Aeroacoustic Characteristics of Static and Dynamic Stall

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Static and dynamic stall and separation noise can be major contributors to the self-noise of fixed and rotating aerofoils. The work presented in this thesis investigates the aerodynamic and aeroacoustic characteristics of a symmetric NACA 0012 aerofoil for a wide range of angles of attack, in particular the changes caused by static and dynamic stall to important flow quantities such as the surface pressure fluctuation spectra. To this end, a Kevlar-walled test section, a dynamic turntable, a highly instrumented NACA 0012 aerofoil, as well as far-field noise measuring devices were designed, built and validated. Subsequently, measurements were conducted in an aeroacoustic wind tunnel facility for a variety of flow conditions to characterise static and dynamic stall, with a focus on the flow field and the hydrodynamic near-field.
The presence of leading edge flow separation leads to profound variations in the aerodynamic and aeroacoustic properties, with several changes observed in the unsteady wall pressure signatures during static stall, including the emergence of spectral peaks, substantial magnitude increases, and a shift to low frequencies. These developments have been linked to various flow field instabilities such as shear layer instabilities and the formation of von K´arm´an vortices in the wake region. Simultaneous measurements of the unsteady surface pressure fluctuations, the attached turbulent boundary layers, and the separated flow fields, have enabled the temporal and spectral relationship between the flow field structures, their location and the surface pressure fluctuations to be examined, which can provide insight into surface pressure fluctuation generation mechanisms. Once the boundary layer has separated from the aerofoil, the predominant surface pressure generating flow structures are no longer detected in the near wall region.
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The dynamic stall phenomenon has been analysed in terms of aerodynamic coefficients and instantaneous pressure distributions. The effects of varying the sinusoidal oscillation parameters (mean angle of attack, oscillation frequency, and oscillation amplitude) on important aeroacoustic properties, such as the far-field noise, unsteady surface pressure and coherence spectra are illustrated. The evolution of the wall pressure fluctuation spectral energy content through the dynamic stall cycle has revealed the dynamic stall vortex to be responsible for the further increases in the magnitude and the considerable spectral variations, in comparison to the wall pressure spectra for static stall.
The vast amount of simultaneously acquired data shed further light on key aeroacoustic characteristics for both the pre- and post-stall regimes of fixed and oscillated aerofoils. This rich database is of high relevance to a number of industries and can in addition be utilised for the validation of LES/WMLES/DNS simulations.
Date of Award29 Sept 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMahdi Azarpeyvand (Supervisor) & Christian B Allen (Supervisor)

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