This paper examines the effect of a finite aperture on wavefronts, especially with regard to aero-optic distortions from a shear layer. When the net deflection of a beam is corrected in real time to bring a beam on target, a common practice in optic applications even if no other corrections are performed, this removal of the off-target tilt and the finite aperture of the beam act as a spatial filter. This restricts the tip-tilt correction and the wavefront correction to separate frequency ranges, causing the rms magnitude of the remaining distortion to be corrected to vary with aperture size. It has also been found that for a shear layer, aperture size is a key factor in scaling the severity of optical distortions, and it is likely to be a key component in scaling other types of optically-aberrating flows.
This paper uses a discrete-vortex code to examine a shear layer's response to forcing at its origin. The code and its thermodynamic overlay have been used in previous studies to predict the optically-aberrating characteristics of relatively-high-Mach-number, subsonic shear layers that can be classified as weakly compressible. The results reported in this study are again directed toward the shear layer's optical characteristics; however, the intent was to use forcing to create periodic aberrating fields, referred to as "regularized" shear-layer aberrations. The study shows that the use of single-frequency forcing produces a regularized shear layer for distances preceding the point where the unforced shear layer's natural frequency occurs. In the case of the forced shear layer, a greater thickness is produced closer to its point of origin until collapsing onto the unforced shear layer thickness past the point of regularization. The aberration periodicity is shown to have lower robustness toward the furthest downstream extent of regularization due to uncontrolled pairing. This region is made more regular by applying both fundamental and subharmonic forcing at the shear layer's origin; however, such subharmonic forcing is sensitive to the phasing of the fundamental to that of the subharmonic.
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