1,720,991 research outputs found
On the effects of fan wake modelling and vane design on cascade noise
No full textThis paper investigates simplified and representative configurations of fan wake-cascade interaction noise, which is a major source of broadband noise in ultra-high bypass ratio turbofan engines. To this end, two-dimensional computational aeroacoustic simulations are performed by using synthetic turbulence and solving the linearised Euler equations. Several fan wake modelling assumptions are investigated, including isotropic turbulence and cyclostationary variations in both turbulent kinetic energy and turbulence length scale. Results indicate that broadband noise mainly depends on the circumferentially-averaged turbulence spectrum that is perceived by the cascade. Furthermore, the turbulence length scale plays a significant role on cascade noise. Variations in the circumferential distribution of the turbulence length scale modify the slope of the noise spectra, and the maximum noise level can vary significantly. A parameter study on cascade noise has also been performed, including variations in the vane count, aerofoil thickness, camber, mean flow Mach number, stagger angle, and inter-vane spacing. It is shown that noise reduction due to vane thickness at high frequencies can be estimated as a linear function of the frequency, thickness, and mean flow speed in the upstream and downstream directions. A modal spectral decomposition has been performed to study the contribution of cut-on circumferential modes to the sound power level. Variations in vane design parameters lead to significant changes in the cut-on modes and modal spectral distributions. However, their effect on the amplitude of the sound power level spectra is normally below 2 dB, which reinforces the suitability of the flat plate assumption for the prediction of cascade noise at an early design stage.</p
Turbulence-cascade interaction noise using an advanced digital filter method
No full textFan wakes interacting with outlet guide vanes is a major source of noise in modern turbofan engines. In order to study this source of noise, the current work presents two-dimensional simulations of turbulence-cascade interaction noise using a computational aeroacoustic methodology. An advanced digital filter method is used for the generation of isotropic synthetic turbulence in a linearised Euler equation solver. A parameter study is presented to assess the influence of airfoil thickness, mean flow Mach number, stagger angle and gap-to-chord ratio on noise. Results are validated against predictions from an analytical method for two-dimensional flat plate cascades. Fan wake modelling is also addressed by extending the advanced digital filter method to account for spatial variations of the turbulence intensit
Fan wake modelling for computational aeroacoustic simulations of turbulence-cascade interaction noise
No full textThe present work addresses the numerical modelling of fan wakes using synthetic turbulence and its influence on turbulence-cascade interaction noise predictions. Initial results show that cascade noise only depends on the circumferentially-averaged turbulence spectra that interact with the cascade. Consequently, isotropic turbulence produces noise predictions with approximately the same level of accuracy than fan wakes with cyclostationary variations in both turbulent kinetic energy and integral length scale. The paper also includes a parameter study on the effects of vane count and camber on cascade noise from thick aerofoils. Numerical results show that vane count may have a significant effect on noise predictions at low frequencies, whereas the effects of camber are negligible
Turbulent-wake-cascade interaction noise simulations using a hybrid sliding mesh method
No full textAccurate predictions 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. Parameters such as the OGV vane thickness, fan wake structure, and advance ratio may affect the sound generation and propagation characteristics. A systematic study of these design parameters via full-scale and three-dimensional simulations is not convenient as it will inherit a significant computational cost. In this work, the fan and OGV geometry is 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-OGV motion, and isotropic 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.</p
Wavy leading edge airfoils interacting with anisotropic turbulence
No full textLeading edge noise reductions caused by serrations have been shown to be sensitive to the length scales of vortical disturbances. In order to improve the understanding of wavy leading edge airfoils as a noise reduction technology, this paper examines the effects of anisotropy on turbulence-airfoil interaction noise by means of computational aeroacoutic simulations. A synthetic turbulence method is used to generate fully three-dimensional, divergence-free, homogeneous anisotropic turbulence, which is injected in a linearized Euler equation solver to model the noise generation. Moderate variations in turbulence length scales, which are representative of the anisotropy in aero-engine fan wakes, are tested for a NACA 0012 airfoil with a wavy leading edge. This work focuses on the noise sources in the near-field by examining the distortion of the turbulent structures and velocity spectra in the vicinity of the noise sources, the unsteady pressure and its spectral density on the airfoil surface, the magnitude-squared coherence between velocity and pressure fluctuations on the noise sources, and the correlation between noise sources along the span for various degrees of anisotropy. Numerical results show that small variations in the turbulence length scales can produce significant changes in the spectral content of the noise sources at the peak and root regions. The loudest noise source is always located in the root region for the cases examined and this source is mainly affected by the transverse velocity fluctuations. To reduce the correlation between noise sources in the peak and root regions, the ratio between the chordwise length scale and the amplitude of the serrations, and the ratio between the spanwise length scale and the wavelength of the leading edge should satisfy lx/(2h)<1 and lz/λ≤0.5, respectively
