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

Abstract

This paper presents the optical results of using combined passive and active flow control to improve the aero-optical environments around a conformal-window, hemisphere-oncylinder turret. Several wavefront sensors were used to directly measure optical distortions at several back-looking angles. It was shown that the hybrid flow control significantly reduces levels of optical distortions for elevation angles as high as 148 degrees at M = 0.3. The paper also devotes considerable attention to overcoming environmental conditions present in large-scale wind tunnels. I. BackgroundN the last decade or so the research in using turrets to transmit laser beams from subsonic airborne platforms has greatly intensified. While a turret provides a convenient mechanical system to point-and-track the laser beam, the wake flow behind this bluff-body turret is both turbulent and complex with many vortical structures present. These structures are known to create significant density gradients and related aero-optical distortions 1 when the laser beam is transmitted through the wake of the turret even at low transonic speeds (see [2] and references therein, for instance). Left untreated, these detrimental aero-optical effects greatly reduce the laser-beam intensity in the farfield.An earlier investigation of the optical environment around a one-foot conformal-window turret for the range of the elevation angles between 60 and 132 degrees 3 in the zenith plane showed the level of optical distortions increases for backward looking angles above 120 degrees. 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. Velocity profiles in the wake downstream of the turret were measured using a single hot-wire and the surface flow topology was studied with oil flow visualization. Optical measurements were performed using a Malley Probe. In order to study the mitigating effects of active flow control the turret was equipped with an array of synthetic jets upstream of the window aperture. In previous studies these actuators were shown to be effective in suppressing turbulence behind a bluffbody turret at Re D = 8·105 [6], as well as behind a 10-inch hemispherical turret 7 , where significant suppression of turbulent fluctuations and reduction in optical distortions were observed up to M = 0.45. The array of synthetic jets introduces small-scale, highly dissipative structures into the attached boundary layer upstream of the window aperture. This flow actuation was shown to lead to flow-separation delay on the window's surface; up to a 10-degree delay in separation was observed to an elevation angle of 139 degrees 4 . A broad-band sup...

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

“…Although accompanying aero-optical flow characterizations are presented in depth by Gordeyev et al 25 , some overall results are shown here, verifying that measured aerodynamic improvements due to the hybrid control are related to the enhancement in the aero-optical transmission through the controlled flow. Thus, it is shown that the Strehl ratio of the beam traversing the separated flow under the hybrid control becomes nearly independent of the elevation angle between γ = 137° -148°.…”

“…Presumably the most important effect is seen at the highest elevation angle (Figures 19e,f), where the flow over the base turret geometry is fully separated upstream from the optical window. Passive and hybrid flow control are capable of fully recovering the non-separated flow condition upstream from the optical window, which were shown to have significant ramification of suppression of the optical aberrations even at this most-backward looking angle 25 . In comparison of short and long partition plates as components in the hybrid flow control, it should be also emphasized that regardless of the plate used, hybrid flow control results are almost identical.…”

“…Much as the static pressure distributions in Figure 19, comparison of spectral properties of the shear layers in Figures 13, 18, and 20 suggests even more that there is a clear advantage of the hybrid flow control over passive and active control in overall suppression of turbulent fluctuations in the shear layer Finally, an illustration of the resulting aero-optical effect of the hybrid flow control is shown in Figure 21. A detailed presentation of the accompanying aero-optical flow diagnostics is done by Gordeyev et al 25 . Here, only the flow control effect is shown in terms of a Strehl ratio SR, i.e., ratio of the laser beam intensity at the detection plane to the theoretical maximum intensity of a perfect imaging system working at the diffraction limit.…”

“…While the primary objective of the current work is to assess the effectiveness of hybrid flow control for suppression of optical aberrations at M = 0.3, the control effectiveness was further characterized at M = 0.4 and 0.5. While the primary focus of this paper is on the aerodynamic effects of active and hybrid flow control, the accompanying direct aero-optical characterization is discussed in great detail in the companion paper 25 . The experimental setup and procedure are described in Section II.…”

Active Flow Control of a Pitching Turret Flow Field Using Closed-Loop Feedback Control

“…12,13 Hybrid control (a combination of active and passive control) on the turret flow field has also been investigated and was shown to increase the separation angle by more than 10 • and was capable of reducing optical aberrations by as much as 40%. 14,15 The objective of the current study was to develop and test closed-loop feedback control algorithms used to drive an active flow control system designed to mitigate aero-optic distortions while pitching the turret. To this end, experimental testing was performed in the Subsonic Aerodynamics Research Laboratory (SARL) at Wright-Patterson Air Force Base (WPAFB) to test a suction based active control system and to develop variety of open and closed-loop feedback control algorithms to determine the most efficient method of reducing aero-optic distortions at Mach 0.3.…”