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Shock waves will form by turning supersonic or locally supersonic flow and result in an increase in the freestream density downstream of the shock. This increase leads to optical distortions that limit the effectiveness of aircraft-mounted laser systems. In this paper, analytic expressions are developed to describe these optical distortions in terms of the optical-path difference (OPD). Pupil-plane disturbances imposed by the shock are studied for two cases: when the shock is parallel to the propagation direction and when the shock is on an angle relative to the propagation direction. Upon propagation from the pupil plane, the analysis shows that shock-induced phase discontinuities can sometimes cause the irradiance pattern in the image plane to bifurcate. Despite a large amount of tilt in the pupil plane, the bifurcated irradiance pattern does not map to a proportional shift in the image plane. The implications that these findings have on Shack–Hartmann wavefront sensor (SHWFS) data are also explored. The results show that least-squares reconstruction from the SHWFS data yield accurate estimates of the change in OPD across the shock when the magnitude of the phase difference [Formula: see text] caused by the shock is between 0 and approximately [Formula: see text]. However, when [Formula: see text], the results show that least-squares reconstruction begins to severely underestimate the change in OPD across the shock. Such results will inform future efforts looking to develop aircraft-mounted laser systems.

A series of flight experiments have demonstrated a novel approach to measuring path-resolved optical turbulence quantities, such as Cn2, along an air-to-ground slant path. This paper describes the ...

The work presented here experimentally measures the tilt imposed on a laser beam by the atmosphere from Shack–Hartmann wavefront sensor measurements collected in-flight. Tip/tilt is imposed on the laser beam by propagating through optical turbulent structures larger than or of the order of the size of the beam diameter. This tip/tilt causes a dynamic, net deflection of the beam in the far field, referred to as jitter, which poses a serious problem for tracking in directed energy applications. The practical measurement of turbulence-induced tip/tilt at altitude is challenging since mechanical contamination in the form of vibrations also manifests as tip/tilt. In this paper, a procedure referred to as the stitching method is used to quantify the turbulence-induced component of tilt without the influence of mechanical corruption. It is found that the measured tilt aligns with what analytic solutions predict and that the turbulent environment through which the beam propagates has Kolmogorov-like characteristics.

Spatially and temporally resolved wave fronts were collected in flight using Airborne Aero-Optics Laboratory at varying subsonic Mach numbers through both the naturally developing boundary layer an...

Surface pressure measurements were taken on a hemisphere-on-cylinder turret in a wind tunnel using pressure sensitive paint and fast response pressure transducers. Four different turret protrusion distances were tested to study the characteristics of the unsteady pressure field on the backside and wake of the turret. Proper orthogonal decomposition was used to identify the dominant spatial surface pressure modes acting on the turret in this parametric study. It was found that the further the turret protruded into the freestream flow, the more the surface pressure field became dominated by spanwise antisymmetric surface pressure distributions resulting from anti-symmetrical vortex shedding at a normalized frequency of approximately StD=0.2. For the case of the partial hemisphere, this anti-symmetrical vortex shedding was essentially absent, insinuating that at some protrusion distance, the surface pressure environment on the turret fundamentally changes. The normalized net force rms was calculated on the turret for each configuration. It was found that the greater the turret protrusion, the greater the net force acting in the spanwise direction.

Optical measurements of a hemispherical turret were conducted in both a wind tunnel and airborne testing environment to measure aero-mechanical jitter imposed onto a laser beam. A hemispherical turret was positioned in the freestream flow at various protrusion distances, Mach numbers, and azimuthal angles. Lasers and accelerometers were used to quantify the mechanical contamination imposed onto the beam due to the fluid-structure interaction of the incoming freestream flow and the protruding turret body. The results from wind tunnel and in-flight testing were compared. It was shown that the wind tunnel and in-flight tests yielded different results both quantitatively and qualitatively. The possible reasons for the discrepancies between these testing campaigns were also discussed.

In this paper, atmospheric optical turbulence strength is estimated for realistic airborne environments using a modified phase-variance approach, as well as a modified slope-discrepancy approach. Realistic airborne environments are generated using wave-optics simulations of a plane wave propagating through increasing strengths of homogeneous atmospheric optical turbulence, both with and without aero-optical contamination (from in-flight wavefront sensor data) and additive-measurement noise. In comparison to the modified phase-variance approach, the results show that the modified slope-discrepancy approach more accurately estimates atmospheric optical turbulence strength over a wide range of conditions. Such results are encouraging for realistic airborne environments because they can be scaled to different freestream conditions as long as the boundary layer is considered canonical.

High speed time-resolved wavefront and imaging measurements were taken synchronously in-flight through both boundary layer and shear layer environments around the Airborne Aero-Optical Laboratory for Beam Control. Instantaneous modulation transfer functions and point spread functions (PSFs), which characterize image degradation, were generated using wavefront data. Instantaneous power-in-bucket ratios were extracted from both the image data and computed from the wavefront data, and the ratios were found to correlate well with each other. The lower power-in-bucket values and related increased blurring that occurred predominantly in the streamwise direction were associated with large-scale, large-amplitude wavefront spatial variations due to large organized vortical structures present in the shear layer. The boundary layer did not create any significant image blurring due to the low level of aero-optical distortions. Finally, spatial autocorrelation functions were extracted from the wavefront data using the stitching method and were used to compute time-averaged PSFs for different aperture diameters.

High speed time-resolved wavefront and imaging measurements were taken synchronously in-flight through the boundary layer and the shear layer environments around the Airborne Aero-Optical Laboratory for Beam Control (AAOL-BC). Instantaneous point spread functions were generated using wavefront data which enabled a relationship between largescale structures present in turbulent flows and resultant instantaneous image degradation to be identified. In this manner, image blurring patterns can be related to specific flow structures which begins to abridge the knowledge gap between treating quantitative wavefront properties and the resultant blurred images.