Edward J. Fitzgerald

Recent advances in aero-optics

Abstract The passage of a planar-wavefront optical beam through a turbulent, index-of-refraction-variant flowfield imprints dynamically changing aberrations on the wavefront; these aberrations have detrimental effects on the performance of optical systems making use of the beam. The study of these aberrations (when the aberrating flowfield is of relatively short propagation length in the near field of the optical system) is referred to as aero-optics. This article begins with a brief discussion of the traditional approach to quantifying aero-optic interactions. It then reviews how the development of high-speed wavefront sensors over the past 10 years has impacted the fluid-dynamics and optics fields. In particular, the fluid-dynamic mechanisms creating optical distortions are now better understood, and successful aero-optic scaling laws have been developed. The article concludes with a description of future trends in aero-optical research.

The optical distortion mechanism in a nearly incompressible free shear layer

The aero-optical distortions caused by compressible flows have been used by researchers for flow diagnostics and accepted by designers of airborne optical systems as a performance penalty. In order to estimate these distortions, an understanding of the optical distortion mechanism is required. This article examines the mechanisms which produce a variable-density field (and accompanying index-of-refraction field) in a nearly incompressible shear-layer flow. The two-dimensional-shear-layer velocity field was approximated using a discrete vortex model. From this ‘known’ velocity field, the pressure and density fields were determined by iteratively solving the unsteady Euler equations. The resulting index-of-refraction field produced simulated schlieren images which closely resemble experimental schlierens. Optical wavefronts computed from the simulation reasonably match the behaviour of large-scale aberrations measured in a transonic wind tunnel. Small-scale distortions in the experimental data may have been caused by boundary layers on the splitter plate and tunnel walls or by three-dimensional effects that were not simulated.

Measurements in a separation bubble on an airfoil using laser velocimetry

An experimental investigation was conducted to measure the reverse flow within the transitional separation bubble that forms on an airfoil at low Reynolds numbers. Measurements were used to determine the effect of the reverse flow on integrated boundary-layer parameters often used to model the bubble. Velocity profile data were obtained on an NACA 663-018 airfoil at angle of attack of 12 deg and a chord Reynolds number of 140,000 using laser Doppler and single-sensor hot-wire anemometry. A new correlation is proposed based on zero velocity position, since the Schmidt (1986) correlations fail in the turbulent portion of the bubble.

Aperture Effects on the Aerooptical Distortions Produced by a Compressible Shear Layer

A recent paper presented time-series measurements of optical wave front distortions produced by a weakly compressible shear layer using the small-aperture beam technique (SABTI. These researchers used a posttest, high-pass, digital filter to remove vibrational noise from their beam-jitter signals, choosing a corner-frequency compatible with their 5-cm test aperture. A reexamination of their raw data is documented, and the proper corner-frequency selection for vibration-corruption removal is systematically treated. The resulting SABT wave front reconstructions recaptured the structures previously reported, but now reveal large-amplitude, time-varying, tilt aberrations not previously of interest to 5-cm-aperture optical-system applications. These tilt aberrations are indicative of large-scale flow structures with a spatial scale larger than the test aperture. This conclusion is supported by the results of a recently published numerical simulation. Experimental evidence is presented that suggests that the measured small-scale distortions may have been caused by temperature discontinuities in the splitter-plate boundary layer that fed into the shear layer

Optical Characterization of the Notre Dame Compressible Shear-Layer Facility

This paper presents the first aero-optical measurements made in the Notre Dame WeaklyCompressible Free Shear Layer Facility. Wavefront measurements were made using the small-aperture beam technique for optical propagation through the shear layer at a distance of 0.5 m downstream from the splitter plate. These wavefronts are the first independent, time-resolved, aero-optical measurements made for propagation through a high-Mach-number shear layer since the measurements made at the Arnold Engineering Development Center in the mid 1990’s. The character of the new wavefronts is similar to that of the AEDC experiments. Further, the wavefronts are in relatively good agreement with those predicted using the Weakly Compressible Model.

Scaling Aerooptic Aberrations Produced by High-Subsonic-Mach Shear Layers

An index-of-refraction model has been shown to produce a reasonable estimate of the aerooptical aberrations measured in a weakly compressible shear layer. This numerical model is a valuable tool both for its insight into the cause of the aberrations and as a source of the parametric data needed to explore and evaluate scaling laws. Scaling relations are developed that describe how the expected aerooptical distortion due to propagation through a free shear layer changes with altitude, that is, static pressure, and/or static temperature. Scaling laws for two cases are developed. In the e rst case, the Mach numbers of the shear layer’ s constituent e ows are held constant. For the second case, the shear layer’ s constituent e ow velocities are instead held constant. The parametric data are also used to evaluate a scaling law suggested by the literature.

Far-field implications of laser transmission through a compressible shear field

Progress has recently been made in both the collection and modeling of fluid-optic disturbances imparted by compressible, shear flows. This field of research, termed Aero-Optics, has been motivated primarily by the development of directed-energy weapons; however, it is equally applicable to a general class of applications involving laser transmission and reception from aircraft. Examples of these-type applications include free-space, laser transmission from ground to air, from air to air and from air to space. In the present paper, we examine the effect of laser transmission through high-Mach-number, subsonic, compressible free shear layers on the ability to focus the beam on distant targets. Time-resolved time series of distorted wavefronts due to propagation through a Mach-0.8 free shear layer collected at the Aero-Optics facility at Arnold Engineering and Development Center, are used as the input to a Fourier-Optics routine that computes time series of far-field irradiance patterns. These patterns are then used to compute the time-averaged Strehl ratio directly, and these are compared to time-averaged Strehl ratios computed using the wavefronts rms Optical Path Differences and the large-aperture approximation. Conclusions are drawn about the appropriateness of using the large-aperture approximation for Aero-Optic-type wavefront aberrations.

Two-dimensional optical wavefront measurements using a small-aperture beam technique derivative instrument

Results obtained from a new, 600-Hz, 624-subaperture, two- dimensional optical wavefront sensor for fluid-optic interaction applica- tions are presented. This instrument can operate at 1 kHz as currently configured and is extendable to 8 kHz with a minor modification. The new sensor uses the small-aperture beam technique for measurements in the streamwise direction, and scanned beams for cross-stream wave- front measurements. Wavefront aberrations produced by a low-speed, heated mixing layer are presented, and the accompanying measurement uncertainty is evaluated.