CFD-based aero-optical performance prediction of a turret

Abstract

An airborne optical system requiring a large field-of-regard will often use a hemisphere or similarly-shaped "turret" to transmit or receive radiation. The aerodynamic flow, however, creates disturbances about the turret resulting in the formation of turbulent boundary and shear layers, and flow separation. The disturbed flow is characterized by optical phase distortions that vary rapidly in time and degrade system performance.With the advancement of Computational Fluid Dynamics (CFD) and computing power, the complex flow field around the turret can be accurately modeled, both spatially and temporally. The density field is extracted from the flow field solution and interpolated within the volume defining the transmitted or received beam. By applying the Gladstone-Dale constant to obtain index of refraction, and integrating along the desired line-of-sight, time-dependent OPD (optical path difference) maps are obtained. These are used as complex field phase modifiers in the wave optics system performance analysis.This methodology was applied to a hemisphere-cylinder turret protruding from a flat plate in Mach 0.3 flow. Timeaccurate flow solutions capturing the flow separation and wake oscillations were obtained. Performance of both conformal and flat transmitting windows was assessed as a function of beam angle.

Airborne Laser Communications: the Challenges of the Propagation Medium

“…For example, as shown in Figure 3, if a hemispherical dome is installed on an aircraft the region in front of the dome experiences a relatively static pressure build-up while the region aft of the dome can experience dynamic turbulence including vortex eddies in the air flow. This is a well-known result and visualizations have been provided in many forums (e.g., see [7,8]). The result of this varying nature is that the effects of the boundary layer have a strong spatial and view angle dependence.…”