One of the last remaining challenges preventing the laminarization of sweptwingsis the control of unstable crossflow vortices. In low-disturbance environments thetransition from laminar to turbulent flow on the swept-wing initially takes the path ofreceptivity, where surface roughness or disturbances in the environment introduce shortwavelengthdisturbances into the boundary layer. This is followed by development andlinear growth of stationary crossflow vortices that modify the mean flow, changing thestability characteristics of the boundary layer. Finally, breakdown to turbulence occursover a short length scale due to the high-frequency secondary instability. The receptivitymechanism is the least understood, yet holds the most promise for providing a laminarflow control strategy. Results of a 3-year flight test program focused on receptivitymeasurements and laminar flow control on a 30-degree swept-wing are presented. Aswept-wing test article was mounted on the port wing of a Cessna O-2A aircraft andoperated at a chord Reynolds number of 6.5 to 7.5 million. Spanwise-periodic, micronsizeddiscrete roughness elements were applied at the leading edge of the swept-wing inorder to excite the most unstable crossflow wavelength and promote early boundary layer transition. An infrared camera was used to detect boundary-layer transition due tochanges in leading-edge roughness. Combined with the IR camera, a new technique ofcalibrating surface-mounted hotfilms was developed for making disturbance-amplitudemeasurements downstream of modulated roughness heights. This technique proved to beeffective at measuring disturbance amplitudes and can be applied in future tests whereinstrumentation is limited. Furthermore, laminar flow control was performed withsubcritically-spaced roughness. A 100% increase in the region of laminar flow wasachieved for some of the conditions tested here.
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