Leading edge noise predictions using anisotropic synthetic turbulence
No full textAn advanced digital filter method is presented to generate divergence-free synthetic turbulence with homogeneous anisotropic velocity spectra. The resulting fluctuating velocity field is obtained through a superposition of anisotropic Gaussian eddies. This method is used to generate a two-dimensional turbulent flow with the key statistics of homogeneous axisymmetric turbulence. This type of turbulence has been reported in aero-engine intakes, fan wakes and open-jet wind tunnel experiments. The advanced digital filter method is implemented in a linearized Euler solver in order to investigate potential effects of anisotropic turbulence on leading edge noise. Computational aeroacoustic simulations are performed for anisotropic turbulence with streamwise-to-transverse length scale ratios ranging from 0.33 to 3 on a number of isolated airfoil configurations, including variations in mean flow Mach number, airfoil thickness and angle of attack. Noise reduction due to airfoil thickness is assessed on a NACA 0012 airfoil at zero angle of attack, showing similar trends for bothisotropic and moderately anisotropic turbulent flows. Effects of anisotropic turbulence on noise become evident for airfoil configurations at non-zero angle of attack
Synthetic turbulence methods for leading edge noise predictions
No full textAn advanced digital filter method to generate synthetic turbulence is presented for efficient two- and three-dimensional leading edge noise predictions. The technique, which is based on the Random Particle-Mesh method, produces a turbulent inflow that matches a target isotropic energy spectrum. The discretized equations for the synthetic eddies, and the input parameters needed to recover the desired turbulence statistics, are presented. Moreover, a simple and fast implementation strategy, which does not require an additional boundary condition, is presented under the frozen turbulence assumption. The method is used in a linearized Euler solver to predict turbulence-airfoil interaction noise from a number of configurations, including variations in airfoil thickness, angle of attack and Mach number. For the first time, noise predictions from a digital filter method are directly compared to those provided by synthetic turbulence based on a summation of Fourier modes. The comparison indicates that the advanced digital filter method gives enhanced performance in terms of computational cost and simulation accuracy. In addition, initial tests show that this method is capable of reproducing experimental noise measurements within 3 dB accuracy
Including wall effects in analytical leading edge noise predictions
No full textAn analytical solution to leading-edge noise produced by a translating two-dimensional flat plate ingesting turbulence in proximity to a hard-wall is presented. This is a relevant problem to calculate the installation noise of open rotors and un-ducted fans. The analytical solution to the problem is given by using Amiet’s flat plate theory in conjunction with the Method Of Images (MOI) to include the effects of the wall. The low frequency, low Mach number limit of the analytical solution is investigated and it is shown that the flat plate in this limit behaves like a compact vertical dipole. The analytical solution is verified by a Computational AeroAcoustic (CAA) simulation that also uses the MOI to simulate a wall. While the MOI gives an approximation of the wall, it does not model all of the effects, such as diffraction from the edges of the flat plate and acoustic shielding due to the presence of the flat plate. These effects, which are ignored in the MOI are quantified using a CAA simulation that models the wall using a hard-slip-wall boundary condition. It is found that the analytical predictions and the CAA simulations using the MOI compare well. However, when the MOI is compared to the CAA simulation using a hard-slip-wall boundary condition, it is found that the MOI does not capture the effect of the shadow zone that is created due to the shielding effect of the aerofoil. The extent of the shadow zone is modified by changing the height of the aerofoil from the wall, and it shown that as the height of the aerofoil from the wall is increased, the shielding effect decreases
CFD study of aerodynamics inputs for engine fan broadband noise
No full textLAUREA MAGISTRALEQuesta tesi di laurea presenta uno studio numerico sul rumore a banda larga derivante dall'interazione tra la scia della ventola e le Guide di uscita (OGV). Si ritiene che questa fonte di rumore a banda larga abbia un importante contributo nei moderni motori turbofan, in particolare durante la fase di atterraggio. Il presente lavoro si concentra sulla previsione del rumore a banda larga di una configurazione semplificata di fan-OGV: il NASA Source Diagnostic Test (SDT) per il quale sono disponibili dati sperimentali aerodinamici e acustici a varie velocità di rotazione.
