Daniel W. Shannon

Effect of Leading-Edge Thickness on High-Speed Airfoil-Turbulence Interaction Noise

Airfoil-turbulence interaction noise is studied to understand how thickness changes in the neighborhood of an airfoil leading edge can modify noise generation. The broadband interaction noise is generated by placing an airfoil in the turbulent boundary layer of a wind tunnel’s rectangular duct test section. Four airfoil leading edge configurations are tested at four different airfoil positions, relative to the duct exit plane. Local changes in mean flow properties are produced by both modifying leading edge thickness, and translating the airfoil axially to positions internal and external to the duct. In each configuration, the maximum airfoil thickness, camber distribution, and aerodynamic suction surface are unchanged. Leading edge thickness changes are confined to nearly the first ten percent of the airfoil chord, along the airfoil pressure surface only. Numerical results for the mean flow show that when the airfoil is internal to the duct, changes in thickness result in small changes in incidence between the flow and airfoil leading edge. However, with changes in the airfoil axial position, more significant changes in incidence occur, with large flow angles exhibited at the furthest external position. Measurements show that noise is reduced by several decibels as leading edge thickness increases, over a frequency range of 2-4 kHz when the airfoil lies within the duct, and 1-4 kHz when the airfoil lies external to the duct. The maximum noise reduction occurs at frequencies where the reduced frequency, based on leading edge thickness, is order one. The difference in the spectral noise reduction when the airfoil is translated internal versus external to the duct, suggests that changes in leading edge flow incidence may contribute to the noise reduction observed, and that the mechanism for noise reduction is not purely controlled by the relative turbulence scale to leading edge thickness relationship. To examine this hypothesis a simple model for turbulence-airfoil interaction noise that includes thickness effects is compared with data. Results show that agreement between the theory and the measurements are quite good when the airfoil lies within the duct and the incidence is small. When the airfoil lies external to the duct, significant variation between the theoretical model, which does not account for incidence, and measurements exists.

Acoustics of a Dual-jet Circulation Control Elliptical Airfoil

The objective of this research was to complete a comprehensive experimental study of the noise generated by a dual-slotted Circulation Control (CC) airfoil. The sound generated by a CC model was measured utilizing a microphone array for relatively high Reynolds numbers ( ~ 800,000) and low free stream Mach number ( < 0.1). Varied jet velocity and free stream velocity values were tested in order to investigate the magnitude and scaling of the various sound sources. These sources include those generated by both the external flow around the airfoil and the jet flow originating from slots at the trailing edge. The interaction of these to flow fields can generate an additional turbulent source mechanism. Reasonable agreement with the analytical models developed by Howe (2002) is shown for spectral shape as well as the free stream and jet velocity dependence. Further investigation, however, is requires in order to better predict the spectral magnitudes.

Unsteady Lift and Radiated Sound Generated by a 2-D Airfoil in a Single Stream Shear Layer

An airfoil subjected to turbulence in the approach stream radiates sound due to the unsteady lift. Prediction of the spectral density of the unsteady lift requires knowledge of the magnitude and correlation length of the chord normal, or upwash, component of velocity. Additionally, a lift function must be specified based on the geometry of the airfoil. The present research experimentally investigated the sound radiated from a thin airfoil placed in a single stream shear layer. The results indicated a substantial level of agreement between the measured and modeled sound spectra with deviations at low frequencies due to anisotropic turbulence. The high frequency magnitudes were found to match the predictions well only if a thickness correction was applied to the unsteady lift function.

Trailing Edge Noise From Blunt and Sharp Edge Geometries

The objective of this study was to experimentally investigate the broadband trailing edge noise generated by a sharp trailing edge geometry and an asymmetric blunt edge. The flow field in the vicinity of the sharp trailing edge was found to be equivalent to that of a flat plate turbulent boundary layer. The interaction of the two boundary layers with the edge was responsible for broadband noise generation. The blunt trailing edge geometry exhibited additional complexity, with turbulent boundary layer separation and sound generated by vortex shedding. The measurement program included hot-wire anemometry, unsteady surface pressure, and radiated sound utilizing two microphone arrays. The boundary layer parameters and wall pressure spectra were used to compute the radiated sound from existing scattering theory. These calculations agreed very well with the array data, with differences typically within 2dB over the frequency range considered valid for the theory.Copyright