Self-determined shapes and velocities of giant near-zero drag gas cavities

Abstract
Minimizing the retarding force on a solid moving in liquid is the canonical problem in the quest for energy saving by friction and drag reduction. For an ideal object that cannot sustain any shear stress on its surface, theory predicts that drag force will fall to zero as its speed becomes large. However, experimental verification of this prediction has been challenging. We report the construction of a class of self-determined streamlined structures with this free-slip surface, made up of a teardrop-shaped giant gas cavity that completely encloses a metal sphere. This stable gas cavity is formed around the sphere as it plunges at a sufficiently high speed into the liquid in a deep tank, provided that the sphere is either heated initially to above the Leidenfrost temperature of the liquid or rendered superhydrophobic in water at room temperature. These sphere-in-cavity structures have residual drag coefficients that are typically less than Embedded Image those of solid objects of the same dimensions, which indicates that they experienced very small drag forces. The self-determined shapes of the gas cavities are shown to be consistent with the Bernoulli equation of potential flow applied on the cavity surface. The cavity fall velocity is not arbitrary but is uniquely predicted by the sphere density and cavity volume, so larger cavities have higher characteristic velocities.

Citation
Vakarelski IU, Klaseboer E, Jetly A, Mansoor MM, Aguirre-Pablo AA, et al. (2017) Self-determined shapes and velocities of giant near-zero drag gas cavities. Science Advances 3: e1701558. Available: http://dx.doi.org/10.1126/sciadv.1701558.

Acknowledgements
This work was supported by the King Abdullah University of Science and Technology. D.Y.C.C. was supported by the Australian Research Council through Discovery Project grant no. DP170100376.

Publisher
American Association for the Advancement of Science (AAAS)

Journal
Science Advances

DOI
10.1126/sciadv.1701558

PubMed ID
28913434

Additional Links
http://advances.sciencemag.org/content/3/9/e1701558http://advances.sciencemag.org/content/3/9/e1701558

Permanent link to this record