Abstract
Research into aerodynamic noise is a fundamental prerequisite for the development of sustainable transport and energy utilisation systems. It arises as the consequence of airflow, in contrast to noise caused by the vibration of solid bodies, and it is a concern to attenuate aerodynamic noise for the benefit of the living environment. The experimental work presented in this thesis is concerned with the reduction of aerodynamic noise produced by the interaction between convected turbulence arising from bodies immersed in subsonic flow and a sharp trailing edge. This is referred to as trailing-edge noise. With the purpose of manipulating the boundary-layer characteristics to reduce the interaction of convected turbulence with the sharp trailing edge and thus trailing-edge noise, surface treatments called finlets were applied on a flat plate and an aerofoil. As a part of this research project, a flat plate rig was designed and manufactured, whereas an existing NACA 0012 aerofoil was used to investigate the effects of finlet treatments. Furthermore, numerous conventional finlet treatments as well as newly conceived surface treatments for application on the aerofoil and the flat plate were produced using rapid prototyping. Performing experiments, extensive data for the treated aerofoil and flat plate were gathered in an aeroacoustic wind-tunnel facility and subsequently used to investigate the effects of the treatments on trailing-edge noise reduction.Building upon the findings of previous works, the optimal parameter ranges for the conventional finlets are identified for the considered configurations using measurements of the far-field sound pressure level. The access to rapid prototyping techniques allowed for the consideration of a wide range of parameters for testing the optimum application position, finlet height, length, spacing between the finlet-wall structures and finlet-profile shape. In particular, relating the boundary-layer velocity profiles to the far-field sound pressure level, it is found that the optimum maximum finlet height depends on the boundary-layer thickness and thus, for aerofoils, on the effective angle of attack.
Previous studies have proven the capability of conventional finlets to reduce trailing-edge noise on aerofoils and flat plates. However, the underlying physical mechanisms are still far from being completely understood. The distinct explanations of the functional principles of conventional finlets found in the literature suggest that their noise-reduction mechanisms differ fundamentally depending on whether they are applied on an aerofoil or a flat plate. Through comparison of the static and unsteady surface-pressure fields as well as the velocity fields on the treated aerofoil and flat plate, this thesis aims to shed light on the commonalities and differences between the effects of finlets applied on an aerofoil and the flat plate. Experimental results for the NACA 0012 aerofoil are compared to a simulation performed by project partners at the Technische Universität Braunschweig. To further deepen the understanding of the altered boundary-layer characteristics induced by the surface treatments, the development of turbulence structures from upstream of the treated area to the trailing edge is traced by means of simultaneous measurements of the unsteady surface-pressure fluctuations and the boundary-layer velocity fields. The pressure-velocity cross-correlations derived from these measurements facilitate the tracking of coherent structures forming at the tapered leading edges of the finlet walls. The coherent structures are observed to further develop along the finlet ridges on top of the treatments. Eventually, a shedding of coherent structures at the rear edges of the finlets is identified, which are convected downstream towards the trailing edge in a certain distance from the surface of the aerofoil and the flat plate. The reduction of trailing-edge noise is found to be likely connected to friction effects along the increased surface area added to the system through the finlet walls, and the interaction of the flow channelled through the finlets with the coherent structures shed at the rear edges of the finlets.
Surface treatments differing from the conventional finlets were designed and tested on the flat plate. For this, certain features related to the capability of conventional finlets to reduce the unsteady surface-pressure fluctuation power spectral density at the trailing edge were extracted. Although the anticipated reduction of the unsteady surface-pressure fluctuation power spectral density due to the application of non-conventional finlets is observed, they show minor to no capabilities to reduce the overall noise emission. This can in large parts be explained with treatment self-noise, which is emitted to the far-field directly from the walls of the finlets. The findings are used to suggest treatment features relevant for effective trailing-edge noise reduction with minimal drawbacks from treatment self-noise. Providing insights into both practical application of finlet surface treatments and the theory of noise-reduction mechanisms induced by the finlets, the outcomes of this work should be of interest to the aerospace engineering as well as the fluid dynamics research community.
| Date of Award | 6 Dec 2022 |
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| Original language | English |
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| Supervisor | B. Zang (Supervisor) & Mahdi Azarpeyvand (Supervisor) |