The article's discussion of golf balls reads completely opposite to standard well-accepted theory. The author writes:
> [The "drag catastrophe"] is when an object is falling so fast that the boundary layer of gas separates off the object and the drag force suddenly drops by a factor of almost 10. The reason why golf balls have dimples is to cause this drag catastrophe to happen at slightly slower speeds, so the ball will travel a lot farther.
This is very confused. Golf balls have dimples to prevent flow separation. The dimples are turbulators meant to induce turbulent flow around the golf ball before the laminar flow would otherwise give way to flow separation. Far from decreasing the drag force, flow separation increases it substantially.
[Also, the term "drag catastrophe" appears to have no relevant hits on Google other than this one article.]
[Edit 1]: The author is well qualified and unlikely to be confused himself; so I don't doubt his conclusion. Reading charitably, turbulent flow might be called a form of separated flow, and this must be what the author means. His coefficient of drag graph supports this interpretation as his "drag catastrophe" would be happening when you would expect a transition flow (from separated laminar to turbulent). Pedagogically he should have more clearly distinguished it from the typical laminar separated flow.
I'd never heard of "drag catastrophe" before either, but I think you're right that he's referring to the onset of turbulent flow that reduces drag on a bluff body.
That's the sudden drop at the right in all of these graphs: www.google.ro/search?q=drag+reynolds+number (same as his own Figure 9, really).
There are a lot of surprising nonlinearities in those graphs. I wish there was a more detailed article/paper somewhere.
I'm confused if a low-enough Reynolds number implies that laminar flow is still happening, how would the dimples on a golf ball even create turbulent flow given they have such a low Reynolds number?
EDIT: Ah, the velocity determines increases so does the Reynolds number. So at around 55 mph, the Reynolds number is high enough to induce turbulent flow.
This article is fascinating and now I understand why golf balls have dimples.
Submissions like this is the reason I frequent HN! A thorough well written and well researched article that point by point dissects an interesting event and comes to a surprising conclusion.
If you come across other articles of this calibre please submit them!
They took a few seemingly unrelated things and uncovered just how intimately intwined they actually are. Although abstract, isn't that essentially what a good hacker does on the daily? I think so.
I came here to post that it was a great article - except for the needless invocation and anthropomorphization of Occam's Razor.
I. It is not clear that a rock falling out is _simpler_ than a meteorite falling out. How exactly is a meteor less simple? Sure, it's rarer and less likely, but it is not simpler.
II. A simple explanation of a how an iPhone works is "magic." But we know that's not the case. Our invocation of Occam is tautological - we only call on it when we think it's right, not when we're making a decision. We note that the article did not begin with: "there must be a simple explanation, because Occam's Razor always works." Instead, it invoked Occam after the fact.
Conclusion: Occam's Razor is a fetishized social construction and carries no magical properties and is of no use in predicted phenomena using a scientific model. It is simply a feel-good mantra like prayer and rosary beads.
This isn't just about the Occam's Razor. The author goes through a thorough analysis of the evidence. Surely a forged video is a simpler explanation, but given the fact that the trajectory and the timing of the rock matches a simulated timing and trajectory and timing of a gravel falling out of the parachute, than the most probable explanation is the stowaway piece of gravel.
That's my not-so-subtle way of claiming a late victory on an internet argument since in this thread all the people claiming that it was more likely that it was something else were systematically downvoted.
On a less narcissistic note it's interesting to see the effect of different articles taking different angles on the comment section, I guess even HN is not proof to confirmation bias and hivemind.
"It turns out that when the rock size is set to about 3 centimeters in the simulation, it passes the skydiver at 12 seconds. This rock size just happens to have the same terminal velocity as the solution we found by matching the velocity seen in the video."
Hm. Poking around[1], drogue/pilot chutes seem to be around 30 inches or 75 centimeters in diameter. I wonder how you'd get a 3cm piece of "gravel" mixed up in that?
The pilot chute can be hastily scrunched up and shoved into the bottom of the container ("backpack") without any adverse effects to parachute deployment, so it's plausible that the rock was stowed by accident.
“It can’t be anything else. The shape is typical of meteorites – a fresh fracture surface on one side, while the other side is rounded,” said geologist Hans Amundsen.
Every hoax just needs one credible expert to put their name to it. This story may not have been a deliberate hoax but it ultimately was a hoax. It is good to see something debunked with some good, old-fashioned science!
I'm pretty sure a hoax requires a deliberate deception.
From the definition:
Noun: something intended to deceive or defraud
Otherwise it's just a mistake, misunderstanding, or other form of "just being incorrect". If they said "we know it's not a meteor, let's tell everyone it is anyway" - then it's a hoax.
