Abstract
The interaction between a turbulent flow and aerofoil can generate significant levels of noise. This aeroacoustic phenomenon affects health, city planning and the environment, as it is found in many engineering applications, such as guide vanes in aircraft engines or wind turbine farms. One strategy to reduce turbulence interaction noise is the use of porous materials on the aerofoil body. In this experimental investigation, the reduction of aerofoil turbulence interaction noise with 3D printed, deterministic porous structures is studied. A NACA0012 profile is used for investigation which features an interchangeable leading edge occupying the first 10% of the aerofoil chord and is immersed in homogeneous, grid-generated turbulence in the aeroacoustic wind tunnel at the University of Bristol. In addition, a multitude of enabling activities are carried out including the design and manufacture of the porous structures; manufacture and instrumentation of the NACA0012 aerofoil; characterization of the deterministic porous structures; a study into quiet grid-generated turbulence for an aeroacoustic facility, and a thorough investigation into the aerofoil turbulence interaction noise mechanism.This study uses triply-periodic minimal surface (TPMS) based porous structures, as the porosity and pore size can be dictated mathematically. Literature defines the permeability of the porous material as a critical parameter in the manipulation of the flow, causing a reduction in the turbulence interaction noise mechanism. Therefore, both the permeability and acoustic absorption characteristics of a large matrix of 3D printed TPMS samples with varying structure, porosity and pore size were tested. The permeability of the TPMS structures demonstrate a cubic relationship with porosity, which is only broken with significant variation in pore size. Secondly, there is a plateau in the permeability-porosity relationship for TPMS for a porosity range of 40%<φ<60%, and is thus used as a benchmark range for the study of aerofoil turbulence interaction noise reduction. Furthermore, it is found that absorption of acoustic waves due to the porous structure is unlikely to contribute to any flow induced noise reduction as no acoustic absorption is evident in the low frequency region where turbulence interaction noise is dominant.
Rigorous study of aerofoil turbulence interaction noise requires a homogeneous turbulent flow that is of the same noise level as the smooth flow in the aeroacoustic facility. In pursuance of quiet turbulence generation in the aeroacoustic wind tunnel, a multitude of passive turbulence grids were characterized for their self-noise and turbulent flow properties. A large contraction ratio between the nozzle dimensions and the grid location is required to ensure a low velocity at the grid to reduce self-noise to a minimum. Variation in the grid geometry allows for control of the turbulence intensity and turbulent length scale without affecting the background noise. However, a large contraction ratio between the grid location and the nozzle results in additional straining of the turbulent structures as they are formed, resulting in an anisotropic homogeneous flow measured with a cross-wire probe. Furthermore, it is found that the power spectral density of the velocity fluctuation measurements made by a single-wire probe for the anisotropic flow demonstrates a good agreement with the Von Kármán spectrum for isotropic turbulence. It is recommended care should be when measuring a turbulent flow with a single-wire as a good fit with the Von Kármán spectrum does not necessitate the presence of isotropic turbulence.
In an effort to understand the physics behind the reduction in the aerofoil turbulence interaction noise mechanism, a thorough investigation of aerofoil turbulence interaction noise with a solid aerofoil is conducted. Comprehensive measurement of the flow field reveals previously unreported observations of turbulence distortion around the leading edge. It is previously reported that a turbulent flow with larger length scales generates elevated levels of turbulence interaction noise. However, new observations from this study shed light on the reason: for a flow approaching the leading edge, smaller coherent turbulent structures interact with the aerofoil leading edge and are destroyed, whereas larger three-dimensional structures are distorted into more two-dimensional like structures warped by the leading edge. Furthermore, through coherence analysis it is demonstrated that the stagnation point has no direct connection to the far-field noise. Near-field to far-field coherence analysis reveals the scale of the turbulent structures impacts the amount of aerofoil chord
that radiates noise to the far-field, with the exclusion of the stagnation point. It is also observed that the coherence between near-field and far-field peaks at the flattest part of the aerofoil. Further insight from velocity-pressure coherence elucidates the complex chain of events from the upstream flow, to turbulent structures interacting with the aerofoil and the subsequent radiated far-field noise.
The use of deterministic TPMS porous structures on the first 10% of the aerofoil chord are shown to reduce aerofoil turbulence interaction noise. The NACA0012 aerofoil is immersed in the homogeneous grid-generated turbulence, and to understand how the properties of porous structure affect the aerofoil turbulence interaction noise mechanism two separate studies are conducted. The first varies the porosity between 40%<φ<60% for the same Schwarz-P TPMS structure type, in turn varying the permeability of the leading edge. The second uses a constant porosity of φ=50% and varies the structure type between the Schwarz-P, gyroid and diamond TPMS, ultimately affecting the pore size and pore orientation whilst having more comparable permeability values. The initial starting porosity of φ=50% demonstrates significant noise reduction which peaks at 7 dB for low frequency. The variation in the porosity between 40%<φ<60% significantly impacts noise reduction performance, where a change in the porosity from φ=50% to φ=40% yields less than 1dB of noise reduction at low frequency compared with the results of the solid leading edge. An increase in porosity from φ=50% to φ=60% does not yield a significant improvement in noise reduction. Increasing porosity promotes flow penetration in the porous leading edge, yet also increases velocity shearing close to the wall downstream of the porous section. Two-point coherence analysis of the spanwise velocity fluctuations approaching the leading edge demonstrate that the porous leading edge causes the turbulent structures to retain the three-dimension structure up to the point of penetration and are not subject to the turbulence distortion caused by the solid leading edge. Keeping porosity constant and changing the porous structure shows little variation to the far-field noise despite a significant change to the flow behaviour in the stagnation region. Previous studies stress the importance of the flow along the stagnation streamline and a reduction in the velocity fluctuation energy as the noise reduction mechanism for a single porous structure type. Evident in this study, where porous structure type is varied, is the drastically different behaviour of the velocity fluctuation along the stagnation streamline, with some leading edge structures demonstrating an increase in the velocity fluctuation energy whilst retaining a noise reduction. This study reveals that the most consistent change in the unsteady flow field is observed with a small spatial movement above the stagnation streamline around the aerofoil leading edge. Each porous structure demonstrates a similar reduction in the energy of the velocity fluctuation at this location in the same frequency range as the noise reduction, when compared with the solid leading edge case. The region around the leading edge of the aerofoil should be considered to assess the reduction in turbulence interaction noise rather than just the stagnation streamline. The research undertaken demonstrates that deterministic TPMS porous treatments can effectively reduce aerofoil turbulence interaction noise. Although this study considers homogeneous turbulence, the results demonstrate that a porous leading edge could be a concept for the reduction of any application of aerofoil-turbulence interaction noise. From the work carried out in this thesis and knowledge from the literature, recommendations are made for future studies in the conclusion for the use of deterministic TPMS porous treatments at the leading edge of an aerofoil for the reduction of turbulence interaction noise.
| Date of Award | 3 Oct 2023 |
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| Original language | English |
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| Supervisor | Mahdi Azarpeyvand (Supervisor) & Beckett Zhou (Supervisor) |