Matthew R. Whiteley

Airborne Aero-Optics Laboratory

Abstract. We provide a background into aero-optics, which is the effect that turbulent flow over and around an aircraft has on a laser projected or received by an optical system. We also discuss the magnitude of detrimental effects which aero-optics has on optical system performance, and the need to measure these effects in flight. The Airborne Aero-Optics Laboratory (AAOL), fulfills this need by providing an airborne laboratory that can capture wavefronts imposed on a laser beam from a morphable optical turret; the aircraft has a Mach number range up to low transonic speeds. We present the AAOL concept as well as a description of its optical components and sensing capabilities and uses.

Aero-Optical Evaluation of Notional Turrets in Subsonic, Transonic and Supersonic Regimes

The aero-optical performance of two notional turret designs, a hemisphere-on-plate and a submerged hemisphere-on-plate, are evaluated at Mach numbers ranging from purely subsonic, transonic, and supersonic using highly-resolved Detached Eddy Simulation (DES). The resolved density fluctuations in the flow fields are captured to compute the unsteady Optical Path Length (OPL) in propagation paths corresponding to a range of beam bearings and elevations. These OPL are processed to produce aperture-averaged wave front error, which are correlated to the different flow features produced around the turrets. In particular, the results show unsteady Shock Boundary Layer Interactions (SBLI), contact surface rollup and intense wake structures for the supersonic conditions. An unsteady transonic shock structure over the turret is observed at the transonic conditions. Large-scale vortical structures are observed in the wake for all Mach numbers. The effect of these various structures upon aero-optical performance is evaluated by comparing aperture-averaged wave front error for different beam bearings and elevations for the different notional turrets. Finally, the wave front error is correlated with unsteady flow features for each turret geometry.

Dynamic modal analysis of transonic Airborne Aero-Optics Laboratory conformal window flight-test aero-optics

Abstract. We discuss spatial-temporal characterizations of recent in-flight Airborne Aero-Optics Laboratory wavefront measurements at transonic speeds (Mach 0.65) with a conformal window turret as a function of turret pointing angle. Using both proper orthogonal decomposition and dynamic mode decomposition modal analysis methods, the flow dynamics are characterized. The conformal window wavefronts show shock formation between 85 deg and 90 deg and shear layer formation at a considerably lower turret aft pointing angle than would be expected at subsonic speeds without shock. At larger aft pointing angles, shear layer vortex roll-up dynamics dominate the aero-optical disturbances. In particular, the spatially and temporally periodic vortices grow in width and magnitude while the corresponding oscillation frequency drops with increasing look-back angle, thus maintaining a near constant vortex convection speed equal to about 0.6 times the free-stream velocity. From these results, a modified form of the aero-optics frequency scaling relation is proposed that yields a Strouhal number independent of turret look-back angle in the portion of the flow dominated by such Kelvin-Helmholtz shear layer vortices.

The Airborne Aero-Optics Laboratory, AAOL

This paper gives a background into aero-optics which is the effect that turbulent flow over and around an aircraft has on a laser projected or received by an optical system. The background also discusses the magnitude of the detrimental effects that aero-optics has on optical system performance and the need to measure these effects in flight. The Airborne Aero-Optics Laboratory, AAOL, fulfills this need by providing an airborne laboratory that can capture wavefronts imposed on a laser beam from a morphable optical turret; the aircraft has a Mach number range up to low transonic speeds. This paper presents the AAOL concept as well as a description of its optical components and sensing capabilities and uses.

Spatial and temporal characterization of AAOL flight test data

This paper discusses spatial-temporal characterizations of recent in- flight Airborne Aero-Optics Laboratory (AAOL) wavefront measurements at transonic speeds (Mach 0.61 - 0.65) with both flat and conformal window turrets as a function of turret look-back angle. Using both proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) analysis methods, the flow dynamics were characterized. The conformal window wavefronts showed shock formation between 85° and 90° that prematurely induced separation at a considerably lower turret look-back angle than would be expected at subsonic speeds without shock. Vortex shedding for the flat window is initiated by the leading edge of the window at a slightly lower angle than the shock-induced conformal window separation. Nevertheless, the vortex shedding dynamics of both flat and conformal window cases were found to be similar. In particular, the vortices grew in width and magnitude with increasing look-back angle. Furthermore, the corresponding shedding frequency dropped with increasing look-back angle maintaining a near constant vortex convection speed equal to about a third of the platform (free-stream) velocity. From these results a new form of the aero-optics frequency scaling relation is proposed that yields a Strouhal number independent of turret look-back angle in the vortex shedding portion of the flow.

