{"id":13983,"date":"2017-12-21T10:34:29","date_gmt":"2017-12-21T10:34:29","guid":{"rendered":"https:\/\/www.revoscience.com\/en\/?p=13983"},"modified":"2020-05-27T06:19:41","modified_gmt":"2020-05-27T06:19:41","slug":"giant-air-pockets-win-drag-race","status":"publish","type":"post","link":"https:\/\/www.revoscience.com\/en\/giant-air-pockets-win-drag-race\/","title":{"rendered":"Giant air pockets win the drag race"},"content":{"rendered":"

Creating teardrop-like gas cavities around metal spheres enables practically friction-free travel through liquids.<\/strong><\/em><\/span><\/p>\n

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Comparisons between 3D-printed plastic torpedoes and teardrop-shaped air cavities that form around metal spheres reveal that gas bubbles can practically eliminate hydrodynamic drag forces.
Copyright : Reproduced from reference 1 under a Creative Commons license \u00a9 2017 AAAS<\/figcaption><\/figure>\n

Fast-swimming fishes and deep-sea submarines share the characteristics of elongated, streamlined forms to help reduce friction and drag in water. A method developed at KAUST that spontaneously generates torpedo-shaped air pockets around objects could help oil tankers and other heavy vessels to benefit from these energy-saving tricks.<\/span><\/p>\n

Researchers looking to minimize hydrodynamic drag often begin by modeling a sphere moving through a nonviscous, incompressible fluid. Under these ideal conditions, the drag force disappears entirely as the ball\u2019s velocity increases\u2014a puzzling behavior not shared by real-world objects. Understanding this paradox through controlled experiments, however, has proved difficult for nearly two centuries.<\/span><\/p>\n

Recently, KAUST\u2019s Ivan Vakarelski and Sigurdur Thoroddsen fired up renewed interest in drag reduction by demonstrating that metal spheres heated to 400C will free-fall twice as fast in liquids as room temperature orbs. The secret lies in a thin bubble of evaporated gas that materializes around the hot balls and stops liquid molecules from sticking and creating friction. But, even after the team stabilized this gas layer with superhydrophobic coatings on the spheres, the drag forces remained much larger than was ideal.<\/span><\/p>\n

\u201cThis prompted us to look for ways to create thicker gas layers because we suspected they would be even more effective in reducing drag,\u201d explains Vakarelski.<\/span><\/p>\n

Serendipity struck when the researchers noticed hot spheres\u2019 free-fall improved dramatically when they were dropped into the liquid instead of being released inside it. High-speed video cameras revealed that at impact, the spheres were enveloped by an air pocket that pinched off into a teardrop-like shape, 5 to 15 times the volume of the metal. The giant air pockets sped down the two-meter-deep liquid chamber and reached a constant velocity\u2014matching predictions of near-zero drag conditions.<\/span><\/p>\n

\u201cIt\u2019s intriguing how the air pocket emerges spontaneously,\u201d says Thoroddsen. \u201cIn our earlier work the gas layer always preserved the form of the solid bodies, but the giant gas cavity self-determines its own shape, which is very close to perfect.\u201d<\/span><\/p>\n

By racing their giant air cavities against identically sized plastic projectiles, the team demonstrated that gases decreased drag coefficients by an order of magnitude compared to solids. Analyses also showed the cavity\u2019s fall velocity can be calculated exactly from the initial sphere density and the volume of trapped air.\u00a0<\/span><\/p>\n

\u201cIt\u2019s impossible to predict how even ordinary objects like golf balls move without experiments or costly simulations,\u201d says Vakarelski. \u201cIn contrast, we showed that sphere-in-cavity can be explained by very simple equations\u2014this could make its way into textbooks.\u201d<\/span> <\/p>\n","protected":false},"excerpt":{"rendered":"

Creating teardrop-like gas cavities around metal spheres enables practically friction-free travel through liquids. Fast-swimming fishes and deep-sea submarines share the characteristics of elongated, streamlined forms to help reduce friction and drag in water. A method developed at KAUST that spontaneously generates torpedo-shaped air pockets around objects could help oil tankers and other heavy vessels to […]<\/p>\n","protected":false},"author":6,"featured_media":13984,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[17],"tags":[],"class_list":["post-13983","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-research"],"featured_image_urls":{"full":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139-150x150.jpg",150,150,true],"medium":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139-300x210.jpg",300,210,true],"medium_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"1536x1536":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"2048x2048":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"ultp_layout_landscape_large":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"ultp_layout_landscape":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"ultp_layout_portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"ultp_layout_square":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"newspaper-x-single-post":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"newspaper-x-recent-post-big":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"newspaper-x-recent-post-list-image":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",93,65,false],"web-stories-poster-portrait":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",500,350,false],"web-stories-publisher-logo":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",96,67,false],"web-stories-thumbnail":["https:\/\/www.revoscience.com\/en\/wp-content\/uploads\/2017\/12\/5139.jpg",150,105,false]},"author_info":{"info":["Amrita Tuladhar"]},"category_info":"Research<\/a>","tag_info":"Research","comment_count":"0","_links":{"self":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/13983","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/comments?post=13983"}],"version-history":[{"count":0,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/posts\/13983\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media\/13984"}],"wp:attachment":[{"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/media?parent=13983"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/categories?post=13983"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.revoscience.com\/en\/wp-json\/wp\/v2\/tags?post=13983"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}