L'obiettivo di questa tesi è duplice. Innanzitutto, sono state testate quattro diverse configurazioni di Computational Fluid Dynamics (CFD) al fine di riprodurre i corretti parametri aerodinamici ai confini del dominio computazionale e accurate statistiche sulla ventola all'avanguardia del OGV. Le statistiche sulla ventola sono importanti, dal momento che sono necessari input per la previsione del rumore a banda larga dal motore. In secondo luogo, i dati aerodinamici vengono utilizzati come input in un modello analitico semplificato (modello di Amiet) per la previsione del rumore di interazione OGV della ventola. Diversi metodi per calcolare la turbolenza della veglia dalla simulazione CFD sono stati testati per valutare la loro influenza sulle previsioni del rumore. Inoltre, è stato utilizzato un metodo di estrapolazione per predire le variabili oltre il piano di miscelazione e per stimare le statistiche di scia della ventola sul bordo anteriore dell'OGV.
Una configurazione CFD è stata mantenuta a causa del suo aumento delle prestazioni rispetto ai dati CFD della NASA e al punto operativo. Le statistiche sulla ventola sono ragionevolmente ben previste e i risultati del rumore sono coerenti con le misurazioni sperimentali della NASA. Tuttavia, il livello di potenza del suono è significativamente sottostimato a velocità più elevata. La propagazione della scia consente di rivalutare la risposta acustica riducendo al minimo gli effetti del piano di miscelazione nelle configurazioni di fan-OGV.This MSc thesis presents a numerical study on the broadband noise from the interaction of the fan wake and Outlet Guide Vanes (OGV). This source of broadband noise is considered to have a major contribution in modern turbofan engines, and in particular during the landing phase. The present work focuses on the broadband noise prediction of a simplified fan-OGV configuration: the NASA Source Diagnostic Test (SDT) for which aerodynamic and acoustic experimental data are available at various rotation speeds.
The objective of this thesis is twofold. Firstly, four different Computational Fluid Dynamics (CFD) configurations have been tested in order to reproduce the correct aerodynamic parameters at the boundaries of the computational domain, and accurate fan wake statistics at the leading edge of the OGV. The fan wake statistics are important, since they are required inputs for the prediction of broadband noise from the engine. Secondly, aerodynamic data are used as an input in a simplified analytical model (Amiet’s model) for the prediction of fan wake-OGV interaction noise. Different methods to compute the wake turbulence from the CFD simulation have been tested to assess their influence on the noise predictions. Furthermore, an extrapolation method has been used to predict the variables beyond the mixing plane and to estimate the fan wake statistics at the leading edge of the OGV.
A CFD setup has been retained due to its increased performance when compared to NASA CFD data and operating point. The fan wake statistics are reasonably well predicted and the noise results are consistent with experimental measurements from NASA. However, the sound power level is significantly underestimated at higher speed. The wake propagation allows to reevaluate the acoustic response by minimizing the effects of the mixing plane in fan-OGV configurations
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