THESIS
2019
xxxv, 152 pages : illustrations (some color) ; 30 cm
Abstract
This thesis investigates the turbulence-aerofoil interaction noise, which is regarded
as a significant contributor to the broadband noise in turbofan engines. In order to
produce affordable means to leading edge noise predictions, synthetic turbulence method
is developed in the Computational AeroAcoustics (CAA) simulations to reproduce
the turbulence statistics, which will then be combined with the Linearised Euler
Equations (LEEs) solved by a high-order CAA code to simulate the noise generation
and propagation.
In this work, a Direct Anisotropic Filter (DAF) method is proposed to produce
desired velocity spectra of three-dimensional anisotropic turbulence, based on a direct
form of anisotropic filter that can be implemented directly to obtain non-Gaussian
spectra. In addition...[
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This thesis investigates the turbulence-aerofoil interaction noise, which is regarded
as a significant contributor to the broadband noise in turbofan engines. In order to
produce affordable means to leading edge noise predictions, synthetic turbulence method
is developed in the Computational AeroAcoustics (CAA) simulations to reproduce
the turbulence statistics, which will then be combined with the Linearised Euler
Equations (LEEs) solved by a high-order CAA code to simulate the noise generation
and propagation.
In this work, a Direct Anisotropic Filter (DAF) method is proposed to produce
desired velocity spectra of three-dimensional anisotropic turbulence, based on a direct
form of anisotropic filter that can be implemented directly to obtain non-Gaussian
spectra. In addition, three-dimensional and pseudo three-dimensional isotropic filters of
von Kármán energy spectrum and Liepmann energy spectrum are also derived based on
the energy spectrum function of isotropic turbulence. The DAF method contains less
model constraints than the existing methods, getting rid of the inconvenience induced
by complex hypergeometric functions and Gaussian superposition with Mach number
dependent model parameters. In addition, compared with the numerical results given
by existing methods, this method offers improvement in accuracy and cost.
In this study, the DAF method is combined with the LEEs in the CAA simulations
to study the turbulence-aerofoil interaction noise, with the results showing good
agreement with benchmarking analytical solutions and experimental measurements.
The numerical study also investigates the effect of anisotropy on leading edge noise for
thick aerofoils, revealing that the anisotropic turbulence stretched in the axial direction
will alleviate the thickness-induced noise reduction. This study also highlights the
contribution of streamwise energy spectra to noise generation when considering the
effects of anisotropy.
The DAF method is also used to investigate the distortion of turbulence spectrum
in the vicinity of the aerofoil leading edge, which has a significant impact on the
turbulence-aerofoil interaction noise. By applying different degrees of anisotropy in
the incident turbulence spectrum, the effects of aerofoil thickness, camber and angle
of attack on distorted turbulence spectrum are investigated in the CAA simulations.
On the basis of the asymptotic results around bluff bodies obtained by the generalised
rapid distortion theory, a theoretical solution of the distorted turbulence spectrum is
proposed considering the influence of anisotropy. Additionally, a modified far field noise
prediction model is proposed by using the distorted turbulence spectrum in the vicinity
of aerofoil leading edge, in order to account for the effect of real aerofoil geometries.
Finally, the DAF method is extended to synthesise the turbulence downstream of
the fan which is highly inhomogeneous and non-stationary, through the combination
with the fan wake model featured by the cyclostationary variations in the turbulent
kinetic energy, which represents a better fit to the physical truth of fan wake-Outlet
Guide Vanes (OGVs) interaction noise. By using the extended DAF method with
cyclostationary anisotropic filter, the effects of cyclostationarity and anisotropy of the
fan wake on the turbulence spectra and far-field noise spectra are investigated.
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