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Title: Active flow control methods for aerodynamics and aeroacoustics : aerofoil trailing-edge noise applications
Author: Maciel Cesar, Luana
ISNI:       0000 0004 9359 4433
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2020
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The improvement of aerodynamic and aeroacoustic characteristics of aircrafts and wind turbines are of fundamental importance for both their performance and operation. The economic and operational viability of aviation and wind energy industry are closely related to the aerodynamic efficiency of their components (which translates directly into costs and profit) and also, to their compliance with the international noise regulations, which are becoming increasingly stringent as it affects detrimentally the population’s health near airports and wind farms. In the aviation industry, aerodynamically generated noise has gained attention from the early 1970’s when the loud turbojet powered civilian aircraft started to operate. From this time, jet noise annoyance has been extensively addressed and studied in the following decades, which resulted in significant progress in our understating of the noise generation mechanisms and development of technologies to reduce the noise. Consequently, other noise generation mechanisms contributing to the total noise emissions, such as the airframe noise, started to gain more attention. This study investigates the use of active flow control techniques to assess the effectiveness of such flow control methods on both the aerodynamic and the aeroacoustic performance of an aerofoil. We are interested in the use of such active flow control methods for reducing trailing-edge noise. While the previous studies have addressed the problem of trailing-edge noise using mostly passive flow control methods, the active flow control technique has received less attention even though it could provide better combined performance from both the aerodynamic and aeroacoustic perspectives. Earlier studies have shown that the application of flow suction and/or blowing could effectively change the turbulent flow structures which are responsible for lift augmentation, drag reduction, stall management and noise scattering. Even though the technique has shown promising outcomes, the literature lacks an in depth understanding of the underlying mechanisms, especially those corresponding to the turbulence statistics for a non-zero pressure-gradient structure. Therefore, the proposed study aims address this gap in the field of aerodynamics and aeroacoustics and provide some reliable and high-quality data and analysis to investigate the effectiveness of flow control techniques for reducing aerofoil self-noise at source. The effects of the different flow control techniques are evaluated through Large Eddy Simulations carried out at the University of Bristol High Performance Computing facility, Blue Crystal. Simulations are carried out for a NACA 0012 aerofoil in a subsonic regime (Mach number of M = 0.087) and chord-based Reynolds number of Rec = 4x105, immersed in a turbulent boundary layer, triggered by a tripping-device. The LES simulations for the clean aerofoil configuration, i.e. in the absence of flow control, was validated thoroughly against experimental data available in the literature for the NACA 0012 aerofoil at the angle of attack of α= 0o. A large number of high-quality LES simulations were then performed to study the effect of flow control upstream of the trailing-edge on the hydrodynamics of the boundary layer and the noise generation mechanisms. The numerical model and setup configurations were determined based on the available computational resources, the data available in the literature and the author mesh and domain size independency studies. Reynolds Averaged Navier-Stokes (RANS) simulations were carried out to initiate the Large Eddy Simulations (LES). The LES turbulence modelling was implemented using the dynamic sub-grid model formulation of Lilly (1992) [1] as closure. The Baseline results are presented and validated against the experiments of Garcia-Sagrado (2007) [2], for the NACA 0012 at the angle of attack of α= 0o. The results of the current LES have shown good agreement with the experimental data. The details of the simulations and the numerical model used for evaluating the changes on the flow structure imparted by the flow control treatments for the Baseline aerofoil, i.e. no flow control, are delineated and discussed in detail in Chapter 3. The effects of the uniform flow suction applied upstream of the trailing-edge on the boundary layer structures are discussed in Chapter 4. The pressure and velocity statistics show that the technique can considerably change the time and length scales of turbulence and the dynamics of both its large- and small-scale turbulent structures. The pressure frequency spectra results computed from the time-dependent pressure fluctuations show reduced power spectrum density magnitudes of the flow control treated cases at low to mid-high frequencies for locations downstream of the flow control device and near the trailing-edge. The increase of the boundary layer structures length- and time-scales are also observed through the spatial-temporal cross correlations analysis of the pressure field. Results are also presented for the convection velocity of the flow structures over the trailing-edge area, showing a reduction relative to the Baseline case. The application of flow blowing for controlling the flow field near the aerofoil trailing-edge is presented in Chapter 5. Results are presented for different flow blowing rate and blowing angles. The evaluation of the pressure and velocity statistics show increased magnitudes of their frequency-energy content at low frequencies while reduced PSD are observed at high frequencies for the highest severity flow blowing case injected perpendicularly to the boundary layer. The use of flow treatments is shown to lead to an increase in both the time and length-scales of turbulence computed through the spatial-temporal cross correlations. The turbulent length-scales show significant augmentation at high frequencies in the vicinity of the trailing-edge. An increase in length-scale was also observed at low frequency ranges, where the inclined uniform flow blowing case with the highest Cμ intensity (Cμ is the flow control severity), shows the more prominent augmentation. The convection velocities decrease considerably for all the cases, where the inclined flow blowing cases show the highest reductions. Furthermore, the streamwise lifespan of the flow structures are seen to rise prominently for the inclined flow blowing. Finally, the concepts of co-flow (zero-net mass flux) and boundary layer periodic excitations (non-uniform inflow and outflow) are evaluated and compared to both the Baseline and the uniform flow control manipulations, as reference. These results are presented in Chapter 6. While the changes on the mean aerodynamic quantities are mild, the alterations on the flow turbulence statistics are found to be substantial. The oscillating (periodic) boundary layer treatments show significant augmentation on the time- and length-scales of the turbulent structures, further than those accomplished by the uniform flow blowing or suction. Overall, the computed pressure autocorrelations exhibit higher width, the spanwise correlations magnitudes are found to be stronger and the decays, smoother. The boundary layer energy-frequency content computed from the measured velocity fluctuations show a frequency shift of energy due to the changes to the boundary layer flow structures. The details are provided in Chapter 6. The presented flow control treatments have shown potential benefits to reduce trailing-edge noise as they have all shown to be able to cause changes to the turbulent flow structures responsible for the trailing-edge noise generation. The effects of the studied flow control methods on the turbulence energy content and its scales can directly influence the noise generation mechanisms. The application of the proposed method will, however, depend on the aerodynamic and aeroacoustic performance and the design requirements of the aero-component. The investigation here provides a good starting point for a better understanding of the mechanisms involved in noise generation and the effect of flow control on boundary layer structures and also demonstrates how such techniques can be later used as an engineering solution for reducing noise from different aero-components.
Supervisor: Gambaruto, Alberto ; Azarpeyvand, Mahdi Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Aeroacoustics ; Aerodynamics ; Flow Control ; Flow Suction ; Flow Blowing ; Turbulence ; Trailing-edge