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Stable hovering of a paper pyramid
Bin Liu, Leif Ristroph, Annie Weathers, Stephen Childress, and Jun Zhang [Phys. Rev. Lett., 108, 068103 (2012)]
We have been studying forward flight and hovering flight for a few years. First of all, our first intend was not to build realistic flyers that fly like birds or insects (well, lately we started to try that as well). Instead, we want to invent some archetype models that present their biological counterparts in the most simplest fashion. The reason, we want to study the essential physical mechanisms that support the animal locomotion and don't want get distracted by the details taken by an individual animal or organism.

In this particular study, we use rigid paper-fold pyramids and them one of them inside an alternative-current (AC) wind tunnel. Shown in the figure (drawn by Bin Liu), a loudspeaker or subwoofer bought from Chinatown in NYC is placed upward. It is powered by a signal generator and a power supply (subwoofer-amplifier), which allows the speaker to operate at a controlled frequency and amplitude. A large cooking-pot, also bought from Chinatown, caps the speaker but the allows the airflow to move through a round openning at the base (now top). With a plastic cylinder and some drincking straws, we built this AC wind tunnel: the airflow moves up and down in a laminar fashion and its amplitude and frequency is set as wish.

It didn't need much fine-tuning to get it work. But to measure the airflow amplitude needs some patience and skills. Once the paper pyramid is placed in the AC airflow, it freely interacts with the air and at some frequency and amplitude it may hover against gravity. This work is reported three times at three different stages of the research. We might work on it again if we find another interesting question. As for the three published papers, they are:

"Hovering of a passive body in an oscillating airflow,"
Physics of Fluids, 18, 117103 (2006)
We study how a shape-changing body hovering in an oscillatory airflow

"Hovering of a rigid pyramid in an oscillatory airflow,"
Journal of Fluid Mechanics, 650, 415 (2010)
We study how a rigid body that hovers in an AC airflow

"Intrinsic stability of a body hovering in an oscillating airflow"
Phys. Rev. Lett., 108, 068103 (2012)
We investigate how an asymmetric body freely hovers and maintains stability

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