What causes pressure in form drag?

  • #1
JTC
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Main Question or Discussion Point

Consider a ball flying through the air.

When there is turbulence, and the flow separates, say on a SMOOTH ball, then in the rear, there is circulation in the wide wake. There is pressure on the front, but no pressure on the rear due to the fact that the fluid is "busy" circulating around. So there is form drag.

When there is NO turbulence (or little) and the flow does not separate as early, say on a golf ball, then there is LESS circulation in the rear in the smaller wake.

So my question is: What gives rise to the pressure in the rear, when there is an "annealed" or "healed" flow (when the dimples on the golf ball make the wake smaller).

Where is the higher pressure coming from in the rear to match the pressure at the front end?

(I am not asking about the dimples on golf balls. I am only asking why the pressure goes up in the rear. Yes, I know there is a smaller wake but that does not explain why the pressure goes up.)
 
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Answers and Replies

  • #3
JTC
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  • #4
JTC
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  • #6
BvU
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Bernoulli should not apply due to the viscous nature.
are you saying Bernoulli does not apply to air ?
 
  • #7
JTC
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are you saying Bernoulli does not apply to air ?
Well, this is where all the confusion for me is.

I would think that no, it does not. But if one is studying effects induced by viscosity, then Bernoulli cannot be used to justify something. I think.
 
  • #8
BvU
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Bernoulli definitely applies (see story about truck and car being pushed towards each other), but not the full extent since there is energy loss.

When I read the Jestocott link or this link, the emphasis is on the size of the wake. But you stll need the pressure in the wake to be lower in order to get that effect of bigger wake ##\Leftrightarrow## more drag.
 

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