Glegg, Stewart A. L.

This thesis has described a Reynolds Averaged Navier Stokes approach to
modeling turbulence in the wake of a cylinder and airfoil. The mean flow, cross stresses,
and two-point space time correlation structure was analyzed for an untripped cylinder
with a Reynolds number based on the cylinder diameter and freestream velocity of
60,000. The same features were also analyzed using this approach for an untripped
NACA 0012 airfoil with a Reynolds number based on the airfoil chord and freestream
velocity of 328,000. These simulation results were compared to experimental and newly
developed models for validation. The ultimate goal of this present study was to create the
two-point space time correlation function of a cylinder and airfoil wake using RANS
calculations which contributes to a larger study where the sound radiated by an open rotor
due to ingestion of turbulence.

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.

This dissertation will consider the sound radiation from forward-facing steps and a three dimensional cylindrical embossment of very low aspect ratio mounted on a plate. Glegg et al (2014) outlined a theory for predicting the sound radiation from separated flows and applied the method to predicting the sound from forward-facing steps. In order to validate this theory it has been applied to the results of Catlett et al (2014) and Ji and Wang (2010). This validation study revealed that the original theory could be adjusted to include a mixed scaling which gives a better prediction. RANS simulations have been performed and used to support the similarities between the forward-facing step and the cylindrical embossment. The simulations revealed that the cylindrical embossment exhibits a separation zone similar to that of the forward-facing step. This separation zone has been shown to be the dominant source of noise on the forward-facing step in previous works and therefore was expected to be the major source of sound from the cylindrical embossment. The sensitivity of this separation zone to the different parameters of the flow has been investigated by performing several simulations with different conditions and geometries. The separation zone was seen to be independent of Reynolds number based on boundary layer thickness but was directly dependent on the height of the cylinder. The theory outlined in Glegg et al (2014) was then reformulated for use with a cylindrical embossment and the predictions have been compared with wind tunnel measurements. The final predictions show good agreement with the wind tunnel measurements and the far-field sound shows a clearly defined directionality that is similar to an axial dipole at low frequencies.

The numerical method presented in this study attempts to predict the mean, non-uniform flow field upstream of a propeller partially immersed in a thick turbulent boundary layer with an actuator disk using CFD based on RANS in ANSYS FLUENT. Three different configurations, involving an infinitely thin actuator disk in the freestream (Configuration 1), an actuator disk near a wall with a turbulent boundary layer (Configuration 2), and an actuator disk with a hub near a wall with a turbulent boundary layer (Configuration 3), were analyzed for a variety of advance ratios ranging from J = 0.48 to J =1.44. CFD results are shown to be in agreement with previous works and validated with experimental data of reverse flow occurring within the boundary layer above the flat plate upstream of a rotor in the Virginia Tech’s Stability Wind Tunnel facility. Results from Configuration 3 will be used in future aero-acoustic computations.

For aerospace and naval applications where low radiated noise levels are a
requirement, rotor noise generated by inflow turbulence is of great interest. Inflow
turbulence is stretched and distorted as it is ingested into a thrusting rotor which can have
a significant impact on the noise source levels. This thesis studies the distortion of
subsonic, high Reynolds number turbulent flow, with viscous effects ignored, that occur
when a rotor is embedded in a turbulent boundary layer. The analysis is based on Rapid
Distortion Theory (RDT), which describes the linear evolution of turbulent eddies as they
are stretched by a mean flow distortion. Providing that the gust does not distort the mean
flow streamlines the solution for a mean flow with shear is found to be the same as the
solution for a mean potential flow with the addition of a potential flow gust. By
investigating the inflow distortion of small-scale turbulence for various simple flows and
rotor inflows with weak shear, it is shown that RDT can be applied to incompressible
shear flows to determine the flow distortion. It is also shown that RDT can be applied to more complex flows modeled by the Reynolds Averaged Navier Stokes (RANS)
equations.

The Federal Highway Administration's development of a new highway noise prediction model (TNM) necessitated the collection of equivalent source height data on moving highway vehicles as a function of frequency. An original method developed by Glegg and Yoon was used in the initial collection of this data. Analysis of this data indicated the measured source height was overestimated at frequencies below 500 Hz. In order to improve the equivalent source height estimates below this frequency two alternative methods were investigated. The first method made use of the coherence function to remove noise from the autospectral density estimate for an array element through the cross spectra of two other elements. This method is called the noise extraction method and was found to be ineffective for this application. However, the second method used matched-field processing, and a significant improvement in the estimated equivalent source heights was achieved for frequencies below 500 Hz.

Aircraft engine fan trailing edge noise prediction is very challenging. To achieve a better understanding of the physics of the propagation problem, the fan has been modeled as an infinite cascade of blades and acoustic monopoles and dipoles have been placed at the trailing edges. The flow has been computed using the Transonic Small Disturbance equation. As soon as the critical Mach number is exceeded by the free stream, a supersonic region that joins two consecutive blades appears. It completely blocks the sound and limits the study to entirely subsonic flow. In this type of flow, a sound propagation simulator has been implemented. The linearized form of Howe's equation is solved by a high frequency method. The ray caustic problem which causes regular ray tracing failure is fixed by interpolating the field on a preset grid. Results are compared with the analytical solution in uniform flow and computations in realistic flow are presented.

This thesis describes a series of measurements that took place over the duration of one year on the South Florida Testing Facility (SFTF) range, Dania, in order to survey the shallow water ambient acoustic environment. Three groups of data sets were taken in December 1998, May 1999 and July 1999. The data was collected using the Ambient Noise Sonar (ANS) that was developed in the Ocean Engineering department at Florida Atlantic University. The ambient acoustic environment was found to be highly variable with two main components. Boat noise was found to primarily be associated with the inlet and a source of snapping shrimp situated on the shallow water 30ft reef became apparent through 24hr observation. The effects of adverse weather were not studied due to the absence of this source during the measurement periods.

Over recent years, the trend in Autonomous Underwater Vehicle (AUV) design has been to reduce vehicle size and cost. On board navigation systems are both large and expensive so alternate solutions for vehicle positioning are required. The thesis explores the performance of a passive platform, the Ambient Noise Sonar (ANS), in remotely detecting, localizing and tracking submersible vessels. This task is achieved by exploiting communication signatures emitted by the moving submersible. The utility modem integrated on the AUV can be operated in a PSK and a MFSK mode. It was demonstrated that the ANS successfully tracks AUVs in both cases. First, the thesis presents the sonar beamformer and shows its potential for tracking by using the AUV communication signals. It describes a scheme developed to enhance the processor performance in a multi-target configuration and clutter. Then, it discusses promising tracking results from experiments conducted in summer and fall 1998, off the coast of South Florida.

The interaction of rotor turbulence with the stator is currently believed to be the predominant mechanism of noise radiation from turbofans in aircraft engines. This thesis presents a general method to compute unsteady 2-D potential flows past a cascade of airfoils. The procedure uses source and vortex distributions on the surface of the airfoils, creation of wakes downstream of the airfoils and non-linear convection of the perturbed flow. These features are designed to satisfy a condition of no-flow through the surface of the airfoils and the Kutta condition at the trailing edge of each of these airfoils. The investigation proves the importance of applying the Kutta condition. It was also shown that an infinite cascade is well approximated by a small number of airfoils and that the non-linear rather than linear convection of vorticity has a large effect on the spectrum of the unsteady lift of an airfoil.