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This paper uses a discrete-vortex code to examine a shear layer's response to forcing at its origin and to develop a relationship between a shear layer's optical characteristics and the commonly used characteristic growth length, vorticity thickness. 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. A weighted-average natural frequency is introduced and used to characterize the unforced shear layer in terms of an optical characteristic length referred to as optical coherence length. It is shown that optical coherence length is related to vorticity thickness by a factor of approximately 3.18. The study also shows that the use of single-frequency forcing produces a regularized shear layer for distances preceding the point at which the unforced shear layer's natural frequency matches the forcing frequency. 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. DOI: 10.2514/1.34244

This paper presents the aero-optical environment around a generic conformal-window turret formed from a hemisphere on a short cylindrical base. A suite of optical instruments consisting of a Malley probe, a conventional two-dimensional Shack-Hartmann wave-front sensor, and a new high-bandwidth, lower-resolution Hartmann wave-front sensor were used to measure the aberrations on the wave front of a laser beam emanating from the turret at various angles in both the forward and aft direction in the turret's zenith plane. The measurements were made over a range of Mach numbers from 0.35 to 0.45. Complementary steady- and unsteady-pressure measurements over a slightly larger range of Mach numbers were also made, along with a surface-flow-visualization study of the complex flowfield over and around the turret. The use of the suite of sensors allowed for the recognition and separation of the aberrating optical environment into components associated with stationary disturbances and convecting disturbances at the frequency of the turret's separated wake and at order-of-magnitude-higher frequencies associated with structures that form in the separated shear layers, respectively. The optical data separated in this way are valuable because of the implications for adaptive optics.

An extensive investigation of the aberrating character of flow over a hemisphere-on-cylinder turret with a flat window was performed. Optical distortions over the window were measured using a two-dimensional wave front sensor and a Malley probe. The Malley probe measurements were complemented with simultaneous hot-wire measurements of the velocity field normal to the window at several points across its diameter. The tests were run for a fixed elevation for several azimuthal angles over a range of Mach numbers. The results provide the levels of unsteady optical aberration across the window, as well as the local thickness, intensity, and convective speed of the separated flow over the window. Results reveal that the optical distortions grow approximately with the square of the incoming Mach number multiplied by the freestream density, [OPD.sub.rms] ~[rho][M.sup.2], as well as with the changing character of the aberrating flow with azimuth angle. DOI: 10.2514/1.24468

In the typical analysis of aero-optical wave-front data, the three lowest-order spatial modes (namely, unsteady piston as well as X and Y tilts) are removed from the experimentally measured wave fronts. These modes are commonly corrupted by mechanical disturbances. In this work, an algorithm called the stitching method was developed that takes advantage of the advective nature of the optical aberrations caused by turbulent structures to recover the unsteady global X-tilt and piston modes from experimental time-resolved wave fronts. One-dimensional modeling and related uncertainty analysis showed that for the wave fronts collected with sufficient sampling frequency, the algorithm is able to correctly recover the aero-optical component of the unsteady X tilt. In this manner, the time series of true wave fronts can be recovered. To further validate the stitching method, spatiotemporal wave-front measurements were conducted on a Mach 0.6/0.1 forced shear layer. The predicted results for the rms of the aero-optical X tilt from the stitching method agree well with the modeled results. Since the stitching method recovers the time series of the aero-optical global X tilt, the global tilt spectra were also computed and presented. This information can be used by system designers to specify the requirements for adaptive-optics system components, such as fast steering mirrors in airborne directed energy systems. [ABSTRACT FROM AUTHOR]

