Chaitanya Paruchuri

Aerofoil geometry effects on turbulence interaction noise

Fan broadband is one of the dominant noise sources on an aircraft engine, particularly at approach. The dominant noise generation mechanism is due to turbulent- aerofoil interaction noise (TAI). This thesis investigates the effect of changes in 2D aerofoil geometry on TAI noise. The main focus of this thesis is to attempt to reduce it through the development of innovative leading edge geometries. The first two chapters of the thesis deals with an experimental and numerical investigation into the effect of aerofoil geometry on interaction noise on single aerofoils and on cascades. Consistent with previous work, they show that variations in aerofoil parameters, such as aerofoil thickness, leading edge nose radius and camber, produce only a small changes in broadband interaction noise at approach conditions. Subsequent chapters deal with the development of innovative leading edge serration profiles aimed at reducing interaction noise. Chapter 4 is a detailed study into the limitations of single-wavelength serrations in reducing interaction noise. The optimum profile is identified. Chapters 5, 6 and 7 all deal with the development of innovative profiles that can provide up to 10dB of additional noise reductions compared to single-wavelength serrations. For each of the profiles investigated a simple model is developed to aid the understanding of their interaction mechanism.

Prediction of Broadband Trailing-Edge Noise Based on Blake Model and Amiet Theory

This paper extends previously published TNO-Blake methods to predict airfoil broadband self-noise due to interaction of a turbulent boundary-layer with a sharp trailing edge. The method presented herein combines Blake’s model to predict surface pressure fluctuations and Amiet model to predict trailing edge noise emission. The Blake method is based on the solution of the Poisson equation as an integral of the mean-shear turbulent source interaction source, over the entire boundary layer thickness. A recent advances in description of streamwise turbulent intensity are employed. Surface pressure spectra are measured with remote microphones and compared to the prediction. In the next step, the wavenumberfrequency spectrum as predicted by the TNO model was utilized as an input to the Amiet model to evaluate the far-field trailing edge noise.

Sensitivity of aerofoil self noise reductions to serration flap angles

The serration amplitude and serration wavelength are traditionally regarded as the primary geometrical variables that can affect the noise performance of an add-on, flat plate type serrated trailing edge. This experimental study investigates another serration geometrical variable, namely the serration flap angle that could potentially affect the self-noise reduction of an aerofoil. The experiment was carried out at Brunel aeroacoustic facility, on a NACA65(12)–10 aerofoil. The serrated flat plates were manufactured to form in several flap angles: 15 o , 10 o , 5 o and 0 o as the reference. Preliminary investigation on the effect of serration amplitude, without the flap angle, confirms with other findings that the largest level of broadband noise reduction is achieved when the amplitude of the serrated flat plate is large. It is also worth reporting that broadband noise can already be reduced even by attaching a large chord length of unserrated, straight flat plate. When the serrated flat plate contains a flap angle, it is generally observed that a flap-up position (positive flap angle) is more favourable for broadband noise reduction, while the opposite is true for the flap-down position (negative flap angle). The best flap-up position is when the positive flap angle is small, at around +5 o . Unfortunately, a small flap-down position, i.e. –5 o is the worst performer amongst the test cases (lowest level of broadband noise reduction at low frequency, and highest noise increase at high frequency). Therefore, even a small misalignment of the trailing edge serration due to the manufacturing defect could potentially degrade (or enhance) the overall aerofoil self-noise reduction because the serration is found to be sensitive to small flap angles.

Analytic solutions for reduced leading-edge noise aerofoils

This paper presents an analytic solution for the sound generated by an unsteady gust interacting with a semi-infinite at plate with a piecewise linear periodic leading edge. The Wiener-Hopf method is used in conjunction with a non-orthogonal coordinate transformation and separation of variables to allow analytical progress. A fully analytic solution is obtained in terms of a modal expansion for the far-field noise which is obtained by summing only a finite number of cuton modes, allowing very quick evaluation. The analytic solution is compared to experimental results for five test case leading-edge geometries. Good agreement is seen indicating the analytic model is capturing the key features of the interaction such as the destructive interference from the tip and root. In four of the five test cases the serrated edges show large reductions of noise compared to the straight edge at mid and high frequencies, however the square wave geometry is seen to be ineffective at noise reduction for high frequencies.

Effect of Leading Edge serrations in reducing aerofoil noise near stall conditions

This paper aims at evaluating the possibility of reducing the noise generated by the separated flow over an aerofoil, due to conditions near to or of stall, by the introduction of serrations on its leading edge. First of all the onset of the separation on the aerofoil surface and the associated noise have been investigated. Different serration geometries as regards wavelength and height are considered, in order to identify the configuration which performs the best in terms of noise.

Prediction of turbulence-cascade interaction noise using modal approach

The turbulent wakes generated by a rotor interacting with outlet guide vanes is one of the dominant broadband noise sources in a turbofan engine. The present paper deals with the prediction of broadband noise due to the interaction of rotor wake turbulence with the OGV. The proposed approach aims to reproduce the two-point cross-spectral statistics of the turbulent velocity fluctuations at the OGV leading edges, which provides a complete specification of the rotor wake turbulence for broadband noise predictions. The approach proposed here is based on a superposition of vortical modes with the appropriate amplitudes. Three different vortical modes are compared for their reconstruction efficiency and accuracy: 1) Fourier modes, 2) Proper Orthogonal Decomposition (POD) modes 3) Normal modes of the Linearized Euler Equations (LEE).

On the robustness of the TNO model for aerofoil self-noise prediction

For attached flow, aerofoil self-noise is dominated by the interaction of a turbulent boundary-layers with a sharp trailing-edge. This present paper presents the robustness of well know TNO model for the prediction of aerofoil self-noise predictions. This paper experimentally investigates the effect of aerofoil thickness on trailing edge self-noise. We show that trailing edge noise exhibits a complex dependence on thickness, where it is observed that, very roughly, the spectral shape shifts towards lower frequencies as thickness is increased. In this paper we attempt to capture this behaviour using the theoretical framework developed by Blake in which trailing-edge noise sources are formulated in terms of the boundary layer quantities. Important parameters in the solution to the Poisson equation are the wall-normal gradient of streamwise velocity, the length scale of the energycontaining turbulent eddies, the normal vertical Reynolds stress, as well as its spectral decomposition. These quantities are obtained from hot-wire measurements in the flow. The robustness of the widely used TNO-blake formulation for predicting aerofoil self-noise has been examined which arises due to uncertainty in the boundary layer profile measurements. Finally, classical flat plate theory is used to predict the far-field radiated noise using surface pressure spectrum predicted from the Blake’s model.