Ronald J. Hugo

Quantification of aero-optical phase distortion using the small-aperture beam technique

We discuss the small-aperture beam technique, a new way of experimentally quantifying the instantaneous optical wave front distortions imposed by propagation through an optically active, turbulent flowfield. We lay out the theoretical basis for the technique and the relationship of the measured jitter of a small-aperture probe beam to optical path difference. A numerical simulation of a two-dimensional heated jet is used to explore the validity of using beam-jitter signals from multiple probe beams to obtain optical path difference in a flow region where eddy production constitutes the major character of the turbulent flowfield.

Time-Resolved Wave Front Measurements Through a Compressible Free Shear Layer

Time series of spatially resolved, aberrated wave fronts distorted by propagation through a compressible shear layer are presented. These wave fronts were measured using the small-aperture beam technique (SABT) applied to the compressible free shear layer facility at Arnold Engineering Development Center. Two viewing stations were investigated at the facility, the ® rst directly downstream of the compressible shear layer’s splitter plate and the second at a location 48.3 cm downstream of the ® rst station. The ® rst station was found to have optically active structures of approximately 2.5 cm in spatial extent, convecting at approximately 160 m/s, and inducing rms aberrationsontheorderof0.06waves.Thesecondstationhadopticallyactivestructuresapproximately5cminsize, also convecting atapproximately 160 m/s, and causing distortions on theorderof0.17waves. TheSABTwasshown to be an effective means of measuring high-bandwidth optical wave fronts in a far from ideal laboratory setting.

Experimental measurement of a time-varying optical path difference by the small-aperture beam technique.

We discuss the use of time series of the jitter angle of multiple, small-aperture probe beams (the small-aperture beam technique, or SABT) as they emerge from a turbulent, optically active flow-field to quantify the time-varying optical path difference (OPD). The flow field studied is that for the transitionally turbulent region of a two-dimensional heated jet. Techniques to construct a complete time series of instantaneous realizations of the OPD are first applied to a numerically generated flow field and then to an experimental flow field. The SABT sensors measurement accuracy is assessed, and its application to flow fields that differ from the numerical heated jet is discussed.

Applicability of the aero-optic linking equation to a highly coherent, transitional shear layer

We investigate the validity of applying a simplified (under the assumptions of isotropic and homogeneous turbulence) aero-optic linking equation to a flow field that is known to consist of anisotropic and nonhomogeneous turbulence. The investigation is performed in the near-nozzle region of a heated two-dimensional jet, and the study makes use of a conditional-sampling experiment to acquire a spatiotemporal temperature field database for the heated-jet flow field. After compensating for the bandwidth limitations of constant-current wire temperature measurements, the temperature field database is applied to the computation of optical degradation through both direct and indirect methods, relying on the aero-optic linking equation. The simplified version of the linking equation was found to provide good agreement with direct calculations, provided that the length scale of the density fluctuations was interpreted as being the integral scale, with the limits of integration being the first two zero crossings of the covariance coefficient function.

Implications of the homogeneous turbulence assumption on the aero-optic linking equation

This paper investigates the validity of applying the simplified (under the assumptions of isotropic and homogeneous turbulence) aero-optic linking equation to a flowfield that is known to consist of anisotropic and nonhomogeneous turbulence. The investigation is performed in the near nozzle-region of a heated two-dimensional jet, and the study makes use of a conditional sampling experiment to acquire a spatio-temporal temperature field data base for the heated jet flowfield. After compensating for the bandwidth limitations of constant-current-wire temperature measurements, the temperature field data base is applied to the computation of optical degradation through both direct methods and indirect methods relying on the aero-optic linking equation. The simplified version of the linking equation was found to provide very good agreement with direct calculations provided that the length scale of the density fluctuations was interpreted as being the integral scale, with the limits of the integration being the two first zero crossings of the covariance coefficient function.

Constant Current Anemometry and Its Impact on Aero-Optical Measurements

however, the measurement hardware has often been expensive and complicated, resulting in the case Paper considers the effects Of constant- where the measurements have been inaccessible to the current wire performance on the estimat,ion of aero- average laboratory, ~l~~id-~~~~~~i~ measurements, optical degradation. A comparison between constant- oll the other hand, have offered advantage of becurrent-wire inferred optical degradation and degra- ing relatively simple to perform, involving little dation measured using direct optical measurement a constant current A~~~~~~~~~ (ccA) systechniques is given. A statistical comparison using rommOn equipment in lnany laboratories, the linking equation suggests that the maguitiide of optical degradation is underestimated in the presence A great deal of work 11% taken advantage of the reiof hardware bandwidth limitations, while tilt. linking ative simplicity provided by assessing optical degra

Acoustic Noise-Source Identification in Aircraft-Based Atmospheric Temperature Measurements

Atmospheric temperature fluctuation data collected from a Grumman Gulfstream II aircraft show features in temperature power spectral density functions that do not follow the expected - 5/3 slope for homogeneous isotropic turbulence. Spectral analysis techniques show that these features result from the upstream propagation of a nondispersive acoustical wave. The source of the acoustical wave, which appears only at flight altitudes greater than 28,000 ft (8530 m), is attributed to engine acoustics in the form of jet screech where vortical structures interact with a quasi-periodic shock cell structure, both in the jet exhaust. The nature of the acoustical disturbance is shown to be dependent on velocity, with increased velocity resulting in a decrease in jet-screech peak frequency. These results are found to be consistent with those of other researchers investigating jet screech in jet flows.

Experimental measurement of a time-varying optical path difference using the small-aperture beam technique

This paper discusses the use of time series of the jitter angle of multiple, small-aperture probe beams as they emerge from a turbulent, optically-active flow field to quantify the time-varying optical path difference (OPD). Techniques to reconstruct a complete time series of instantaneous realizations of the OPD are first applied to a numerically-generated flow field and then to an experimental flow field. The flow field studied was that for the transitionally- turbulent region of a heated, two-dimensional jet. From these OPD histories spatial and temporal frequencies characterizing the OPDs are extracted. The relevance of these results to adaptive-optic devices is discussed.

Application of an acoustic noise removal method to aircraft-based atmospheric temperature measurements.

An acoustic noise removal method is used to reject engine acoustical disturbances from aircraft-based atmospheric temperature measurements. Removal of engine noise from atmospheric temperature measurements allows a larger wave number range to be fit while quantifying the magnitude of atmospheric temperature turbulence. The larger wave number range was found to result in a more statistically certain spectral slope estimate, with up to a 50% reduction in the standard deviation of measured spectral slopes. The noise removal technique was found to break down under conditions of weak atmospheric temperature turbulence where the engine acoustical disturbance can be several orders of magnitude larger than atmospheric temperature turbulence.

Shedding Characteristics of a Low Aspect Ratio Probe Mount in Subsonic Flow

Wind tunnel and flight tests have been used to investigate the vortex shedding characteristics of a low aspect ratio probe mount. Wind tunnel tests examined the nature of the flow at Mach 0.5, while flight tests examined the flow over Mach numbers ranging from 0.55 to 0.8. The frequency at which the shedding occurs agrees well with the Strouhal numbers measured by other investigators. The shedding is shown to be caused by shock-induced separation on the probe mount for Mach numbers in excess of approximately 0.78. It was concluded that the shedding process does not have a large effect on the measurement of atmospheric temperature fluctuation strength using a constant current anemometer upstream of the shedding activity.