Active Flow Control of Turbulence for Airborne Directed Energy Weapons

The application of airborne and ground-based high power lasers as Directed Energy (DE) weapons is under consideration for the Joint Strike Fighter (JSF). Depending upon the integration selected for the laser, varied forms of turbulence is encountered alongthe beam propagation path, e.g. turbulent boundary layer, free shear layer, or wake. Flowfield turbulence creates a highly non-uniform and time-varying, three-dimensional, fluctuating density field, and causes a propagating optical wavefront to incur phasedelays and become aberrated. Aero-optical wavefront errors produce beam spreading in high-energy laser weapon beams, which drastically reduces target irradiance and lethality for laser weapons and impairs laser-guided bomb accuracy. Optical degradationby turbulent flow, and compensation by adaptive optics, is compounded by the broad spectrum of turbulence frequencies, which can exceed 10 kHz for typical fighter flowfields. The time-dependent interaction of an optical wave front with a turbulent flow ispoorly understood. The Phase I program will provide fundamental characterization of this interaction through small-scale beam propagation experiments, complimentary LES modeling & analytical description of beam degradation. This paves the path forimplementation of high-frequency, active flow control of turbulence around the DE weapon with the goal of reducing the demand upon the adaptive compensation system.The proposed SBIR research will lead to an actuator design and integration concept, whichcan be directly transitioned to the JSF program. Beyond DE weapons, the flow control technology for reduction of optical degradation by turbulence is of great relevance for reduction of bore-sight-error (BSE) for a tracking system or elimination of blur inthe case of an imaging system. From the perspective of flow control, the high frequency actuator is relevant to the current research in aircraft exhaust noise reduction and aircraft plume infra-red (IR) signature reduction.