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Old 02-24-2011, 07:53 PM   #131 (permalink)
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Theory

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Originally Posted by some_other_dave View Post
Ah, yes--the "F-104 Aerodynamic Theory", now known as the "Space Shuttle Aerodynamic Theory". Simply stated as, "Throw anything hard enough, and it will fly."

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Old 02-24-2011, 10:40 PM   #132 (permalink)
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I was also thinking about the speed vs drag issue -- if a certain trailing angle works at a low speed, then it might be too steep for a higher speed? The atmospheric pressure can push air at a certain rate, and given a little longer time (i.e. at a lower speed) then wouldn't a steeper trailing angle work just as well at low(er) speeds?

In other words, is there a specific speed used to develope this ideal profile?
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Old 02-25-2011, 12:07 AM   #133 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
...The atmospheric pressure can push air at a certain rate, and given a little longer time (i.e. at a lower speed) then wouldn't a steeper trailing angle work just as well at low(er) speeds?....
Hi Neil,

As you move through the air faster, and the air is split apart by the moving body, the local pressure at the split area is also higher.

As the body passes and the air starts moving back to it's original spot again, the higher local pressure can move the air back to it's original location faster because of the higher local pressure.

There are limits to this of course, but the speed in which the air moves back again is not proportional to the speed at which the body moves through the air.

BicycleBob and AeroHead have elaborated on this before.

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Not quite applicable, but I typically use subsonic orifices to calibrate equipment on occasion and it's interesting to note that the air velocity increases due to inlet pressure are not linear at subsonic values.

If you double the inlet gage pressure to the orifice, the resultant flow rate is only increased about 50%, not double like you would expect. As you get towards sonic flow this ratio slows down even more.

Jim.

Last edited by 3-Wheeler; 02-25-2011 at 02:26 PM..
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Old 02-25-2011, 09:13 AM   #134 (permalink)
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Hmmm, I missed that part of the discussion, I guess. It is odd to think that the air is "bumped" out of the way and has a spring affect? One would think that a given volume moves out of the way a given distance, and then moves back.

There must be a repelling force between the molecules in the air that is acting as a spring. So, it is not just the static volume of air that gets displaced, but rather the speed and shape of the front that has a lot to do with it.

So, then the shape of the front does have an indirect affect on drag -- blunt front ends "punch" the air harder, and may *improve* the air flow on the trailing side, by creating higher pressure? But at high(er) speeds, this leading pressure wave becomes critical, and the front must be pointy to split the air, rather than "punch" it?
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Old 02-25-2011, 10:04 AM   #135 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hmmm, I missed that part of the discussion, I guess. It is odd to think that the air is "bumped" out of the way and has a spring affect? One would think that a given volume moves out of the way a given distance, and then moves back.

There must be a repelling force between the molecules in the air that is acting as a spring. So, it is not just the static volume of air that gets displaced, but rather the speed and shape of the front that has a lot to do with it.

So, then the shape of the front does have an indirect affect on drag -- blunt front ends "punch" the air harder, and may *improve* the air flow on the trailing side, by creating higher pressure? But at high(er) speeds, this leading pressure wave becomes critical, and the front must be pointy to split the air, rather than "punch" it?
Neil, I was satisfied about the relative less importance of front shape when I started hauling a (small) motorcycle across the front of my truck. My Scamp 19' fifth wheel camper is not ideal aero-wise but it has generous radius and is slightly tapered and therefore does better wake management than square boxy trailers as evidence by MPG numbers from both camps. I was worrried MPGs would take a big "hit" with the bike on front but it didn't hurt bad at all as it turned out. I was pleasantly surprised and relieved as I found this setup the safest and most comforting way to haul the bike along with the trailer (which trumps fuel economy actually).





It's anecdotal but I don't think irrelevant field data.
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Old 02-25-2011, 12:19 PM   #136 (permalink)
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Originally Posted by t vago View Post
I saw some interesting sights during this last winter snowstorm a few days ago. People around here generally don't brush off more than the bare minimum of snow from their cars, and of course they like to drive on the interstate with all of this snow blowing off.

On quite a few of these cars, I happened to see snow aerodynamically shape itself to the roofs of these cars, such that the roof was effectively raised a few inches. That makes me think that for maximum gain, maybe the leading edge of this template ought to be matched to the windsheild of a given vehicle, and then aero extensions ought to be built to match the template (even if it raises the roof a bit).

