Active Control and Optical Diagnostics of the Flow Over a Hemispherical Turret

47th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

Vukašinović

et al. 2009

Self Cite

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

Section: A Baseline Optical Resultssupporting

confidence: 71%

mentioning

confidence: 99%

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

47th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

Vukašinović

et al. 2009

Self Cite

“…The far-field Strehl Ratio, SR, defined as the ratio between the far-field intensity with optical aberrations present, normalized by the maximum diffraction-limited intensity in the absence of optical distortions, can be estimated using the Large-Aperture Approximation as SR = exp (-(2πOPDrms/λ) 2 A point worth noting is that it is important to have the partition plate only upstream of the turret, but not downstream. Experiments with a hemisphere-on-surface 7 showed that despite the fact that the hemisphere is aerodynamically less protruding than the hemisphere-on-cylinder, aero-optically a hemisphere on a surface or plate is more aberrating than when it is elevated from the surface by a cylindrical base. 2 The quality of 2-D WFS data, after significant post-processing, was found to be acceptable for most of the elevation angles.…”

Section: Discussionmentioning

confidence: 99%

“…5 [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.…”

Section: Imentioning

confidence: 99%

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

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

Vukašinović

et al. 2010

Self Cite

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...

“…The shock creates strong unsteady density gradients in the flow and promotes an earlier separation of the flow off the turret. All these shock-related features add additional aero-optical distortions to the outgoing beam [5,8]. Figure 1 shows results of the numerical simulations of the flow for a hemispherical turret for incoming Mach numbers of M = 0.7 and 0.9.…”

Section: Introductionmentioning

confidence: 99%

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

Burns

51st AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

et al. 2013

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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.