Adaptive compliant structures for fluid flow control : a 'catastrophic' approach

Structural adaptivity is an enabling concept when an engineering system requires trade-offs between stiffness, strength, weight and functionality. Adaptive devices, that sense and react to their environment, allow the system to change geometry and/or material properties in response to external stimuli. As such, they have huge potential for a broad range of engineering applications. Mechanical instabilities and elastic nonlinearities are one of the emerging engineering design methods to facilitate shape-adaptation and multistability. Although nonlinear adaptive structures have captured the interest of the research community, they have not yet found their way into industry. The complexity of nonlinear systems and lack of controlled actuation and precise design guidelines, contribute to practical skepticism towards the application of these kind of morphing structures. This thesis illustrates a 'catastrophic' approach, in the sense of based on Catastrophe Theory principles, for the development of passively adaptive structures. Shape-changing devices are obtained by embracing elastic instabilities deriving from buckling of pre-compressed panels. A taxonomy of nonlinear post-buckling behaviours is defined and exploited as a design framework to obtain geometrical changes and multistability. The introduction of asymmetries into buckled morphing components enables tailoring of fold and cusp catastrophes, as well as the identification of regions of stability and instability. The definition of design spaces delimited by folds and cusps permits an intuitive design approach for embedding multistability and shape-adaptation into structures. This design method is used to develop a morphing inlet that can morph and adapt shape in response to external stimuli and varying environmental conditions. In particular, the inlet is conceived to be open at low airspeeds, such that air can flow freely into a connected duct. When the airspeed increases above a pre-defined critical value, the adaptive component-a glass-fibre composite plate-snaps passively and closes the inlet to the duct, such that air ceases to flow into the channel. Experimental results from wind tunnel tests replicate design features such as folds and cusps, validating the usefulness of the 'catastrophic' approach. In particular, the location of the cusp catastrophe in the post-buckling regime is of interest to the designer, as it defines the onset of the hysteretic and snapping behaviours used for shape adaptation. The general applicability of this design framework is further demonstrated by applying the same technique to the optimisation of an aircraft high-lift device component-a leading edge slat-cove filler for airframe noise reduction. The slat cove filler is designed to passively deploy and retract into the gap created by the high-lift device, in a manner that considerably reduces induced noise, while maintaining the benefits of increased lift. Given the generality of the underlying physical principles, the system's parameters can be tailored to fit specific operating conditions, and consequently, are suitable for a wide range of applications where shape adaptation and passive actuation are desired.