustic wind tunnel facility at the University of Bristol. The purpose of the facility is to enable near-and far-field acoustic and aerodynamic studies on a variety of different aerodynamic components and to examine diverse noise control techniques. The facility comprises a large acoustic chamber, anechoic down to 160 Hz, and a temperature controlled closed-circuit wind tunnel with an open test section. The wind tunnel features two interchangeable rectangular nozzles with a partially shared contraction. Both nozzles are shown to possess a high flow quality with high flow uniformity and low turbulence intensity of 0.09% and 0.12% for the smaller and larger nozzle, respectively. The maximum attainable flow speeds are 40 m/s for the larger nozzle and 120 m/s for the smaller nozzle corresponding to Reynolds numbers of 2.7 million and 8.1 million per meter, respectively. In this paper, we will present various aerodynamic and acoustic results to characterize the performance of the facility. The background noise levels are found to be sufficiently low and the far-field noise measurements from a flat plate, a round cylinder and a NACA 0012 airfoil compare favorably to existing experimental observations.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. The possibility of trailing edge noise reduction using active flow control is addressed in this paper. The boundary layer is altered with the help of uniform perpendicular and inclined flow injection and suction on a long flat plate upstream of a sharp trailing edge. The flow quantities have been measured using particle image velocimetry, hot-wire anemometry, and the surface pressure fluctuations using flush mounted miniature pressure transducers. It was shown that the proposed flow control methods are capable of reducing the noise over a wide range of frequencies. It was also revealed that the perpendicular blowing is effective in shifting the energy containing turbulent structures away from the wall, resulting in a significant reduction in the surface pressure spectra. The inclined blowing control method was capable of further reducing the surface pressure spectra, while maintaining low input power requirement. The perpendicular suction control was shown to be effective in the reduction of pressure fluctuations in the mid frequency regions, which quickly penetrates to the lower frequency downstream. The inclined suction control has also lead to significant reductions in the surface pressure fluctuations over a wide range of frequencies, while the required suction rate was kept considerably low. It has also been shown that the proposed active flow control methods can reduce the spanwise correlation length of the turbulent structures near the trailing edge. In general, results have shown that all proposed active flow control techniques can effectively alter the boundary layer structure and reduce trailing edge noise. Nomenclature

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms The use of surface treatments as a passive flow control method for trailing edge noise reduction is considered in this paper. In order to investigate the effects of different types of surface treatments on surface pressure fluctuations, eddy convection velocity and spanwise length scale, a long flat-plate model, equipped with several streamwise and spanwise surface pressure microphones has been designed and built. Measurements have been carried out for a variety of 2D and 3D surface treatments. The flow behavior downstream of the surface treatments is also studied by employing a single probe hotwire anemometer. Results indicated that the use of finlets with coarse spacing leads to a favorable reduction in PSD at mid to high frequencies and an undesirable increase in spanwise length scale. In the case of finlets with fine spacing, the high frequency pressure fluctuations have been effectively suppressed and the spanwise length-scale has been reduced, but with a penalty of low to mid frequency surface pressure elevation. Furthermore, it is found that the ratio of the finlets spacing to the boundary layer thickness is a critical parameter for achieving maximum surface pressure PSD reduction and the finlets spacing should be in the order of the boundary layer inner region. It has also been shown that the proposed novel 3D surface treatments have better aeroacoustic performance than the standard 2D ones in terms of the reduction in the surface pressure power spectral density, the longitudinal and lateral coherence and eddy convection velocity.

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