Characterization and Aero-Optic Correction of a Forced Two-Dimensional Weakly Compressible Shear Layer

Phase-Locked-Loop Adaptive-Optic Controller and Simulated Shear Layer Correction

“…The alternative AO controller design, simulations, and preliminary experimental testing documented in this paper are the result of several years of research conducted at the University of Notre Dame [7,[9][10][11]. It has long been known that large-scale structures that naturally "roll up" are the primary cause of optical aberrations in free shear layers [12][13][14].…”

“…Furthermore, it was uncertain whether regularizing the shear layer's large-scale structures would in turn create a more regular, and consequently more periodic, emerging wavefront. As a result, a numerical investigation [9], followed by high-speed shear layer wind-tunnel experiments [10,11], was conducted at the University of Notre Dame. These studies showed that regularizing the large-scale structures in a highspeed shear layer, along with their associated optical wavefronts, was indeed possible.…”

“…High-speed forced shear layer experiments conducted at the University of Notre Dame demonstrated the ability to regularize a region of large-scale structures through mechanical forcing at its origin [11]. Subsequently, Rennie et al were able to successfully show that significant AO corrections could be achieved by applying a feedforward predictive waveform to a forced two-dimensional weakly compressible, shear layer once the corrective waveform was manually synchronized with the regularized large-scale structures [10]. This paper outlines the design, construction, and preliminary experimental testing for a PLL AO controller used to automate the synchronization process referred to previously in which a predictive conjugate correction was synchronized with the forced shear layer's aberrating wavefront.…”

“…A region of regularized, more predictable large-scale structures and optical wavefronts must first be created by applying flow control to the shear layer itself [9][10][11]16,17]. Recent wind-tunnel experiments conducted at the University of Notre Dame have demonstrated the ability to generate such a region in a high-speed shear layer using mechanical forcing applied at the splitter plate [11].…”

“…The high-frequency content and unpredictable nature of this type of flow severely degrades beam quality, adversely affecting system performance. And at present, the resulting high-frequency aberrations imposed on the optical beam as it propagates through a free shear layer cannot be successfully mitigated using the conventional AO approach due to bandwidth limitations caused by system gain requirements and latencies [1,9,10]. As such, research efforts over the past several years at the University of Notre Dame have been investigating an alternative approach to performing AO corrections to a beam experiencing highfrequency disturbances caused by a free shear layer [9][10][11].…”