[+] [-] tc_|12 years ago|reply
> [The "drag catastrophe"] is when an object is falling so fast that the boundary layer of gas separates off the object and the drag force suddenly drops by a factor of almost 10. The reason why golf balls have dimples is to cause this drag catastrophe to happen at slightly slower speeds, so the ball will travel a lot farther.
This is very confused. Golf balls have dimples to prevent flow separation. The dimples are turbulators meant to induce turbulent flow around the golf ball before the laminar flow would otherwise give way to flow separation. Far from decreasing the drag force, flow separation increases it substantially.
[Also, the term "drag catastrophe" appears to have no relevant hits on Google other than this one article.]
[Edit 1]: The author is well qualified and unlikely to be confused himself; so I don't doubt his conclusion. Reading charitably, turbulent flow might be called a form of separated flow, and this must be what the author means. His coefficient of drag graph supports this interpretation as his "drag catastrophe" would be happening when you would expect a transition flow (from separated laminar to turbulent). Pedagogically he should have more clearly distinguished it from the typical laminar separated flow.
[+] [-] certainly_not|12 years ago|reply
That's the sudden drop at the right in all of these graphs: www.google.ro/search?q=drag+reynolds+number (same as his own Figure 9, really).
There are a lot of surprising nonlinearities in those graphs. I wish there was a more detailed article/paper somewhere.
[+] [-] debt|12 years ago|reply
EDIT: Ah, the velocity determines increases so does the Reynolds number. So at around 55 mph, the Reynolds number is high enough to induce turbulent flow.
This article is fascinating and now I understand why golf balls have dimples.
[+] [-] jessaustin|12 years ago|reply
Ah, the term should be "drag crisis":
[+] [-] yukichan|12 years ago|reply
[+] [-] mixmax|12 years ago|reply
If you come across other articles of this calibre please submit them!
[+] [-] debt|12 years ago|reply
[+] [-] thedrbrian|12 years ago|reply
http://www.check-six.com/Crash_Sites/Tiger138260
[+] [-] Osmium|12 years ago|reply
[+] [-] jpswade|12 years ago|reply
[+] [-] chillingeffect|12 years ago|reply
I. It is not clear that a rock falling out is _simpler_ than a meteorite falling out. How exactly is a meteor less simple? Sure, it's rarer and less likely, but it is not simpler.
II. A simple explanation of a how an iPhone works is "magic." But we know that's not the case. Our invocation of Occam is tautological - we only call on it when we think it's right, not when we're making a decision. We note that the article did not begin with: "there must be a simple explanation, because Occam's Razor always works." Instead, it invoked Occam after the fact.
Conclusion: Occam's Razor is a fetishized social construction and carries no magical properties and is of no use in predicted phenomena using a scientific model. It is simply a feel-good mantra like prayer and rosary beads.
[+] [-] runarberg|12 years ago|reply
[+] [-] amirmc|12 years ago|reply
[+] [-] simias|12 years ago|reply
That's my not-so-subtle way of claiming a late victory on an internet argument since in this thread all the people claiming that it was more likely that it was something else were systematically downvoted.
On a less narcissistic note it's interesting to see the effect of different articles taking different angles on the comment section, I guess even HN is not proof to confirmation bias and hivemind.
[+] [-] mcguire|12 years ago|reply
Hm. Poking around[1], drogue/pilot chutes seem to be around 30 inches or 75 centimeters in diameter. I wonder how you'd get a 3cm piece of "gravel" mixed up in that?
[1] http://www.chutingstar.com/skydive/chernis-collapsible-main-...
[+] [-] sublimino|12 years ago|reply
[+] [-] darkhorn|12 years ago|reply
Sorurce http://youtu.be/jfEdEIwhj6s
[+] [-] thix0tr0pic|12 years ago|reply
[+] [-] markbnj|12 years ago|reply
[+] [-] Gracana|12 years ago|reply
[+] [-] Theodores|12 years ago|reply
“It can’t be anything else. The shape is typical of meteorites – a fresh fracture surface on one side, while the other side is rounded,” said geologist Hans Amundsen.
Every hoax just needs one credible expert to put their name to it. This story may not have been a deliberate hoax but it ultimately was a hoax. It is good to see something debunked with some good, old-fashioned science!
[+] [-] sophacles|12 years ago|reply
From the definition:
Noun: something intended to deceive or defraud
Otherwise it's just a mistake, misunderstanding, or other form of "just being incorrect". If they said "we know it's not a meteor, let's tell everyone it is anyway" - then it's a hoax.
Let's not dilute the word.
[+] [-] unknown|12 years ago|reply
[deleted]
[+] [-] dvirsky|12 years ago|reply