Numerical Prediction of the Spatial and Temporal Characteristics of the Aero-optical Disturbance Produced by a Helicopter in Hover

by Ryan T. Kelly Aero-optical disturbances produced from turbulent compressible flow-fields can seriously degrade the performance of an optical signal. At compressible flight speeds these disturbances stem from the density variations present in turbulent boundary-layers and free-shear layers; however helicopters typically operate at incompressible speeds which nearly eliminates the aberrating effect of these flows. For helicopter platforms the sources of aberration originate from the high subsonic flow-field near the rotor blade tips in the form of rotor-tip vortices and from the high temperatures of the engine effluence. During hover the shed rotor-tip vortices and engine effluence convect with the rotor wake encircling the airframe and subsequently a helicopter mounted optical system. The aero-optical effects of the wake beneath a hovering helicopter were analyzed using a combination of Unsteady RANS (URANS) and Large-Eddy Simulations (LES). The spatial and temporal characteristics of the numerical optical wavefronts were compared to full-scale aero-optic experimental measurements. The results indicate that the turbulence of the rotor-tip vortices contributes to the higher order aberrations measured experimentally and that the thermal exhaust plumes effectively limit the optical field-of-regard to forwardand side-looking beam directions. This information along with the computed optical aberrations of the wake can be used to guide the development of adaptive-optic systems or other beam-control approaches.