A series of experiments was conducted to evaluate the aerodynamic and acoustic performance of a supersonic inlet with a modified auxiliary door geometry. A 1/14 scale model of an axisymmetric, mixed-compression, supersonic inlet designed for civil transportation was used in conjunction with a 10.4cm (4.1 in.) turbofan engine simulator to test a new inlet door geometry designed to reduce flow distortion and noise radiation. The new door geometry uses door passages with increased circumferential span to improve the distribution of the flow entering through the doors. In addition, the new design employs sonic flow velocity at the inlet throat and a converging flow passage in the auxiliary doors to attenuate propagating fan noise through the choking effect. To provide a basis for comparison, a baseline door geometry representative of current designs was also tested. The experiments were conducted at simulated aircraft takeoff engine speeds under static conditions. Steady-state measurements of the inlet flow field were made along with far field acoustic measurements of the fan noise. The results show the new door geometry is successful in reducing circumferential flow distortion at the fan entrance by a factor of 2.3 compared to the baseline configuration. In addition, far field radiation of the blade passing frequency tone and overall noise is reduced by an average of 4dB(SPL) in the forward and aft sectors (0° to 110° from the inlet axis). As a compromise for the distortion and acoustic improvements, the overall inlet total pressure recovery is reduced by approximately 2% with the new auxiliary doors. / Master of Science

A turbulent combustion noise model based on first principles is developed in this thesis. The model predicts (1) the pressure time series, (2) Sound Pressure Level (SPL) spectrum, (3) Over-All Sound Pressure Level (OASPL), (4) the thermoacoustic efficiency, (5) the peak frequency, and (6) the sound power of combustion generated noise. In addition, a correlation for sound power is developed based on fundamental burner and fuel variables known to affect the acoustic characteristics of turbulent combustion. The predicted pressure time series exhibits consistency with reality in that it has no steady component. It also confirms speculation in the literature that the predominant noise mechanism in open turbulent flames results from a "transition burning" phenomenon at the flame front. The predicted Sound Pressure Level spectrum, Over-All Sound Pressure Level, and the thermoacoustic efficiency are in excellent agreement with the results available in the literature. The shifts in the peak frequency with basic burner and fuel parameters are consistent with experimental observations from the literature. The disagreements between the predicted and the observed exponents of fuel and burner parameters for sound power are shown to be well within the standard deviation of the experimental observations. Certain areas for further analytical research on the combustion noise mechanism are identified. / Master of Science

The two-noise source model for predicting jet noise claims that the radiated jet noise is composed of two distinct sources one associated with the small-scale turbulence and another associated with the large-scale turbulence. The former source is claimed to radiate noise predominantly at larger angles with respect to the downstream jet axis, whereas the large-scale turbulence radiates predominantly at the shallower angles. A key objective of this effort is to experimentally validate this model using correlation and coherence measurements. Upon the successful validation of the two-noise source model for jets exhausting from multiple nozzle geometries driven at Mach numbers ranging from subsonic to supersonic, a three-microphone signal enhancement technique is employed to separate the contribution of the small-scale turbulence from that of the large-scale turbulence in the far-field. This is the first-ever quantitative separation of the contributions of the turbulence scales in far-field jet noise measurements. Furthermore, by suitable selection of far-field microphone positions, the separation of the contribution of any internal or core noise from that of the jet-mixing noise is achieved. Using coherence-based techniques to separate the contributions of the small-scale turbulence, large-scale turbulence, and any internal or core noise from far-field exhaust noise measurements forms the backbone of this effort. In the application of coherence-based multiple-microphone signal processing techniques to separate the contributions of the small-scale turbulence, large-scale turbulence, and any internal or core noise in the far-field, research efforts focus on three techniques (1) the coherent output power spectrum using two microphones, (2) an ordinary coherence method using the three-microphone technique, and (3) the partial-coherence method using five microphones. The assumption of jet noise incoherence between correlating microphone is included in each of these methods. In light of the noise radiation mechanisms described within the framework of the two-noise source model and their spatial characteristics as experimentally determined in the far-field, the assumption of jet noise incoherence is evaluated through a series of experiments designed to study jet noise coherence across a variety of nozzle geometries and jet Mach numbers ranging from subsonic to supersonic. Guidelines for the suitable selection of far-field microphone locations are established.

This thesis explores one possible method of preventing helicopter crashes caused by brownout using the noise generated by the helicopter rotor as an altimeter. The hypothesis under consideration is that the helicopter's height, velocity, and obstacle locations with respect to the helicopter, can be determined by comparing incident and reflected rotor noise signals, provided adequate bandwidth and signal to noise ratio. Heights can be determined by measuring the cepstrum of the reflected helicopter noise. The velocity can be determined by measuring small amounts of Doppler distortion using the Mellin-Scale Transform. Height and velocity detection algorithms are developed, optimized for this application, and tested using a microphone array. The algorithms and array are tested using a hemianechoic chamber and outside in Georgia Tech's Burger Bowl. Height and obstacle detection are determined to be feasible with the existing array. Velocity detection and surface mapping are not successfully accomplished.

