A Study of Secondary Flow Generation Within a "Virtual-Duct"

For an airborne optical beam-director the use of a spherical turret is a common choice, however, turrets are far from ideal from an aero-optical standpoint. This is due primarily to two dominant aero-optical features, consisting of the separation region on the aft of the turret and, at high enough Mach numbers, the presence of a shock. In order to mitigate or eliminate these dominant optically-active flows over a spherically-shaped turret, the “virtual-duct” technology was designed as a passive flow-control approach. The virtual-duct is designed as a fence with a diffusion section and a contraction section with contours generated using 5^th order polynomials. Previous investigations revealed that the virtual-duct also generated secondary flows on the turret surface, in particular, oil-flow visualization showed regions next to the virtual-duct fences that were swept clear of oil that was pushed towards the centerline of the hemisphere. Further investigation showed that these oil-free regions were caused by streamwise vorticies that form in the corners of the virtual-duct at the junction between the virtual-duct fences and the turret surfaces.

This dissertation describes research into the mechanism behind the generation of these corner vorticies, their general behavior, and their effect on the virtual-duct aero-optical performance. Results are shown for steady flow measurements performed in low-speed flow (M approximately 0.12) for virtual-ducts of varying curvature on; flat plates with different boundary layer growth distances, and on a hemisphere

of diameter, D = 0.3048 m. Oil-flow visualization, surface pressure, and five hole flow angle probe measurements were conducted to study the secondary flow corner vorticies. Results show that these vorticies are generated from a local corner separation within the diffusion section of the virtual-duct, and are dependent on the curvature and hence possibly the adverse pressure gradient in the diffusing section. The strength of the corner vorticies do not seem to be dependent on the underlying model surface, showing circulation that is the same whether the virtual-duct fences are mounted on a flat plate or hemisphere.

The research also experimentally demonstrates that the virtual-duct is performing as intended by countering the effects of turret curvature by increasing the centerline pressure coefficient and delaying separation off the aft of the turret, however performance is reduced by the presence of the corner vorticies. The flow measurements also show that the initial corner separation that generates the corner vorticies is local to the intersection of the virtual-duct and the mounting surface, and not severe as the flow reattaches within the contraction section of the virtual-duct.

Lastly, wavefront measurements were performed at M = 0.3, 0.4, and 0.5 for one set of virtual-duct fences on a hemispherical turret with diameter D = 0.254 m. It is shown that there are regions of large optical distortions over the turret aperture where the corner vortices are present. This suggests that the virtual-duct should be designed to eliminate the corner vorticies or displace them off the aperture, as their affect cannot be neglected. Outside the corner vortex regions, there are low optical aberrations showing that the virtual-duct substantially improves aero-optical performance, but better performance could be obtained without the corner vorticies.