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Old 06-03-2007, 08:52 AM   #21
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This has got me wishing we were in the same room with a white board so we could just work together.

I want to ask some questions. They are challenge questions because that is what we scientists do when we have these discussions. Please do not take any of them the wrong way... I am hoping to learn more about what you are thinking (and what I am missing) and maybe point out some additional ideas for you.

So 1st observation is on the Mig... which is kind of driving your thought I guess. The change in the surface to affect an airflow change to stop/start stuff going into the engines does not necessarly mean that it had a macro affect on airflow characteristics. I am not claiming it didn't... or did... just that the conclusion cannot be drawn. These same aerodynamic changes elsewhere on the Mig, or on a car for that matter, may or may not have produced any measurable change. Again, not claiming did or didn't, just insufficient data.

On the golf ball, agree completely. Most folks don't understand this subltely and many aero primers ignore it completely. But part of why it works is it's velocity, air pressure, size relationships. ie it is fast and small... cars and big and slow... but this said, the whole vortex generator idea must have merit since too many smart people have put them on airplanes (and other "really" smart, or "not so really" smart people have put them on cars.... I just don't know which at this point but do have my suspicions :-).

Now to the meat of your stuff. I like the way you are thinking. I have also been thinking about ways to alter the exterior flow, although my thoughts have gone down the active/dynamic route rather than static, but we are thinking about solving the same problem. Couple of comments, and these are not specific to your stuff, but general to the problem (and thus apply). I'd like your thoughts.

From what I recall of my fluid dynamics education, frictional drag (if we should call it that) of a fluid is not a function of the surface it is passing at all, but rather a function of the fluid viscosity. The reason for this is the fluid, exactly at the surface is not really moving... here is a reasonable link:

(http://www.answers.com/topic/viscosity

So friction drag is not because the fluid is sliding across the surface, but because the moving surface is grabbing on to the closest layer and causing the fluid to slide across itself. So any change in surface must "alter" aerodynamic flow to have any effect... ie. big enough to introduce turbulent into streamlined flow, or separated into turbulent flow.... but since both of these are bad with the primary effect, one must ensure the gain is comparable elsewhere. Again, back to vortex generators.

>>Conventional wisdom says that the smoother the surface
>>the better the vehicle slips through the air (closing of gaps, removal of >>wipers, etc.).

Agreed, but I think you really mean "smoother the shape".

>>In the same vein, an introduction of a surface with less surface
>>area would produce less wind resistance.

The former does not lead to this conclusion, however, and I can't figure out how the math does either. Intuitively it does make sense I agree... but I don't see how it maps to any of the equations... but I am not expert here so let me know what I am missing. It certainly does seem to me that less sufrace area leads to less fluid friction and thus to less engergy dissipation, but the fluid force equation on a body doesn't take this into account.... ie a long cylinder experiences the same force effect as a short cylinder.

Lasty, to further motivate you and applaud your's (and others) thoughts in these areas.... this is an area of physics that still is not understood.

http://www.physics.teleactivities.ne..._problems.html

Just think about the equations... they all have things like 'coefficient of friction" and other determined constants.... there are no F=MA or E=MC'2 equations, which means we do not have any base understanding of how things work, therefore someone like you or other's may discover something important through tinkering and thinking about this.

OK, so just a mention of what I have been thinking about... simply add an electric fan or two or more with the appropriate shrouding to positively alter the airflow at the bad portions of the flow "AND" provide some base force thrust as well... like put a couple of the HW750 RC jet fans and get up to about 30 lbs of thrust (think how long your glides would be when 30 lbs is shaved off the overall force drag).... AND, AND (this is the part I am wondering about)... the force thrust is produced from air flow that positively alters the external Cd. as well... but I might be hoping for too much.. appreciate your thoughts here cuz a few people think I am nutz :-)
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Old 06-03-2007, 04:42 PM   #22
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Crap I had a really really great reply then closed the tab instead of the one next to it so I have to reconstruct it. I forgot my golden rule: always highlight and copy posts every few seconds. Poop. Okay reconstruction time.

Quote:
So 1st observation is on the Mig... which is kind of driving your thought I guess. The change in the surface to affect an airflow change to stop/start stuff going into the engines does not necessarly mean that it had a macro affect on airflow characteristics. I am not claiming it didn't... or did... just that the conclusion cannot be drawn. These same aerodynamic changes elsewhere on the Mig, or on a car for that matter, may or may not have produced any measurable change. Again, not claiming did or didn't, just insufficient data.
My first reply was stunningly great. Ah, I hate reconstructing documents.

All I know is that Belyakov reported that was the reason for the failures "vortexes caused by the coating of the aircraft dictated removal of certain aerodynamic enhancements as well as physical modifications to the intake system".

Quote:
OK, so just a mention of what I have been thinking about... simply add an electric fan or two or more with the appropriate shrouding to positively alter the airflow at the bad portions of the flow "AND" provide some base force thrust as well... like put a couple of the HW750 RC jet fans and get up to about 30 lbs of thrust (think how long your glides would be when 30 lbs is shaved off the overall force drag).... AND, AND (this is the part I am wondering about)... the force thrust is produced from air flow that positively alters the external Cd. as well... but I might be hoping for too much.. appreciate your thoughts here cuz a few people think I am nutz :-)
The Brabham BT46 was a Formula One racing car in 1978. The cars were powered by a flat-12 Alfa Romeo engine. The original 'A' model had "flat panel" heat dispersing units for getting rid of the heat from the oil and engine coolant. Didn't work. So, for the Swiss Grand Prix the Brabham team rolled out, tada the fan car. A very powerful fan was mounted in the car that sucked air from under it creating an additional amazing 1,000 pounds or so (that's what the rumor is, anyways) of additional lift. The car raced in exactly one race, whipped everyone and was then withdrawn from the tour.

So, my friend you may take it one step further. Take a large duct, mount the opening to it intersects the entirety of the vehicle and accelerate the air through the duct out the back of the vehicle. You could use a passive venturi effect to do this and cool the air at the same time. