This paper introduces conceptual design principles for a novel class of adaptive structures that provide both flow regulation and control. While of general applicability, these design principles, which revolve around the idea of using the instabilities and elastically nonlinear behaviour of post-buckled panels, are exemplified through a case study: the design of a shape-adaptive air inlet. The inlet comprises a deformable post-buckled member that changes shape depending on the pressure field applied by the surrounding fluid, thereby regulating the inlet aperture. By tailoring the stress field in the post-buckled state and the geometry of the initial, stress-free configuration, the deformable section can snap through to close or open the inlet completely. Owing to its inherent ability to change shape in response to external stimuli—i.e. the aerodynamic loads imposed by different operating conditions—the inlet does not have to rely on linkages and mechanisms for actuation, unlike conventional flow-controlling devices.

Mechanical instabilities and elastic nonlinearities are emerging engineering means for designing shape-changing devices. In this paper, we exploit the taxonomy of post-buckling behaviours of a glass-fibre panel to design an adaptive air inlet that passively regulates the fluid flow into a connected duct. The adaptive component controls the inlet aperture by snapping open and closed depending on the velocity and pressure of the surrounding fluid. Sensing, actuation and control is entirely governed by the characteristics of the post-buckled component. Post-buckling stresses—induced in the composite panel by suitably applying compressive and bending loads—create the intrinsic characteristics of stability required for the panel to snap-through and snap-back. Furthermore, the associated post-buckled shape creates the aerodynamic pressure fields required for actuation. This design concept is explored and validated here by means of wind tunnel experiments. The passive actuation and control mechanism presented is particularly valuable for fluid control applications where simplicity and mass-minimisation are critical.

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Exploiting mechanical instabilities and elastic nonlinearities is an emerging means for designing deployable structures. This methodology is applied here to investigate and tailor a morphing component used to reduce airframe noise, known as a slat-cove filler (SCF). The vortices in the cove between the leading edge slat and the main wing are among the important sources of airframe noise. The concept of an SCF was proposed in previous works as an effective means of mitigating slat noise by directing the airflow along an acoustically favorable path. A desirable SCF configuration is one that minimizes: (i) the energy required for deployment through a snap-through event; (ii) the severity of the snap-through event, as measured by kinetic energy, and (iii) mass. Additionally, the SCF must withstand cyclical fatigue stresses and displacement constraints. Both composite and shape memory alloy (SMA)-based SCFs are considered during approach and landing maneuvers because the deformation incurred in some regions may not demand the high strain recoverable capabilities of SMA materials. Nonlinear structural analyses of the dynamic behavior of a composite SCF are compared with analyses of similarly tailored SMA-based SCF and a reference, uniformly thick superelastic SMA-based SCF. Results show that by exploiting elastic nonlinearities, both the tailored composite and SMA designs decrease the required actuation energy compared to the uniformly thick SMA. Additionally, the choice of composite material facilitates a considerable weight reduction where the deformation requirement permits its use. Finally, the structural behavior of the SCF designs in flow are investigated by means of preliminary fluid-structure interaction analysis.

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