Programmable aerodynamic drag control in morphable cylinders

We study active aerodynamic drag control on morphable cylinders, whose topography can be modified pneumatically. Our design was initially inspired by the Saguaro cactus, which possesses an array of axial grooves that are thought to help reduce drag. In our analog experiments, we fabricate samples covered by an outer elastomeric film that is supported by a rigid inner skeleton. This cylindrical skeleton comprises either (i) a series of axial grooves or (ii) a hexagonal arrangement of holes. Decreasing the sample’s inner pressure deforms the outer film to produce (i) axial grooves or (ii) a pattern of dimples, whose depth can be varied on demand. First, we combine mechanical experiments and finite element simulations to characterize the depth of the topographic features vs. pneumatic loading. Secondly, we perform wind tunnel tests to measure the aerodynamic drag coefficient (as a function of Reynolds number) of the samples, for different surface depths. Finally, we leverage the morphable nature of our samples to customize their drag profile, in a fully ‘programmable’ manner.