Comparison of Aero-Optical Measurements from the Flight Test of Full and Hemispherical Turrets on the Airborne Aero-Optics Laboratory

Abstract

The optical environment around both a hemisphere-on-cylinder turret and a hemisphereonly turret with flat and conformal windows was characterized at both subsonic and transonic speeds. Data was taken from Mach 0.4 to 0.65 at altitudes from 15,000 ft to 30,000 ft to analyze scaling laws, with a focus on Mach numbers of 0.5 and 0.65. A 25 kHz Shack-Hartmann wavefront sensor was used to measure the optical aberrations around the turret. Data were primarily collected while the two planes slewed by, giving statistics at several azimuthal/elevation angles with additional data taken at fixed angles. Additionally, dynamics of a local shock appearing on the conformal-window turret at the transonic Mach number are presented and discussed.

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

“…The shock-induced separation changes the pressure gradient immediately downstream the shock, and for cases where the relatively weak shock was formed over the region with small pressure gradients, such as the weak moving shock and the strong moving shock cases, the interaction between the shock and the shock-induced flow separation leads to unsteady pressure gradients downstream of the shock, Gordeyev at al AIAA-2013-0717 7 American Institute of Aeronautics and Astronautics which results in the unsteady shock motion. A similar shock unsteadiness was observed over the hemisphere-oncylinder turret at incoming transonic speed of 0.6 [14] and in the shock-induced separation on the wall of a slightlyoverexpended supersonic nozzle [16]. When the shock becomes stronger, it is less-sensitive to the small wakerelated pressure gradients downstream of it, thus becoming stationary.…”

Efficacy of predictive wavefront control for compensating aero-optical aberrations

“…23,24 With the conformal window at subsonic speeds (Mach ≤ 0.55), this separation can occur as far back as α ¼ 115 deg to 120 deg over the turret. 5,22 Then, from about α ¼ 7 deg on up, the peak at St ¼ 0.5 diminishes and a peak at St ¼ 1.8 begins to dominate. After α ¼ 93 deg, the flow begins to reattach at the back edge of the flat window and the rms WFE begins to drop.…”

“…1 As in remote imaging, laser beam projection, and astronomical viewing applications, adaptive optics (AO) are required to compensate such aero-optical aberrations. 5 Accordingly, the spatio-temporal nature of the aero-optical aberrations is a function of the shape of the turret, the turret pointing geometry, and the flight conditions. 2,3 Such common, conventional AO control systems consist of a fixed-gain, linear-filter control law.…”

The Comparison of Unsteady Pressure Field over Flat- and Conformal-Window Turrets using Pressure Sensitive Paint

“…Finally, various flow control mechanisms have been studied on this geometry [7,8,9]. There has also been substantial research into the difference in aero-optical performance between flat and conformal windows on turrets [1,2].…”

“…The hemisphere-on-cylinder turret has been studied extensively in recent years. There has been a large parametric study of the aero-optical properties of this geometry in flight using the Airborne Aero-Optics Laboratory, AAOL [2,3]. Additionally a variety of CFD studies have been performed on the turret geometry [4][5][6][7].…”