ReCoVor

Abstract: Kirigami-inspired surfaces passively deform in fluid flows, offering a novel approach for applications in adaptive aerodynamic control, drag reduction, and renewable energy harvesting. We experimentally investigate how the cutting pattern can be used to control aerodynamic forces.  First, the cutting pattern and base material’s Young’s modulus govern the sheet’s effective stiffness, which sets its resistance to fluid-induced deformation, as quantified by the Cauchy number. As the kirigami opens, the resulting flow permeability causes the drag to fall below the quadratic force-velocity scaling typical of rigid bodies.  Second, specific cutting patterns trigger out-of-plane buckling of the elements created by the cuts. We show that by strategically designing the pattern to control the rotation and orientation of these buckled elements, we can generate and finely tune lift forces. Critically, we demonstrate independent control over lift and drag: drag on a kirigami can be varied by more than a factor of two at a constant lift, and lift can be adjusted with minimal change in drag. This work establishes a direct link between kirigami geometry, its resulting flow-induced deformations, and the aerodynamic performance, paving the way for smart, adaptive surfaces for flow manipulation.

Abstract: Flapping-wing turbines (FWTs) – oscillating wings rather than radial rotors – represent an innovative clean renewable source and have shown competitive power extraction efficiencies relative to conventional rotary turbines, as demonstrated in literature with high-fidelity simulations and limited experiments on physical prototypes. Although interest in FWTs is growing, design optimization, adaptability and reliability have proven difficult for field implementation due to the high-order parameter design space that defines interactions between aerodynamics, wing motion, and energy extraction. Current engineering design approaches involve extensive computer simulations coupled with optimization solvers or trial-and-error experiments which are both time and resource intensive tools. The motivation for the current work was to provide insights into the governing dynamics informing practical design guidelines. An experimental setup was developed and equipped with synchronized high precision force measurements along with streaked particle imaging for Leading Edge Vortex (LEV) evolution observations. A parameter space sweep across varied reduced frequencies, pitch amplitudes, Reynolds numbers, and blockage ratios was conducted to assess the impact of LEV and kinematic motion synchronization on performance. Complementing the experimental study, a low-order unsteady flow solver was used to predict performance trends between optimal cases with LEVs present versus cases with attached flow as Reynolds number increased. Results from the solver were compared against the experiments instantaneous force data and streaked particle images to validate the methodology before extending the parameter space to higher simulated Reynolds numbers. The results indicate that while overall optimum performance often coincides with the presence of an LEV, comparable performance can be achieved under attached flow conditions, which is desirable for practical design and implementation of these systems.