An experimental investigation of unsteady surface pressure on single and multiple airfoils

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&feet; chord and spans the 6&feet; 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, a 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 $wr=wbU\infty$ < 10 a reduction in unsteady lift of up to 7 decibels (dB) occurs while an increase occurs for ω_r > 10 as the angle of attack is increased. The reduction in unsteady lift at low ω _r 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.* (Abstract shortened by UMI.); *This dissertation is multimedia (contains text and other applications not available in printed format). The accompanying CD requires the following system requirements: Windows 95 or higher; Adobe Acrobat; Windows MediaPlayer or RealPlayer.