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Cyclopath
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PostWed Aug 14, 2019 7:35 am 
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There have been prior efforts to construct an "invisibility cloak" for objects in a flowing fluid, which have typically involved micro-pumps or similar active components. What's unique about a new design from scientists at Seoul National University in South Korea is that it is a passive cloak. A ring of 523 small pillars deflects an incoming fluid flow so that the middle of the ring, where the object or obstacle is—a cylinder, in this case—experiences no drag. The lack of a downstream wake demonstrates that the cylinder is effectively "cloaked."

A second, independent group of scientists at Beijing University and Xiamen University in China came up with a slightly different passive approach for its hydrodynamic cloak. That's because the goal of these scientists was not to eliminate drag but reduce the amplitude of water waves within a channel. Inspired by optical waveguide versions of invisibility cloaks, they created their own hydrodynamic version of a gradient index: two thin platforms running along the length to a wave tank. This effectively redirects an incoming fluid flow, such that waves in shallower regions propagate more slowly, ultimately leaving the center of the tank free of waves entirely.

DOI: Physical Review Letters, 2019. 10.1103/PhysRevLett.123.074501

DOI: Physical Review Letters, 2019. 10.1103/PhysRevLett.123.074502

https://arstechnica.com/science/2019/08/scientists-create-invisibility-cloaks-to-hide-objects-from-water-waves/
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MtnGoat
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PostWed Aug 14, 2019 9:04 am 
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That is some funky applied science and a great read! thanks for posting

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Stefan
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PostWed Aug 14, 2019 4:40 pm 
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I bet the people who design those boats for the "America's Cup" have already contacted those scientist.....

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Malachai Constant
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PostWed Aug 14, 2019 5:08 pm 
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This has been known for years examples are a sharks skin and a golf ball. I did many patens a few years ago for a local company and the DOD. Oh America’s Cup racers already use similar tech.

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Cyclopath
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PostWed Aug 14, 2019 9:10 pm 
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Can you expand on the golf ball thing?  There's a company making bike wheels with rims that are dimpled like a golf ball, for aerodynamic advantage.  I have other, flush ones, and always thought that "micro-vortex shedding" stuff was pure marketing.
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Malachai Constant
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PostWed Aug 14, 2019 11:30 pm 
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It has to do with shifting between latimer and turbulent flow. Vortexes transfer energy from the moving object to the fluid in which it is moving. The difference is readily measured in a wind tunnel or flow tank. It changes the skin friction and makes objects more slippery. They even have swimsuits with small ridges for athletes. Active hulls have slots connected to pumps to increase speed.

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cdestroyer
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PostFri Aug 16, 2019 7:28 am 
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navy uses something called air lubrication, which makes bubbles along the hull to reduce the friction between hull/water..
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Cyclopath
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PostFri Aug 16, 2019 7:45 am 
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Malachai Constant wrote:
It has to do with shifting between latimer and turbulent flow. Vortexes transfer energy from the moving object to the fluid in which it is moving. The difference is readily measured in a wind tunnel or flow tank. It changes the skin friction and makes objects more slippery. They even have swimsuits with small ridges for athletes. Active hulls have slots connected to pumps to increase speed.

This stuff can be so counterintuitive.  I have an aero bike jersey, it's patterned in a way that speed of mine like mesh.  Every time I look at it, I think the skin friction should be higher not lower.  But my power to speed ratio says it's the fastest one I've got.
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