The dynamic response of a zero-pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally investigated. The experimental TBL had a sufficiently low momentum thickness Reynolds number, such that there were no energetic organized large-scale structures in the outer region, as evidenced from measurements of the premultiplied wavenumber-frequency spectra. The periodically pulsed plasma actuator, placed inside the outer region of TBL, introduced a synthetic large-scale structure, and the boundary-layer response to this synthetic structure downstream of the actuator at select wall-normal and streamwise locations was investigated. The turbulence amplitude modulating effect of the synthetic large-scale structure, within the inner and log-linear regions of the boundary layer, was isolated and analyzed using a phase-locked analysis. The dynamic interaction of the synthetic large-scale structure and smaller-scale turbulent motions was quantified using a modulation coefficient, and a strong positive correlation within the inner and log regions of the boundary layer was measured. The streamwise development of the synthetic large-scale structure and its modulating effect on the near-wall turbulence across several streamwise locations are described. Profiles of the phase speed at these streamwise locations were extracted and were found to be constant within the log region, further confirming a strong coupling between the near-wall fluctuations in turbulence intensity and the synthetic large-scale motions introduced in the outer region. Overall, the turbulence amplitude modulation effect induced by the synthetic large-scale structure was found to be dynamically similar to the large-scale modulation measured in canonical TBLs at higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Aero-optical measurements of a zero-pressure-gradient, supersonic boundary layer along the test-section wall at M = 2.0 were performed using a Malley probe. The Malley probe captured both the amplitude of optical distortions and the convective speed. The convective speed of the optically active structures inside the supersonic boundary layer was found to be 0.84 of the freestream speed. The deflection-angle spectra were found to collapse with the local displacement thickness. The streamwise correlation function for the supersonic boundary layer revealed the presence of a pseudoperiodic structure with the typical size of 1.5 of the local boundary-layer thickness. A new model was developed to describe aero-optical effects of both the subsonic and the supersonic boundary layers. Finally, this new model and several other theoretical scalings were tested in the attempt to collapse both subsonic and supersonic boundary-layer aero-optical results. [ABSTRACT FROM AUTHOR]

Wall-modeled large-eddy simulations of turbulent boundary-layer flows over a flat plate at Mach 3.5, 7.87, and 13.64 were carried out, and the aero-optical distortions resulting from density fluctuations were investigated. The conditions for the Mach 3.5 case match those of a direct numerical simulation in the literature. The Mach 7.87 conditions are representative of the Hypersonic Wind Tunnel at Sandia National Laboratories, and the Mach 13.64 conditions match those of the Arnold Engineering Development Complex Hypervelocity Tunnel 9. The wall-modeled large-eddy simulations were validated using available experimental and DNS reference data. The normalized root-mean-square optical path difference for all three cases is in good agreement with respective data obtained from reference direct numerical simulations and experiments (within 1.53% for M∞=3.5, 1.25% for M∞=7.87, and 12% for M∞=13.64, compared to numerical data). Above M∞=5.0, the normalized path difference obtained from the simulations is above the value predicted by a semi-analytical relationship by Notre Dame University. With increasing Mach number, the bulk contribution to the total optical distortion shifts toward the boundary-layer edge. In addition, a proper orthogonal decomposition reveals that the nature of the dominant density fluctuations, which are located near the boundary-layer edge, changes from three-dimensional to two-dimensional. Understanding how the coherent structures contribute to the optical path difference may pave a way toward devising active flow control strategies geared toward a reduction of the optical path difference. [ABSTRACT FROM AUTHOR]

This paper develops a spatial input-output approach to investigate the dynamics of a turbulent boundary layer subject to a localized single frequency excitation. One-way spatial integration is used to reformulate the problem in terms of spatial evolution equations. The associated input-output operator is then used to examine the effect of localized periodic actuation at a given temporal frequency, based on an experimental setup in which an active large scale structure is introduced into the outer layer of a turbulent boundary layer. First, the large-scale structures associated with the phase-locked modal velocity field obtained from spatial input-output analysis are shown to closely match those computed based on hot-wire measurements. The approach is then used to further investigate the response of the boundary layer to the synthetically generated large-scale structures. A quadrant trajectory analysis indicates that the spatial input-output response produces shear stress distributions consistent with those in canonical wall-bounded turbulent flows in terms of both the order and types of events observed. The expected correspondence between the dominance of different quadrant behavior and actuation frequency is also observed. These results highlight the promise of a spatial input-output framework for analyzing the formation and streamwise evolution of structures in actuated wall-bounded turbulent flows. [ABSTRACT FROM AUTHOR]