Then again, that may drop C(d) at the expense of increased frontal area. Just a thought...
There has been work done on this, and it is in Hucho's book. You can drop the Cd of the vehicle by raising and rounding the roof of an automobile by up to several inches. However, when you take the increased frontal area into account, the CdA actually increases steadily, meaning there is no real gain to be had here as long as the vehicle had attached flow to begin with.
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Old 02-25-2011, 02:24 PM   #137 (permalink)
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To tack on a bit about your motorcycle on the front. ...I learned the hard way not to put a bike on the back of an aerodynamic car. By mounting a bicycle with 26" spoked tires on the back on my Insight and driving across the United States, I got gas tanks in the mid 30's MPG, worst I ever got. Best tank on the way home with the bike disassembled and in the car with me, 73MPG! With the same bike mounted on the back of a car that positioned the bike better over the trunk that had mediocre gas mileage, a 1995 Prizm(Toyota Corolla clone, although possibly slighty better aero), I got better gas mileage than the Insight and managed close to 40MPG for the round trip with the bike on the back while using power much more freely since I wasn't avoiding the use of IMA assist the whole time and maintaining higher speed climbing hills without leaving top gear.
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Old 02-25-2011, 02:41 PM   #138 (permalink)
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Quote:
Originally Posted by NeilBlanchard View Post
Hmmm, I missed that part of the discussion, I guess. It is odd to think that the air is "bumped" out of the way and has a spring affect? One would think that a given volume moves out of the way a given distance, and then moves back.
At rest, the air exerts a pressure of 15 psi on our bodies as we walk through the air. This being at sea-level.

As we walk through the air, the air molecules are "displaced" by the presence of our bodies and move out of the way as we walk.

If one had a sensitive enough pressure instrument, you could measure the pressure rise of the air impacting the front of our bodies as we walk down a hallway for example. There is a slight but perceptible increase in air pressure as we walk. You can feel this pressure change if someone else walks by you will you are standing still.

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There must be a repelling force between the molecules in the air that is acting as a spring. So, it is not just the static volume of air that gets displaced, but rather the speed and shape of the front that has a lot to do with it.
You can think of the air itself as a series of molecules that are all attached to each other via small springs, as a lattice structure. Picture yourself standing on the outside of this structure and pushing on the molecules at the edge. All the other surrounding molecules and springs will move in response to your input. The result of this will be a pressure wave that propagates across the entire lattice structure in an attempt to dissipate the pressure you put at the edge.

When your car body moves through the air, the same thing is happening with the displaced air molecules by your car moving them aside. The result of this movement creates a pressure increase in the local air molecules, and this pressure increase attempts to move the molecules back to their original position, but has to wait until your car body passes through that area.

The drawback to this whole scenario, is that once the molecules are moved apart and under localized pressure, they have to be "placed" back in their original position slowly over time to minimize energy losses. This comes in the form of turbulent flow and creates high energy losses. And this is where a streamlined shape like Aeroheads Template comes into play.

The shape of the Template in the tail area, "eases" the air molecules slowly back into their original position as best as possible, with minimal energy losses. In a perfect world, there would only be skin losses due to the shape moving through the air and no turbulent air losses. A trip to the airport to look at wings and air fuselages puts this whole exercise into reality.

Jim.

Last edited by 3-Wheeler; 02-26-2011 at 12:02 PM..
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Old 02-25-2011, 02:53 PM   #139 (permalink)
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Quote:
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There has been work done on this, and it is in Hucho's book. You can drop the Cd of the vehicle by raising and rounding the roof of an automobile by up to several inches. However, when you take the increased frontal area into account, the CdA actually increases steadily, meaning there is no real gain to be had here as long as the vehicle had attached flow to begin with.
Hi Wyatt,

The fact that the snow did accumulate just above and beyond the top edge of the front window glass, means to me that there was slightly detached flow in this area. The snow tends to "stick" in areas where there is low molecular pressure, especially on an otherwise smooth surface shape.

There were several times this winter that the rear deck of the tail was completely covered with a thin film of snow, yet the angular tail leading to that spot had none. Sorry but I did not get any pictures of this.

Jim.
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Old 02-25-2011, 07:26 PM   #140 (permalink)
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angle

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Originally Posted by NeilBlanchard View Post
I was also thinking about the speed vs drag issue -- if a certain trailing angle works at a low speed, then it might be too steep for a higher speed? The atmospheric pressure can push air at a certain rate, and given a little longer time (i.e. at a lower speed) then wouldn't a steeper trailing angle work just as well at low(er) speeds?

In other words, is there a specific speed used to develope this ideal profile?
Neil,once the boundary layer goes over to turbulent the Cd is fixed for a body up until around 250-mph for cars,when compressibility issues come into play. Angles, good at any sub-transonic velocity will not alter drag anywhere within this velocity domain.
In fluids,the barometric pressure will affect air density,which is part of the drag equation.
The pressure drag,which is what we're going after,is ruled by separation.If you can eliminate separation you've eliminated pressure drag.
If you get an angle which will work at 25 mph,she'll still be good at 225.

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