Flapping Elastic Straws

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One of the interesting challenges in fluid dynamics is the coupling of aerodynamic forces with structural forces. This could be the result of external flow, as with aeroelastic flutter on aircraft or architecture, or internal flow, as with the video above. Here researchers blow air through compliant cylindrical shells–think of a straw made of an elastic solid like latex–and observe the vibrations that result. Depending on the flow rate and material properties, different vibrational modes can be activated. The first mode behaves much like a garden hose that’s not being held; it vibrates wildly back-and-forth. The second mode wobbles the mouth of the shell open and closed, whereas the third mode forms three “flaps” that vibrate inward and outward. Each of these modes behaves very differently, and, for practical applications, it’s important for engineers to be able to predict, control, and account for these kinds of structural behaviors under aerodynamic loading. (Video credit: P. Zimoch et al.)

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