Fluidic Control of a Turret Wake, Part II: Aero-Optical Effects

Abstract

This paper, along with a companion paper [1], investigates the effects of flow control actuation on aero-optical distortions in the near wake of a 0.6 meter in diameter hemisphereon-cylinder turret model placed on the side wall of a wind tunnel. The aero-optical environment was characterized using a Malley probe. The main objective of the current work was to assess the effectiveness of the active flow control in mitigating optical aberrations over a conformal optical window mounted on the turret for backward-looking elevation angles at subsonic Mach numbers. The paper presents the analysis of the aerooptical environment present for the baseline flow around the turret and the influence of that the different active-flow-control configurations have on that environment. I. BackgroundURRETS provide convenient ways of pointing and tracking laser beams from airborne platforms; however, the turret creates a separated turbulent region of the flow, which, even at relatively-low subsonic speeds, starts to distort an otherwise planar emerging laser beam [2][3][4]. This, in turn, leads to the laser beam's unsteady defocus and jitter at the target [5]. The turbulent flow behind the turret is, in general, quite complex [4] and hard to control. However, it has been shown [2,4,6] that the main cause of aero-optical distortions for moderate back-looking angles is the shear-layer structures that form shortly after the flow separates from the hemispherical portion of the turret. One way to reduce the aero-optical degradation on the laser beam at these angles is to delay separation by manipulating the boundary layer just upstream of where separation would be in the absence of flow control. Recently, this approach was successfully demonstrated on a 0.25 meter hemispherical turret [7] where synthetic jetactuators mounted flush on the hemispherical surface were able to delay the separation, reducing levels of optical distortion by as much as 45% for M=0.4.Similar, but more powerful, synthetic-jet actuators were tested on a generic hemisphere-on-cylinder turret to see whether they could reduce optical aberrations behind a larger, 0.6 meter in diameter turret. The companion paper [1] discusses in detail the actuators' design, their locations on the turret and actuation cases tested, as well as actuation effects on surface pressure and wake velocity profiles for different actuation cases. This paper complements that paper and focuses on aero-optical measurements using a Malley probe.T

Hybrid Flow Control of a Turret Wake, Part II: Aero-Optical Effects

“…2 The quality of 2-D WFS data, after significant post-processing, was found to be acceptable for most of the elevation angles. Also, 2-D WFS results were found to be quite consistent with results taken with the Malley probe in the previous studies of the optical studies around the turret 5 . Data from the distorted grating wavefront sensor after proper reduction were also found to be consistent with data from other sensors; however DGWFS wavefronts were found to be easily corrupted by the residual beam jitter on the laser beam and clipped-beam effects.…”

“…During previous turret tests in SARL using a Malley probe a vibration-isolation system consisting of inner-tubes was used 5 . For the current tests the optical table was isolated from most of the tunnel-induced highfrequency vibrations (~50-300 Hz) using vibration-isolation boots; however, it was discovered that some low frequencies, notably in the range of 10-20 Hz were still present on the optical table and all optical components.…”

“…To measure optical aberrations at higher elevation angles and investigate strategies to mitigate these large levels of optical distortions at high back-looking angles, as well to address Reynolds number effects and scaling issues, a two-foot conformal window turret was tested for a range of back-looking angles from 129 to 149 degrees and Mach numbers between 0.3 and 0.5 4,5 . The turret was instrumented with static ports to measure the location of the flow separation line.…”

Hybrid Control of a Turret Wake, Part I: Aerodynamic Effects

“…The present investigation builds on the findings of previous work on active control of flow over a turret by direct small-scale excitation of the flow by arrays of surface-mounted synthetic jets 17,18 . Although actuation at the frequencies comparable with the natural wake frequencies is often used in the applications aimed at a flow separation delay over aerodynamic surfaces, there is reason to believe that excitation of the large-scale, organized motions in the wake by such a control could induce even higher optical aberrations 5 in spite of a possible favorable effect in the static (mean) sense.…”

“…Numerical work of Morgan and Visbal 16 emphasized that distributed porous suction over forward turret surface significantly delayed separation off the hemisphere at M = 0.4 and consequently reduced the wake extent and its turbulent intensity. Vukasinovic et al 17 and Gordeyev et al 18 measured both aerodynamic and aero-optical effects of synthetic jet active control of the flow over a turret, nominally at M = 0.3 and Re D = 4.5 •10 6 . It was shown that aerodynamic improvements in separation delay and suppression of turbulent energy within the wake resulted in about 30% suppression in optical aberrations as measured directly by the Malley probe.…”