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Old 06-24-2009, 08:15 PM   #1 (permalink)
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Maximum angle for boat tail?

Hi

for something like a boat tail, what is the maxum angle you can use before you start to get seperation of air flow?

I had read someplace one time, but I can not find it again.

Thanks
Allen

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Old 06-24-2009, 10:04 PM   #2 (permalink)
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I think you need to specify a speed as well. Slow speeds for example might not even induce separation on a SUV.

I believe CD has a direct relationship with speed
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Old 06-24-2009, 10:14 PM   #3 (permalink)
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Speed affects the Reynold's number, which has a small effect over the range we are concerned with. The classic answer to afterbody taper is 15 deg. top and sides, and 4 on the bottom. Those are average best numbers, but can be affected by many other factors.
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Old 06-24-2009, 10:24 PM   #4 (permalink)
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I understand that part, but (and this is just me visualizing airflow, not from any formal education on the matter) it seems to me that the faster a vehicle moves, the less the air has the ability to form around the vacuum side of a moving body.

So at say 60 mph, the air may remain attached but at 100 mph the air doesn't have time to fill in behind a vehicle, effectively producing a higher CD.

With all that being said (and provided it's true) I'm unsure if the Force_drag formula already accounts for that or if it's more of an approximation because the CD varies as a function of velocity.


Sigh, I should check my library and see if they have Hucho's book so I can get more up to speed on this stuff, rather then trying to visualize what is happening =)

Anyways, thanks if you can clear this up for me!
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Old 06-25-2009, 02:36 AM   #5 (permalink)
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At higher speeds, the pressure differences also increase. Intuition is not a good guide here, but eventually, reading the same thing over and over makes it seem normal.
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Old 06-25-2009, 08:50 AM   #6 (permalink)
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I understand about the pressure changes.... but is my previous post accurate?
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Old 06-25-2009, 09:18 AM   #7 (permalink)
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At sub sonic speeds the flow around a object really doesn't change all that much. At least thats what Hucho says. And since he knows more about aerodynamics than this whole board combined, I tend to trust him.
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Old 06-25-2009, 10:30 AM   #8 (permalink)
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>I understand about the pressure changes.... but is my previous post accurate?

I think that the transition you are intuiting might be happening around 1 or 2 MPH. You won't find it in the relevant literature, so I'm going to stop repeating myself, and sum up wih "No."
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Old 06-25-2009, 11:33 AM   #9 (permalink)
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This might help. Try to visualize things from the air perspective. It is standing still, a mostly stationary air mass. The vehicle moves into the space and creates a high pressure moving the air out of the way (no compression sub-sonic). Behind the vehicle is a low pressure zone (air already moved out of the way). This high pressure air wants to move into the low the most efficient way it can. The air hugs the vehicle every chance it gets unless a shape forces it further away. As the vehicle passes the stationary point you have visualized, the air collapses back into the low pressure wake. How rapidly, depends on the relative pressures which is related to the speed of the vehicle.

Hope this helps. Critique and correction of my depiction is welcome.
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Old 06-25-2009, 01:28 PM   #10 (permalink)
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