This work proposes to address the computation of flow-generated noise, including the high frequency components in an acceptable computational time, relative to industrial applications. In this framework, hybrid methods based on aeroacoustic analogies are relevant to predict the corresponding sound, in term of computational time and easy implementation, and are separating the sources of sound computation from the sound propagation itself. Both parts of the hybrid method are then investigated.<p><p>Firstly, the sources of sound should be computed accurately, reproducing most of the energy content of the turbulent flow. The present study is mainly based on LES computations, that are a good compromise between the accuracy and the time required to obtain the sources, especially for low Mach number application, where incompressible flow computations can be used. The influence of the computational parameters is then studied through the investigation of the influence of the boundary conditions, flow solver, mesh refinement, LES subgrid-scale model and computational domain definition. <p><p>Secondly, the sound sources propagation is computed using classical analogies based most of the time on free field Green's functions. They are limited to the low frequency range of the emitted sound spectrum, where the source is compact, if incompressible acoustic sources are used. They are then proposed to be combined with Amiet's theory for airfoil noise to cover the remaining high frequency part of the sound spectrum. The limitations of Amiet's theory are analysed through the geometrical and acoustical far-field assumptions. Furthermore, an inverse strip method is proposed to extend Amiet's theory to spanwise varying flow conditions and taking correctly into account the spanwise wave number effects. <p><p>The leading-edge noise mechanism is studied through the interaction of the turbulent region of a jet with a NACA0012 airfoil. The hybrid method, consisting in an incompressible LES computation combined with Curle's analogy, showed its limitation to the low frequency range of the sound spectrum when low-order CFD are used. The Amiet's theory, based on the modelisation of the upstream velocity spectrum, provides a good sound prediction at high frequency, as far as the airfoil thickness is accounted for. <p><p>The Trailing-edge noise mechanism is studied through the flow around a mid-span cut of a blade (CD airfoil) of an automotive cooling fan. <p>Three different acoustic methods are used :Curle's analogy is using wall-pressure fluctuations over the airfoil surface, Ffowcs-Williams and Hall's analogy is using volumetric velocity informations around the trailing-edge and Amiet's theory for trailing-edge noise is using the wall-pressure spectrum around the trailing-edge and the corresponding spanwise correlation. The methods using wall-pressure informations are shown to directly transfer the informations of the wall-pressure spectrum to the sound spectrum. Differences are appearing in the higher frequency range where the airfoil starts to be non compact, invalidating the use of Curle's analogy at such frequencies compared to the other methods, taking scattering effects into account implicitly in their formulations. <p><p>Finally, in order to reduce the computational cost, Amiet's theory for trailing-edge noise is proposed to be driven from steady RANS computations. Two methods to compute the wall-pressure spectrum from boundary-layer informations are studied. Their respective robustness and reliability are analysed in an uncertainty quantification framework, in case of varying velocity profiles upstream the airfoil. Both models showed similar results and tendency compared to the wall-pressure LES spectrum, as far as the flow topology remains similar. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Wind energy is one of the fastest-growing renewable energy technologies, and horizontal axis wind turbines (HAWT) have been the most common device to convert wind kinetic energy into electrical energy. As the capacities of wind turbines and scales of wind farm constructions are rapidly increasing over time, environmental impacts of wind energy are becoming more relevant and raising more attention than ever before. One of the major environmental concerns is noise emission from wind energy facilities, especially low-frequency noise and infrasound that allegedly cause so-called wind turbine syndrome. Therefore, a numerical simulation program capable to predict low-frequency noise and infrasound emission from wind turbines is a useful tool to aid future wind energy development. In this study of this thesis, a computer program named TDRIP (Time Domain Rotor Infrasound Prediction) is developed based on acoustic analogy theories. Farassat’s formulation 1A, a solution to Ffowcs Williams-Hawkings (FW-H) equation, is implemented in the TDRIP program to compute aerodynamically generated sound. The advantage of this program is its capability to simultaneously compute infrasound emission of multiple wind turbines in time domain, which is a challenging task for other aerodynamic noise prediction methods. The developed program is validated against results obtained from computational fluid dynamics (CFD) simulations. The program is then used to compute aerodynamic noise emitted from wind turbine rotors. The effects of wind direction, wind turbine siting, and phase of wind turbine rotation on consequent aerodynamic noise are investigated. Results of aerodynamic noise computation imply that wind turbine siting configuration or wind turbine phase adjustment can help reducing noise level at certain locations, which make the program ideal to be integrated into wind farm siting or control tools.

No description available.

This project is intended to demonstrate the current state of knowledge in the prediction of the tonal and broadband noise radiation from a Sevik rotor. The rotor measurements were made at the Virginia Tech Stability Wind Tunnel. Details of the rotor noise and flow measurements were presented by Wisda et al(2014) and Murray et al(2015) respectively. This study presents predictions based on an approach detailed by Glegg et al(2015) for the broadband noise generated by a rotor in an inhomogeneous flow, and compares them to measured noise radiated from the rotor at prescribed observer locations. Discrepancies between the measurements and predictions led to comprehensive study of the flow in the wind tunnel and the discovery of a vortex upstream of the rotor at low advance ratios. The study presents results of RANS simulations. The static pressure and velocity profile in the domain near the rotor's tip gap region were compared to measurements obtained from a pressure port array and a PIV visualization of the rotor in the wind tunnel. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2015. / FAU Electronic Theses and Dissertations Collection

In this study, acoustic noise radiation through a duct in frequency domain is analyzed. Frequency domain linearized Euler equations are solved for turbofan exhaust noise propagation and radiation. The geometry in studied cases is assumed as axisymmetric. The acoustic waves are decomposed into periodic azimuthal modes. Characteristic boundary conditions, and buffer zone boundary conditions are employed. Iterative type pseudo time integration is employed. Nonuniform background flow effect on the radiation pattern is experienced. All computations are performed in parallel using MPI library routines in computer cluster. Results proved that the one with the buffer zone has a better radiation characteristic than the characteristic one because of absorbtion of spurious waves. It is seen that the efficiency of the buffer zone is frequency dependent.

No description available.