Aero-optical jitter estimation using higher-order wavefronts

Abstract. Wavefront measurements from wind tunnel or flight testing of an optical system are affected by jitter sources due to the measurement platform, system vibrations, or aero-mechanical buffeting. Depending on the nature of the testing, the wavefront jitter will be a composite of several effects, one of which is the aero-optical jitter; i.e., the wavefront tilt due to random air density fluctuations. To isolate the aero-optical jitter component from recent testing, we have developed an estimation technique that uses only higher-order wavefront measurements to determine the jitter. By analogy with work done previously with free-stream turbulence, we have developed a minimum mean-square error estimator using higher-order wavefront modes to compute the current-frame tilt components through a linear operation. The estimator is determined from computational fluid dynamics evaluation of aero-optical disturbances, but does not depend on the strength of such disturbances. Applying this technique to turret flight test data, we found aero-optical jitter to be 7.7±0.8  μrad and to scale with (ρ/ρSL)M2 (∼1  μrad in the actual test cases examined). The half-power point of the aero-optical jitter variance was found to be ∼2u∞/Dt and to roll off in temporal frequency with a power law between f−11/3 and f−10/3.

Temporal properties of the Zernike expansion coefficients of turbulence-induced phase aberrations for aperture and source motion

Zernike polynomials are often used to analyze turbulence-induced optical phase aberrations. Previous investigations have examined the spatial and temporal characteristics of the expansion coefficients of the turbulence-induced optical phase with respect to these polynomials. The results of these investigations are valid only for certain geometries and atmospheric models and do not take into account the effects of relative motion between the sensor and the object of interest. We introduce a generalized analysis geometry and use this aperture-and-source geometry with conventional methods to arrive at a general expression for the inter- aperture cross correlation of the Zernike coefficients. Aperture-and-source motion considerations are introduced to derive an expression for the temporal cross correlation and cross power spectra of these expansion coefficients. Temporal correlation and spectrum results are presented for several low-order Zernike modes, given certain aperture-and-source motions.

Efficacy of predictive wavefront control for compensating aero-optical aberrations

Abstract. Imaging and laser beam propagation from airborne platforms are degraded by dynamic aberrations due to air flow around the aircraft, aero-mechanical distortions and jitter, and free atmospheric turbulence. For certain applications, like dim-object imaging, free-space optical communications, and laser weapons, adaptive optics (AO) is necessary to compensate for the aberrations in real time. Aero-optical flow is a particularly interesting source of aberrations whose flowing structures can be exploited by adaptive and predictive AO controllers, thereby realizing significant performance gains. We analyze dynamic aero-optical wavefronts to determine the pointing angles at which predictive wavefront control is more effective than conventional, fixed-gain, linear-filter control. It was found that properties of the spatial decompositions and temporal statistics of the wavefronts are directly traceable to specific features in the air flow. Furthermore, the aero-optical wavefront aberrations at the side- and aft-looking angles were the most severe, but they also benefited the most from predictive AO.

Influence of aero-optical disturbances on acquisition, tracking, and pointing performance characteristics in laser systems

We have modeled the imaging performance of an acquisition, tracking, and pointing (ATP) sensor when operating on a high-speed aircraft platform through a turreted laser beam director/telescope. We applied standard scaling relations to wavefront sensor (WFS) data collected from the Airborne Aero-Optics Laboratory (AAOL) test platform operating at Mach 0.5 to model aero-optical aberrations for a λ = 1 μm wavelength laser system with a Dap = 30 cm aperture diameter and a 90 cm turret diameter on a platform operating at 30 kft and for speeds of Mach 0.4-0.8. Using these data, we quantified the imaging point spread function (PSF) for each aircraft speed. Our simulation results show Strehl ratios between 0.1-0.8 with substantial scattering of energy out to 7.5× the diffraction-limited core. Analysis of the imaging modulation transfer function (MTF) shows a rapid reduction of contrast for low-to-mid range spatial frequencies with increasing Mach number. Low modulation contrast at higher spatial frequencies limits imaging resolution to > 2× diffraction-limit at Mach 0.5 and approximately 5× diffraction-limit at Mach 0.8. Practical limits to usable spatial frequencies require higher image signal-to-noise ratio (SNR) in the presence of aero-optical disturbances at high Mach number. Propagation of an illuminator laser through these aero-optical aberrations produces intensity modulation in the incident target illumination on scale sizes near the diffraction-limit of the transmitting laser aperture, thereby producing illumination artifacts which can degrade image-contrast-based tracking algorithms.

Measurement of Beacon Anisoplanatism Through a Two- Dimensional Weakly-Compressible Shear Layer

An experimental investigation was conducted into the effectiveness with which aero-optic aberrations imposed on a collimated reference beam could be evaluated using a point-source beacon. The experiments were conducted in the University of Notre Dame’s Compressible Shear-Layer Wind Tunnel which was used to create an optically-active shear-layer flow with high-speed Mach number of 0.8. Anisoplanatic effects included a difference in wavefront shape between the (spherical wavefront) beacon and the (planar wavefront) reference beam, and a difference in the regions of the flow sampled by the beacon and reference beams. A modal compensation approach was used to minimize the anisoplanatism between the beacon and reference wavefronts, which showed that the best compensation results were obtained when the shear layer was regularized using mechanical forcing.