Passive Flow Separation Control for High Lift

Abstract

A current focus in the quest for improvement in fuel efficiency in aircraft design is the simplification of complex high lift systems. A simpler high lift system in combination with flow separation control is potentially able to meet the current high lift performance requirements, while still reducing overall complexity. Passive flow separation control is promising for its inherent simplicity. This is in line with the main goal of simplification of the high lift system. This thesis covers an exploratory and experimental investigation of a novel type of passive vortex generator: the internal vortex generator. Its distinctive feature, an internal structure for the generation of vortices below the surface, is hypothesized to create strong vortices low inside the boundary layer. Due to this increase in effectiveness the internal vortex generator can potentially outperform external vortex generators. As no literature is currently available on this novel concept, different designs are evaluated using numerical analysis. The wedge type internal vortex generator, similar to a backward wedge external vortex generator, is found to produce the most stable and strong vortices, which are inherently close to the surface. Its close resemblance to the well-documented backward wedge external vortex generator allows for a detailed design that is based on a literature study. Moreover, it allows for an interesting reference case to be present during the experiment test. The internal vortex generator is experimentally tested against its geometrically equivalent external vortex generator. Both vortex generator types are equipped on the same single element high lift model and tested in the Low Speed Low Turbulence Wind Tunnel at the Delft University of Technology. Force measurements indicate a more than 3% increase in maximum lift compared to baseline, while the geometrically equivalent external vortex increases the maximum lift with less than 1%. When the boundary layer is artificially tripped at 0.067 chord length, the internal vortex generator increases maximum lift with 33% compared to baseline, while the external vortex generator increases the lift coefficient with 16%. Due to practical limitations the force measurements do not accurately represent a real-world condition. Critical analysis, strengthened by oil visualizations and XFOIL simulations, suggest that the wedge-type internal vortex generator is capable of achieving more lift than the force measurements indicate. Moreover, the internal vortex generator concept is found to inhibit unique qualities that can be exploited in future research. Overall, the internal vortex generator concept shows to be a viable improvement to regular vortex generators. Therefore, future research is highly encouraged.