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.

Trailing edge serrations are a widely used passive technique for the suppression of aerodynamic noise from wind turbines. Despite their popularity, no reliable engineering prediction tool has yet been developed to estimate the noise reduction for different serrations. This paper concerns the development of an engineering noise prediction tool, based on a recently developed mathematical model. Results show that the new model has several advantages over Howe's model, as it can take both destructive and constructive sound interference effects into account. Two surface pressure wavenumber-frequency models are implemented, namely Chase and TNO models, to demonstrate the sensitivity of the model to boundary layer characteristics. The boundary layer parameters needed in the wavenumber-frequency models are obtained using RANS CFD simulations. Far-field noise comparisons are provided between the proposed prediction tool and experimental data for a NACA0018 airfoil. A parametric study regarding the boundary layer changes of serrated airfoils signifies the need for more reliable wavenumber-frequency models. The results presented in the paper show that the proposed engineering tool can provide a fairly accurate estimate of the noise reduction performance of serrated airfoils, but its accuracy relies heavily on the availability of reliable near-field boundary layer information.

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The present study describes an experimental investigation of the aerodynamic and aeroacoustic characteristics of a sinusoidally oscillated NACA 0012 airfoil. The experiments were conducted in an aeroacoustic wind tunnel with a uniquely designed Kevlar-walled test section. Prior to experiments, these Kevlar walls were calibrated carefully and shown to provide reliable and accurate aerodynamic and aeroacoustic measurements. Two different regimes of interest, namely the pre-and post-stall angle of attack regimes, have been examined for lift curve polars, far-field noise spectra and unsteady surface pressure spectra. Interestingly, when the lift curve polar hysteresis is small at pre-stall angles of attack, the unsteady surface pressure spectra of the oscillating airfoil can be predicted with satisfactory accuracy using a position-based weighted averaging approach from its static counterparts. On the other hand, such a method becomes invalid at post-stall angles due to the presence of a significant dynamic stall hysteresis. Instead, an increase in the mean surface pressure and far-field noise spectra is observed at dynamic stall conditions. Furthermore, a short-time Fourier transform analysis reveals that the increase of the surface pressure spectra is a direct result of the periodic production and convection of dynamic stall vortices.

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