Survey of Optical Environment over Hemisphere-on-Cylinder Turret Using Suite of Wavefront Sensors

Gordeyev, Stanislav; Post, Laurens Van der; McLaughlin, Thomas; Ceniceros, Juan M.; Jumper, Eric J.

doi:10.2514/6.2006-3074

Cited by 12 publications

(14 citation statements)

References 6 publications

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“…Also, they grow with the incoming Mach numbers and the freestream density as = SL M 2 . A similar trend was observed in other shear-layer-dominated flows [11,14] and provides a very useful scaling law to compare data from different experiments or to extrapolate data to a different flow regime. To perform a correct comparison, a steady-lensing component and an instantaneous tip/tilt were removed in the postprocessing analysis from both the 2-D wave front data and the Malley probe results.…”

Section: Discussionmentioning

confidence: 50%

“…The phase values hover around zero up to 1 kHz and then become linearly increasing with frequencies above 1 kHz. This indicates that jitter data below 1 kHz are due to stationary effects such as vibrations, stationary-lensing, separation-bubble breathing, or far-field effects of the necklace vortex around the base of the turret, discovered for similar turret geometries [11]. No doubt, stationary optical aberrations carry important information about the aberrating character of the flow; however, for the stationary effects, the phase argSf is zero, and the computed convective speeds, as stated previously, are formally infinite.…”

Section: Malley Probementioning

confidence: 99%

“…al. [4] and has now been shown to give extremely accurate measurements of OPD rms [6,8,[10][11][12][13][14]. The instrument consists of two closely spaced, small-aperture (typically, 1_mm) beams (3-10 mm apart) that are aligned, front beam to aft beam, in the streamwise direction (see Fig.…”

Section: Malley Probementioning

confidence: 99%

“…For this study, all stationary effects were filtered from the data. Although a Malley probe offers an accurate measure of the convective component of the aberrating signal, other instruments should be used to investigate the nature of the stationary structures [8,11]. Above 1 kHz, the phase slope is nonzero and its slope can be used to compute the mean convective speed of optically active structures.…”

Section: Malley Probementioning

confidence: 99%

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Aero-Optical and Flow Measurements Over a Flat-Windowed Turret

2007

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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. HEN an otherwise-collimated laser beam passes through a variable-index-of-refraction turbulent flow, its wave front becomes dynamically aberrated (unsteady). These aberrations degrade the beam's ability to be focused in the far field, thereby reducing the system utility of the beam that may be used for communication, interrogation, and targeting or as a directed-energy weapon. When the laser platform is an aircraft, the two main causes of beam degradation are the thin layer and immediate airflow around the aircraft, referred to as the aero-optic problem [1], and the intervening, orders-of-magnitude-longer propagation path through the atmosphere to the target, referred to as the atmosphericpropagation problem. Modern beam-control, adaptive-optic methods appear to now be able to mitigate the atmosphericpropagation effects on the beam; however, both the spatial and temporal bandwidths of the aero-optic problem place it well outside the capabilities of these traditional approaches [2]. It has only been a decade since the first time-resolved wave front measurements for propagation through a relevant aero-optic flowfield were made [3]; before that time, aero-optic propagation environments were characterized by limited time-unresolved interferograms and indirectly inferred from hot-wire anemometry techniques [1,2]. In general, the paucity of such characterizations that were available treated the aero-optic problem as a stochastic problem and reduced the measurements to very unspecific measures of optical degradation such as OPD rms . Such measures, although providing an estimation of the degradation that might be expected, provided little in the way of higher-order information about the aberrating environment's aberration coherence length (spatial bandwidth) and temporal bandwidth over relevant laser-beam apertures. The lack of such characterizations made it impossible to either infer the far-field degradation in the point-spread function or address the requirements for adaptive-optic mitigation schemes.The ability to collect copious spatial and temporal wave front information through relevant aero-optical-type flowfields changed abruptly with the invention by Malley et al.[4] of a new approach to interrogating these fields with a direct optical method that used a single, small-aperture laser beam at a single location in the larger ...

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Section: Discussionmentioning

confidence: 50%

Section: Malley Probementioning

confidence: 99%

Section: Malley Probementioning

confidence: 99%

Section: Malley Probementioning

confidence: 99%

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Aero-Optical and Flow Measurements Over a Flat-Windowed Turret

2007

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“…By application of the adaptive feed-forward control, laser peak irradiance is shown to increase by a factor of 2.5 or more compared to classical AO feedback control. Sampling frequency of an adaptive optics control sensor (Hz) k 1 Gain of adaptive control loop augmentation…”

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confidence: 99%

Adaptive Laser Compensation for Aero-Optics and Atmospheric Disturbances

Whiteley

Gibson

2007

38th Plasmadynamics and Lasers Conference

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Performance of adaptive optics (AO) compensation for beam control is quantified via wave-optical propagation, sensing, and control methods using wind tunnel measurements 1 of aero-optical disturbances in addition to Kolmogorov turbulence distributed over a laser path. For Kolmogorov turbulence the residual phase variance scales as (fG/f 3dB ) 5/3 , where fG is the Greenwood frequency for the propagation path 2 and f 3dB is the error-rejection bandwidth of the classical AO control. It is shown that the residual phase variance with classical AO control, when normalized to the open-loop, scales as (fA/f 3dB ) γ , where γ is an arbitrary power and fA is a characteristic frequency of the aero-optical disturbance determined from a linear fit of the compensation data with increasing bandwidth. AO system latency degrades performance, especially with high-bandwidth control. When operating at a fixed but modest sampling frequency with appreciable latency, AO compensation performance can be significantly enhanced by application of an adaptive control augmentation based on lattice filtering of the residual wavefront sensor gradients, as was implemented in the wave-optics simulations. A classical controller operating at 200 Hz bandwidth with > 400 µsec latency has a limited ability to compensate the aero-optical and free-stream disturbances. By application of the adaptive feed-forward control, laser peak irradiance is shown to increase by a factor of 2.5 or more compared to classical AO feedback control. Strehl ratio degradation associated with wavefront aberrations other than tilt (higher-order)

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