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Title:Scattering of an entropy disturbance into sound by a linear cascade of turbine stator blades
Author(s):Mishra, Ashish
Advisor(s):Bodony, Daniel J.
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Indirect combustion noise
Abstract:It is known that modern turbofan engines produce more noise from their exhaust than can be accounted for by jet noise alone. Part of the extra noise coming from the exhaust of the engine is associated with the combustor. Apart from direct combustion noise, the combustor is known to have an indirect source of noise in the form of the interaction of convecting entropy disturbances with the turbine blades. This indirect noise generation is due to the (i) acceleration of the convected disturbance by the mean flow and (ii) satisfaction of the wall-boundary condition on the turbine blades. The indirect combustion noise is known to be present in modern gas turbine engines but its specific details are not known, including its overall contribution to the acoustic signature of the engine and its influence on the combustor. The present work focuses on carrying out direct numerical simulations for various cases of interaction between the entropy disturbance and turbine stator vane. Combustor-produced entropy disturbances have been simulated both in the form of a plane wave and localized high frequency pulse. DNS results for the interaction of an entropy disturbance with turbine stator vanes are presented and the resulting pressure disturbance field is analyzed. The DNS results are used to verify the analytical predictions of actuator disk theory. The actuator disk theory (ADT) ignores the geometric details of a blade under the assumption that wavelengths of all the disturbances are much longer than the chord length of the blade and replaces it with a discontinuity at position x0, referred in this thesis as the origin of the noise source. Actuator disk theory only uses the inlet and outlet flow Mach number and flow angle to compute the pressure disturbance field caused by an input entropy disturbance. One feature of this two-dimensional theory is the presence of evanescent modes below the cut-off wavelength, which decay in amplitude away from the blade, and also the independence of cut-on modes with the upstream and downstream distance from the blade. It is observed that for a low-frequency planar input entropy wave the decay rates for modes of larger wavelengths are very well predicted but as wavelengths goes smaller and smaller, matching between ADT predictions and DNS results worsens. The measured decay rate for smallest wavelengths is invariably lower than the ADT predictions. Comparisons between cut-on modes have been performed using the plane-mode values of pressure disturbance spectrum. It is observed that ADT predictions work reasonably well for low-frequency waves both upstream and downstream of the blade. The location of x0 has been computed using DNS results and it is found that ADT predictions are reasonable if we keep leading edge of the blade as origin of noise source for upstream traveling waves whereas trailing edge is better suited location for x0 when it comes to downstream traveling waves. At higher frequencies the accuracy of ADT predictions suffers.
Issue Date:2012-02-06
Genre:Dissertation / Thesis
Rights Information:Copyright 2011 Ashish Mishra
Date Available in IDEALS:2012-02-06
Date Deposited:2011-12

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