The effects of laser plasma discharge on a separating boundary layer

Modification of the separation and drag characteristics of a laminar airfoil using a remotely located high-power laser was experimentally investigated in a low speed, low-turbulence wind tunnel. It was proposed that pulsed laser energy could be used to cause a disturbance in the boundary layer of a laminar airfoil thus modifying the flow by inducing a cross exchange of momentum within the boundary layer. The result is a unique zero net mass flux and zero net momentum flux actuator for delaying separation.;A 500 mm chord length laminar airfoil was designed and fabricated to closely emulate in the wind tunnel the flight characteristics of a Cessna 177 at cruise speed. At zero degrees angle of attack the airfoil was determined to have an incipient laminar separation bubble on its lifting surface between 67 and 80% chord. By focusing the collimated laser beam emitted from a 900mW Q-switched YAG laser a strong plasma pulse was generated from the optical breakdown of the air. The plasma was focused to a location approximately 2 mm in front of the airfoil. High temperature plasma produced by each pulse generates a shockwave and volume of heated turbulent air which interacts with the airfoil and travels along its lifting surface. Particle Image Velocimetry (PIV) system was used as the primary data collection method. Determining the effects of the plasma on the flow over the airfoil requires measuring the behavior deep within the boundary layer which is typically less than 2 mm thick. Custom optics were used to capture flow behavior in a 6 mm x 6 mm field of view along the centerline of the airfoil. The PIV system was electronically triggered by the plasma laser. By varying the trigger delay time a range of data captured the onset, peak and decay of the plasma with fine spatial resolution.;Results of this investigation show that a burst of turbulence generated by the pulsed plasma exchanges significant momentum between the freestream and the incipient separation bubble. This exchange of momentum is sufficient enough to reattach the separated flow for a period lasting 7 orders of magnitude longer than the duration of the plasma pulse itself.