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.
> [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.