Mechanical Engineering - Theses

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Start date: 2020 × End date: 2023 × Subject: Computational fluid dynamics × Author: Deuse, Mathieu Maurice P ×

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    High-fidelity numerical investigation of different mechanisms of aerofoil self-noise
    Deuse, Mathieu Maurice P ( 2020)
    The self-noise of an isolated controlled-diffusion aerofoil is investigated using direct noise computations. The motivation is to investigate the multiple sources of aerofoil self-noise on a realistic compressor blade geometry, the physical mechanisms leading to the generation of sound, and the influence of compressibility effects. The use of direct noise computation allows to obtain a detailed and accurate picture of both the hydrodynamic and acoustic fields. The high-order finite difference solver HiPSTAR is used to conduct the numerical simulations. In the context of the present study, the capabilities of HiPSTAR have been extended with an overset grid framework. The framework employs a novel algorithm for the generation of the composite grid, i.e. to identify the discretisation, interpolation and non-physical points. This algorithm was designed to minimise and simplify the user input, while maintaining the flexibility to handle complex setups. An explicit fourth-order Lagrange interpolation scheme is used, so that the formal order of accuracy of the finite difference scheme used in the flow solver is matched. An instability linked to the possible presence of uncoupled numerical solutions on separate grids sharing the same physical location is discussed, and a modification of the composite grid generation algorithm that prevents this instability is introduced. The final overset grid method is validated with two test cases: the convection of an isentropic vortex and the Taylor-Green vortex. The solver is used to conduct four large eddy simulations and one direct numerical simulation of the flow around the controlled-diffusion aerofoil. The angle of attack is 8 degrees, the chord-based Reynolds number is 10000, and results obtained for four values of the free-stream Mach number [0.2, 0.3, 0.4, 0.5] are compared. For those flow parameters, the pressure side is fully laminar, whereas a separation bubble is present on the suction side close to the leading edge that promotes transition to turbulence. The size of the separation bubble is found to increase with the Mach number. Two noise sources are observed, one at the trailing edge and one in the leading edge transition/reattachment region. The first has a broadband, low frequency spectrum, while the second displays a tone whose frequency depends on the local Mach number. Because the leading edge separation bubble is very small, the associated tone frequency is high and requires a significantly finer grid to faithfully resolve the acoustic propagation than what is typically deemed sufficient. Finally, cross correlations between the surface pressure and the far-field pressure reveal that the pressure fluctuations reaching the trailing edge are initially generated in the transition/reattachment region, which indicates that the trailing edge noise is a consequence of the pressure fluctuations generated by the separation bubble.