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This paper presents experimental studies of aero-optical distortions due to a turbulent boundary layer over a range of subsonic speeds with the underlying wall both heated above and cooled below the adiabatic wall temperature. A statistical scaling model, based on extended strong Reynolds analogy is derived and shown to correctly predict experimentally observed results. The temperature mismatch between the flow and the wall was shown to have a profound effect on the level aero-optical aberrations, as the heated wall amplifies them and the cooled wall significantly reduces distortions. The importance of an inclusion of the pressure term in explaining the boundary-layer density fluctuations with the cooled wall is also discussed. [ABSTRACT FROM AUTHOR]

In-flight wavefront measurements around a flat-window turret at subsonic Mach numbers are analyzed in instantaneous and time-averaged sense. In addition to the root-mean-squared levels of aerooptical distortions, higher-order spatial statistics are calculated, and their dependence as a function of the viewing angle is discussed. Given the optical data obtained, the applicability of the commonly used large aperture approximation (LAA) is revisited. We show that, for all angles, the LAA consistently underestimates the time-averaged Strehl ratio, so the LAA should be used very cautiously. Some reasons for these discrepancies are traced to non-Gaussian spatial distribution of the optical wavefronts. A different approximation for computing time-averaged Strehl ratios is proposed, and the results are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article provides a critical review of aero-optics with an emphasis on recent developments in computational predictions and the physical mechanisms of flow-induced optical distortions. Following a brief introduction of the fundamental theory and key concepts, computational techniques for aberrating flow fields and optical propagation are discussed along with a brief survey of wave-front sensors used in experimental measurements. New physical understanding generated through numerical and experimental investigations is highlighted for a number of important aero-optical flows, including turbulent boundary layers, separated shear layers, and flow over optical turrets. Approaches for mitigating aero-optical effects are briefly discussed. [ABSTRACT FROM AUTHOR]

• Novel flow control device introduces large-scale structure into turbulent boundary layer. • Boundary layer response to synthetic structure measured using particle imaging velocimetry. • Synthetic large-scale structure modulates amplitude of near-wall turbulence. • Modulation occurs as traveling wave overlying near-wall physics. • Boundary layer behaves like higher-Reynolds number flow with synthetic structure. The dynamic response of a zero pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally studied using continous laser particle imaging velocimetry (PIV). In previous experiments using a single hot-wire, it was shown that the synthetic large-scale structure (LSS) introduced by the plasma-based actuator, located in the outer region of TBL, had a strong modulating and reorganizing effect on the near-wall turbulence. In the study reported here, an actuator, optimized for the experimental TBL, was placed at the upper boundary of the log-linear region of the TBL to produce a spanwise uniform, periodic synthetic LSS in order to study the response of the TBL to these large-scale perturbations. Planar PIV over a narrow streamwise region was used to measure the time-resolved, two-dimensional velocity downstream of the actuator at a series of streamwise point locations. Using PIV, the modulating effect of the synthetic LSS on the near-wall turbulence is described in more detail. The results are discussed and compared with previous hot-wire measurements and numerical simulations of the actuated TBL. [ABSTRACT FROM AUTHOR]

Effects of hybrid flow control and its active and passive components on the aerodynamic characteristics of flow over a 0.254-m-diam conformal optical aperture embedded in the hemispherical cap of a cylinder turret model (D = 0.61 m) are investigated at M = 0.3-0.5 and ReD = 4.4-7.4 x 106. Resulting mean flows are characterized by surface static pressure distributions and oil-flow visualizations, while the separated-flow dynamics are assessed by hot-film measurements. Active flow control is effected by arrays of piezoelectrically driven synthetic jet modules distributed in multiple arrays upstream from the aperture. Active flow control is further assisted by global flow alterations induced by a passive forward partition plate, and, when combined, constitute hybrid flow control. It is shown that the hybrid flow control combines the positive effects of its component control elements to yield superior results in any cumulative aerodynamic aspect of the separated flow. This cumulative effect of the actuation is manifested by concomitant delay of flow separation and active, dissipative suppression of turbulent motions downstream of separation. It is also demonstrated by means of direct two-dimensional wave-front measurements that the overall aerodynamic improvements correlate with substantial suppression of optical aberrations through the separated flow. Furthermore, estimated Strehl ratios for the laser beam indicate that nearly invariant Strehl ratio is established within the range of tested aperture elevation angles, yielding improvement of about 50% for the highest elevation angle. [ABSTRACT FROM AUTHOR]

Effects of direct small-scale actuation on aerodynamic and aero-optical characteristics of flow over a hemisphere-on-cylinder turret model (D = 0.61 m) with a round 0.254-m-diam conformal optical aperture are investigated at M = 0.3 and ReD = 4.46.106 (with additional measurements at M = 0.4 and 0.5). Flow control is effected by arrays of piezoelectrically driven synthetic jet modules. The cumulative effect of the actuation is manifested by a concomitant delay of flow separation and active, dissipative suppression of turbulent motion downstream of separation. Effects of actuation on aero-optical distortions are assessed from the flow dynamics, using surface oil visualization, static pressure distributions, and hot-film measurements within the separated flow domain. In addition, the suppression of optical distortion across the separated flow is measured directly using a Malley probe over a range of elevation angles. These measurements show that for a fixed actuation level, the active suppression level of spectral components of the optical distortion is about 30% within a resolved frequency band 0.5 < f < 25 kHz at M = 0.3. [ABSTRACT FROM AUTHOR]