Accurate computations of broadband noise generated by fan-wake-OGV (outlet guide vane) interaction remain challenging due to the wide range of acoustic and turbulent length-scales. A systematic study of the design parameters and turbulence properties via full-scale and three-dimensional simulations is not convenient due to the high computational cost. The fan and OGV geometry can be represented as an unwrapped two-dimensional cascade with periodic upper and lower boundaries. A two-dimensional Euler solver with a hybrid sliding grid method is applied to account for the fan wake motion, and turbulence is synthesised and injected upstream of sliding surface to model the fan-wake. The numerical results are compared to an analytical model that is an extension of Amiet’s flat-plate noise mode...[ Read more ]

Accurate computations of broadband noise generated by fan-wake-OGV (outlet guide vane) interaction remain challenging due to the wide range of acoustic and turbulent length-scales. A systematic study of the design parameters and turbulence properties via full-scale and three-dimensional simulations is not convenient due to the high computational cost. The fan and OGV geometry can be represented as an unwrapped two-dimensional cascade with periodic upper and lower boundaries. A two-dimensional Euler solver with a hybrid sliding grid method is applied to account for the fan wake motion, and turbulence is synthesised and injected upstream of sliding surface to model the fan-wake. The numerical results are compared to an analytical model that is an extension of Amiet’s flat-plate noise model to a cascade of flat plates. The parametric studies were conducted using both isotropic and anisotropic turbulence for various fan wake advance ratio, turbulence wake and stagger angles, etc.

For realistic cascades, there are relative motions between the rotating fan blades and stationary OGVs. The upstream-travelling sound waves can be scattered by the moving solid surfaces of the fan blades, which alters the sound distribution. This scattering process is called the blockage or shielding effect, and it is influenced by the fan speed, blade geometry and spectral content of the sound. By employing the sliding grid method, the physics due to the relative motions between the fan blades and OGVs is captured. The blockage and scattering effect on the acoustic power is further analysed for the frequency and modal transferring phenomenon, contributing to the alternation of the sound features due to the presence of the rotor blade.

For the industrial use of the accurate engine broadband simulations, the parametric studies usually involve a large scale of test parameters. Among all time consumptions, much is costed for the grid generation due to the complex geometries. The immersed boundary method (IBM) is implemented to overcome these drawbacks by the direct modification to the governing equation. The geometry is placed directly on the Cartesian grid without the need to generate a body-fitted grid. This method is implemented to both Navier-Stocks and Euler equations for acoustic simulations. The immersed boundary method substantially reduces the human efforts in generating mesh at the cost of an increase in computational time and storage. To validate the effectiveness of IBM method in high-order simulations, the broadband leading edge noise is measured and verified with the Euler solver which shows that the accuracy for noise prediction is promising.

In conclusion, this thesis presents the numerical studies conducted on turbulence-cascade simulations to model the broadband noise, which is the major broadband source from an aero-engine. By the implementation of the sliding grid technique, the rotor blockage effect is further simulated to account for the influence of rotating blades on the noise propagation. To enable the fast setup and meshing for vast parametric studies for broadband engine noise predictions, the immersed boundary method is implemented to the high-order acoustic solver.

Keywords: sliding grid, cascade noise, synthetic turbulence, immersed boundary method