Application of Variable Leading-Edge Roughness for Transition Control. on Swept Wings

For transition to turbulence in crossflow-dominated swept-wing boundary layers, details of the strongly nonlinear nature of the stationary primary instability are well understood both experimentally and theoretically. It is the particular action of the nonlinearities that makes possible a transition control scheme using subcritically spaced leading-edge surface roughness arrays. What is desired is a means by which this scheme can be generalized to all cases of crossflow-dominated transition. To this end, two elements are presented here: a experimental examination of the process by which the primary instability generates a high-frequency secondary instability that leads to breakdown and a prototype system of pneumatically driven surface roughness actuators that can provide the correct control input for the transition suppression scheme. The key experimental results are that both mode shape and growth data show that the secondary instability is indeed the key factor in triggering breakdown of crossflow boundary layers and that the location of the secondary instability mode explains the nature of breakdown patterns observed in naphthalene flow-visualization experiments. The actuator system is shown to be capable of producing a transition effect.