This dissertation presents measurements of the far-field noise associated with the interaction of grid-generated turbulence with a series of airfoils of various chord lengths, thicknesses, and camber. The radiated noise was measured for a number of angles of attack for each airfoil to determine the effects of angle of attack on the leading edge noise. Measurements are compared with numerous theories to determine the mechanism driving the production of leading edge noise. Calculations were also made using a boundary element method to determine the effects of airfoil shape on the unsteady loading spectrum on the different airfoils to attempt to explain the far-field noise. Measurements of the unsteady surface pressure on a single airfoil were made for a number of angles of attack to determine the effects of wind tunnel interference corrections on the unsteady surface pressure. These measurements were compared with those of Mish (2003) to determine the effects of the interference correction. An attempt was also made to correlate the unsteady loading on the airfoil with the far-field noise. The airfoils studied were a 0.203-m chord NACA 0012, a 0.61-m chord NACA 0015, a 0.914-m chord NACA 0012, a 0.914-m chord DU96, and a 0.914-m chord S831. All airfoils spanned the entire 1.83-m height of the test section. Measurements were made using the Virginia Tech Stability Wind Tunnel in its acoustic configuration with an anechoic test section with side walls made of stretched Kevlar fabric to reduce aerodynamic interference. Measurements were made in grid-generated turbulence with an integral length scale of 8.2-cm and a turbulence intensity of 3.9%. Far-field noise measurements were made at Mach numbers of 0.087 and 0.117 with various configurations of up to 4 Bruel and Kjaer microphones mounted at an observer angle of 90° measured from the wind tunnel axis. Unsteady surface pressure measurements were made on the NACA 0015 airfoil immersed in the same grid generated turbulence used in the far-field noise study. An array of microphones mounted subsurface along the airfoil chord and a spanwise row was used to measure the unsteady surface pressure. These measurements were made at angles of attack from 0 through 16° in 2° increments. Far-field noise measurements of the leading edge noise show a consistent angle of attack effect. The radiated noise increases as the angle of attack is increased over the frequency range. These effects are small for large integral scale to airfoil chord ratios. The larger airfoils have been shown to generate significantly less leading edge noise at high frequencies, but this effect does not appear to be solely dependent upon the leading edge radius. The leading edge noise can be predicted with accuracy using the method of Glegg et al. (2008). Unsteady surface pressure measurements have been shown to be largely independent of the wind tunnel interference correction as shown by comparison with Mish (2008). The same low frequency reduction described by Mish was seen for an interference correction that was nearly 30% larger. The unsteady sectional lift spectra have been shown to be related to the far-field noise spectra by a factor close to the dipole efficiency factor; however, no correlation could be found between the instantaneous unsteady surface pressure and the radiated noise. The spanwise averaged unsteady pressure difference spectra have been shown to be related to the far-field noise spectra by the dipole efficiency factor. / Ph. D.

This dissertation presents measurements of unsteady surface pressure on airfoils encountering flow disturbances. Analysis of measurements made on an airfoil immersed in turbulence and comparisons with inviscid theory are presented with the goal of determining the effect of angle of attack on an airfoils inviscid response. Unsteady measurements made on the surface of a linear cascade immersed in periodic flow are presented and analyzed to determine the relationship between the blades inviscid response and tip leakage vortex strength. Measurements of fluctuating surface pressure were made on a NACA 0015 airfoil immersed in grid generated turbulence. The airfoil model has a 2' chord and spans the 6' Virginia Tech Stability Wind Tunnel test section. Two grids were used to investigate the effects of turbulence length scale on the surface pressure response. A large grid which produced turbulence with an integral scale 13% of the chord and a smaller grid which produced turbulence with an integral scale 1.3% of the chord. Measurements were performed at angles of attack from 0 to 20. An array of microphones mounted subsurface was used to measure the unsteady surface pressure. The goal of this measurement was to characterize the effects of angle of attack on the inviscid response. Lift spectra calculated from pressure measurements at each angle of attack revealed two distinct interaction regions; for reduced frequencies < 10 a reduction in unsteady lift of up to 7 decibels (dB) occurs while an increase occurs for reduced frequencies > 10 as the angle of attack is increased. The reduction in unsteady lift at low reduced frequencies with increasing angle of attack is a result that has never before been shown either experimentally or theoretically. The source of the reduction in lift spectral level appears to be closely related to the distortion of inflow turbulence based on analysis of surface pressure spanwise correlation length scales. Furthermore, while the distortion of the inflow appears to be critical in this experiment, this effect does not seem to be significant in larger integral scale (relative to the chord) flows based on the previous experimental work of McKeough (1976) suggesting the airfoils size relative to the inflow integral scale is critical in defining how the airfoil will respond under variation of angle of attack. A prediction scheme is developed that correctly accounts for the effects of distortion when the inflow integral scale is small relative to the airfoil chord. This scheme utilizes Rapid Distortion Theory to account for the distortion of the inflow with the distortion field modeled using a circular cylinder. Measurement of the unsteady surface pressure response of a linear cascade in periodic disturbance is presented. Unsteady pressure was measured on the suction and pressure side of two cascade blades with an array of 24 microphones (12 per blade side) mounted subsurface. The periodic disturbance was generated using a pair of vortex generators attached to a moving end wall. Measurements were made for 8 tip gaps (t/c = 0.00825, 0.0165, 0.022, 0.033, 0.045, 0.057, 0.079, 0.129) and phased averaged with respect to the vortex generator pair position. This measurement was motivated by the results presented by Ma (2003). The work of Ma (2003) suggested that tip leakage vortex shedding in the presence of a periodic disturbance is heavily influenced by the inviscid response of the cascade blade. This conclusion was arrived at by Ma's (2003) observation that as the tip gap is increased the amount of fluctuation in the tip leakage vortex circulation increases dramatically, in fact, many times the circulation in the inflow vortices. Unsteady pressure measurements reveal that the blade response involves a complex interaction of both inviscid response and viscous phenomena. However, a close relationship between unsteady tip loading and tip leakage vortex circulation is revealed suggesting the inviscid response is significant in determining the tip leakage vortex circulation. Additionally, predictions using inviscid theory agree well with measured levels of unsteady tip loading. As such, inviscid theory may be useful for predicting the tip leakage circulation and perhaps, pressure fluctuations in the tip leakage vortex. / Ph. D.