Author

Man Zhang

Date of Award

2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Committee Chair

Kader Frendi

Committee Member

S. S. Ravindran

Committee Member

Babak Shotorban

Committee Member

Sarma Rani

Committee Member

Jason Cassibry

Subject(s)

Theory of wave-motion., Fluid dynamics., Aerofoils.

Abstract

Several important engineering problems have been tackled in this dissertation: a. the spectral characteristics of turbulent wall-pressure fluctuations, b. wave propagation in sensor ports and c. effect of the airfoil leading edge waviness on flow structure and noise. Significant contributions have been in all three areas. a. In the wall-pressure fluctuation study, an in-depth review of the literature is presented and the most notable existing semi-empirical models are evaluated against experimental data and observations. Based on this scrutiny of the experimental data and the different models, a framework is proposed for the development of a new semiempirical model. The proposed model anchored by the wavenumber-frequency spectrum of the wall-pressure fluctuations satisfies all the framework conditions and gives better agreement with experimental data compared to all other existing models. In particular, the model’s coherence contours are elliptic in shape in agreement with theory. The model captures the convective peak of the wavenumber-frequency spectrum accurately and the spectrum is wavenumber white for wavenumbers below the convective peak in agreement with experimental data and numerical results. In addition, the proposed model is given in both wavenumber-frequency and space-frequency forms which makes it easy to use in any engineering application. Moreover, all the empirical constants are judicially selected based on experimental data and observations. This is not the case of other models in the literature which are often given in one form, wavenumber-frequency or space-frequency, and lack a clear definition of its empirical constants. Our theoretical study showed the sources contributing to the wall-pressure fluctuations to be the fluctuating normal stresses and no contribution from shear stresses. b. Measurement of pressure fluctuations in sensor ports is often carried out in harsh environments such as a combustion chamber. However, correlating the sensor port measurement to the conditions in the chamber in terms of pressure fluctuations has not been done. In this study, models are developed for both constant and variable temperature conditions that give the pressure fluctuation in the chamber using measured data in the sensor port. Predictions from the constant temperature model are compared extensively to experimental data and good agreement is obtained for most comparisons. For the variable temperature model, no measurement data is available for comparison. c. The problem of passive control of the nearfield noise radiated by an airfoil is motivated by the desire to reduce airframe noise. In this study, two hybrid RANS-LES computations are carried-out on two different airfoils, one with a straight leading edge and the other with a wavy leading edge. The results show that the noise radiated by the wavy-leading airfoil is 4 to 10 dB lower than that radiated by the straight leading airfoil. The source of this reduction is believed to be due to the ability of the wavy-leading edge airfoil to breakup large coherent structures and form the streamwise vorticles along the trough.

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