Active aerodynamic drag reduction on morphable cylinders

Abstract

We study a mechanism for active aerodynamic drag reduction on morphable grooved cylinders, whose topography can be modified pneumatically. Our design is inspired by the morphology of the Saguaro cactus (Carnegiea gigantea), which possesses an array of axial grooves, thought to help reduce aerodynamic drag, thereby enhancing the structural robustness of the plant under wind loading. Our analog experimental samples comprise a spoked rigid skeleton with axial cavities, covered by a stretched elastomeric film. Decreasing the inner pressure of the sample produces axial grooves, whose depth can be accurately varied, on demand. First, we characterize the relation between groove depth and pneumatic loading through a combination of precision mechanical experiments and finite element simulations. Second, wind tunnel tests are used to measure the aerodynamic drag coefficient (as a function of Reynolds number) of the grooved samples, with different levels of periodicity and groove depths. We focus specifically on the drag crisis and systematically measure the associated minimum drag coefficient and the critical Reynolds number at which it occurs. The results are in agreement with the classic literature of rough cylinders, albeit with an unprecedented level of precision and resolution in varying topography using a single sample. Finally, we leverage the morphable nature of our system to dynamically reduce drag for varying aerodynamic loading conditions. We demonstrate that actively controlling the groove depth yields a drag coefficient that decreases monotonically with Reynolds number and is significantly lower than the fixed sample counterparts. These findings open the possibility for the drag reduction of grooved cylinders to be operated over a wide range of flow conditions.

Elasticity and stability of shape-shifting structures

“…Buckling bilayer plates can be utilized to generate shape-shifting structures [40] that may be used as soft grippers [140]. Buckling of shells has provided an intriguing way to control global shape [141,142,12] and local patterning [143,144,145,146], reduce aerodynamic drag [147,148], generate lock-and-key colloids that can selectively aggregate [149], form liquid crystal shell actuators [150], and pave the way for buckling microswimmers [151].…”

Wind-resilient civil structures: What can we learn from nature

“…Such strategies include adding O-rings and helicoidal wires and supplementing external momentum to the boundary-layer flow (Lim and Lee 2004;Lin et al 2016;Morrison et al 2016;Kim and Yoon 2017) to alter the boundary-layer behavior and wake dynamics. Bioinspired flow control strategies have emerged as one of the new tools to address this issue (Guttag and Reis 2017;Rinehart et al 2017). For instance, the morphable cylinder inspired by the Saguaro cacti will be discussed in the section on Saguaro cacti.…”

“…Nevertheless, an engineered system can go beyond the limits to create a versatile mechanism to better adapt to wind effects. A recent study by Guttag and Reis (2017) examined aerodynamics of morphable grooved cylindersa grooved cylinder of a pneumatically controlled cavity ratio. The morphable cylinder's drag was consistently lower than fixed samples at an Re up to 10 6 .…”

Programmable Aerodynamic Drag on Active Dimpled Cylinders

“…[6] In our previous work, we performed wind tunnel experiments on cylinders with axial grooves, whose depths could be pneumatically controlled. [22] Through careful study of the effect of the groove depth on the drag coefficient, we were able to use the tunable nature of the groove depth to minimize the drag coefficient over a range of Reynolds numbers around the critical regime. [22] In addition to modifying the shape of the surface, other active methods for controlling the drag have been studied.…”

“…[22] Through careful study of the effect of the groove depth on the drag coefficient, we were able to use the tunable nature of the groove depth to minimize the drag coefficient over a range of Reynolds numbers around the critical regime. [22] In addition to modifying the shape of the surface, other active methods for controlling the drag have been studied. One heavily studied active drag reduction strategy is the use of oscillatory motion.…”