Donald J. Wittich

Revised Scaling of Optical Distortions Caused by Compressible, Subsonic Turbulent Boundary Layers

New measurements of optical aberrations caused by a compressible subsonic turbulent boundary layer are presented. These new measurements are based on more-accurate Malley Probe measurements made possible by developing new wind- tunnel test sections that greatly reduced vibration corruption that imprints itself on the Malley Probes small-aperture, probe-beam deflection-angles. These new measurements give more-accurate levels of optical distortions for laser propagation through boundary layers, lowering previously-obtained values by ~ 30%. Using a similarity analysis based on these new measurements, a rigorous approach of extracting boundary-layer-related optical information from partially corrupted spectra is proposed and demonstrated by applying it to previously-obtained, vibration-corrupted measurements. Optical spanwise and streamwise length scales for a boundary layer are also presented and these statistical data is used to develop a method of creating realizations of 2-D wavefronts.

Aero-Optical Mitigation of Shocks Around Turrets at Transonic Speeds Using Passive Flow Control

Experimental studies of the aero-optical effects around a partially-protruding cylindrical turret for a range of incoming transonic Mach numbers are presented and discussed. Spatially-temporally resolved wavefronts were collected using a high-speed Shack- Hartmann sensor and flow visualization was performed with a Schlieren system. Different flow regimes with a local shock, either a steady or an unsteady one, were described for the baseline case and the shock dynamics was found to be sensitive to a local flow speed. In addition, several passive flow control devices, consisted of a single spanwise row of vertically-placed small-diameter pins or porous screens, were tested in order to mitigate detrimental unsteady-shock-related aero-optical effects. It was found that passive flow control devices with large blockage values slowed the flow near the cylinder surface down to subsonic speeds by introducing total pressure losses in the wall region upstream of the cylinder, thus eliminating the shock formation over a wide range of transonic Mach numbers and significantly improving aero-optical environment at some elevation angles.

Shack-Hartmann Wavefront Measurements of Supersonic Turbulent Boundary Layers in the TGF

Aero-optical environment of the supersonic flow in Trisonic Gasdynamic Facility (TGF) wind tunnel at Wright-Patterson AFB was experimentally measured using a high-speed wavefront sensor. Temporally- and spatially-resolved wavefronts were collected at a range of Mach numbers between 1.5 and 3.0 and the range of Reynolds numbers between 1 and 4 million per foot. Several data reduction techniques, including multi-point spectral crosscorrelation method, were introduced to analyze results and important statistical information about the turbulent boundary layer was extracted and discussed. A novel method was presented for using the frozen flow assumption to recover time-resolved wavefront measurements from spatially resolved 2-D wavefronts that are under-sampled in time. The criteria for using this technique were discussed in detail.

Estimation of Aero-Optical Wavefronts Using Optical and Non- Optical Measurements

6Air Force Research Laboratory, Directed Energy Directorate, Kirtland AFB, NM 87117 In this paper we present two algorithms for estimating the aero-optical aberration of a transonic flow around a 2-D turret based on Malley probe signals or pressure signals from few selected points. These two algorithms use Artificial Neural Networks and Linear Stochastic Estimation of varying model orders to estimate Proper Orthogonal Decomposition modal coefficients. These estimated coefficients are then used to reconstruct an estimated wavefront. This estimated wavefront is subtracted from the true wavefront to obtain a simulated reduction in the overall level of optical aberration. Reductions of up to 48% are achieved for both models. A robustness analysis is also performed, in which it is found that the algorithm is not sufficiently robust to changing flow conditions. Solutions are proposed for further investigation.

Investigation of Shock Dynamics on a Hemisphere Using Pressure and Optical Measurements

The aero-optical environment and pressure field around a hemispherical turret were experimentally studied in a wind tunnel between Mach numbers 0.5 and 0.65. The shock motion based on timeresolved wavefronts and shadowgraphs were analyzed and the average shock location, its spatial extension and temporal spectra were extracted. The typical shock frequency was at StD ~ 0.3 for M=0.63; these results correlate well with flight tests. When the shock was present on the hemisphere, pressure data both on the hemisphere and in its wake were found to be correlated in range of StD = 0.1..0.3, suggesting a global lock-in mechanism. The shock strength was found to be larger when the shock moves in the upstream direction, compared to the downstream motion. Possible links between the pressure around the hemisphere and shock dynamics were also discussed.

Studies of Flow Topology around Hemisphere at Transonic Speeds Using Time-Resolved Oil Flow Visualization

This paper presents the results of the study of surface flow topology over a hemisphere using time-resolved oil flow visualization at a range of Mach numbers between 0.2 and 0.7. Several cameras were used to simultaneously record temporal evolution of oil luminescence on and around the hemisphere. Perspective Transformation Matrix technique was used to reconstruct the spatial distribution of oil luminescence over the “virtual” surface; this approach allows studying oil flow visualization patterns from angles, not directly accessible in the experiment. The separation region downstream of the hemisphere was found to continuously decrease with the increasing Mach number and a formation of a large vortical structure at the separation line near the bottom of the hemisphere was observed at M = 0.7.

Study of Unsteady Surface Pressure on a Turret via Pressure-Sensitive Paint

Fast-response pressure-sensitive paint (PSP) was used in this work for a study of the unsteady surface pressures resulting from the complex separated flow over a hemispherical turret model. The turret includes several distinct features such as a flat window and crevices that are based on functional requirements, but also introduce interesting additional compact unsteady flow features. Three high-speed cameras imaged the paint luminescence at 2 kHz, producing pressure time histories over the entire model. These pressure data were integrated over the model surface in order to determine unsteady loads. Fast-response PSP successfully resolved very small pressure features that had aperiodic fluctuations of at least several kilohertz.

Spatial–temporal-covariance-based modeling, analysis, and simulation of aero-optics wavefront aberrations

We introduce a framework for modeling, analysis, and simulation of aero-optics wavefront aberrations that is based on spatial-temporal covariance matrices extracted from wavefront sensor measurements. Within this framework, we present a quasi-homogeneous structure function to analyze nonhomogeneous, mildly anisotropic spatial random processes, and we use this structure function to show that phase aberrations arising in aero-optics are, for an important range of operating parameters, locally Kolmogorov. This strongly suggests that the d5/3 power law for adaptive optics (AO) deformable mirror fitting error, where d denotes actuator separation, holds for certain important aero-optics scenarios. This framework also allows us to compute bounds on AO servo lag error and predictive control error. In addition, it provides us with the means to accurately simulate AO systems for the mitigation of aero-effects, and it may provide insight into underlying physical processes associated with turbulent flow. The techniques introduced here are demonstrated using data obtained from the Airborne Aero-Optics Laboratory.

Passive Shear Layer Regularization Experiments in Wind Tunnels and Feed-Forward Adaptive-Optic Correction

Shear layer regularization is a fundamental requirement in a Feed-Forward, Adaptive-Optic (FFAO) wavefront correction scheme applied to a beam passing through the shear layer. „Passive regularization‟ exploits the self-sustained oscillations of a shear layer over a resonant cavity, thereby eliminating the need for active flow control actuation. In wind tunnel tests, a strong acoustic resonance coupled with the cavity shear layer feedback mechanism produced a robust, predictable shear layer motion. The unsteady pressure at the upstream wall of the cavity was periodic enough to be used as a reliable phase reference. This phase reference was used to drive a phase-locked wavefront acquisition system and, ultimately, a deformable mirror which applied a feed-forward wavefront correction. The source of the strong acoustic resonance resulted from trapped duct modes, a result of the particular cavity and wind tunnel geometry combination. Unsteady pressure data indicated that this otherwise undesirable source of resonance can be mitigated by lining the wind tunnel wall opposite the cavity with an acoustically absorbent material.

Statistical analysis of Airborne Aero-Optical Laboratory optical wavefront measurements

Abstract. The Airborne Aero-Optical Laboratory has produced a large database of aero-optical measurements with a high-speed, high-resolution Shack Hartmann wavefront sensor. The data have been collected over a wide range of flight conditions. An analysis of the statistical characteristics of the subsonic and early transonic data is performed to assess the adequacy of the spatial and temporal resolution of the data. Sample rate requirements for a minimum variance phase estimator are also explored. The techniques employed are validated by application to measurements of optical atmospheric turbulence where results can be anticipated based on established Kolmogorov statistics.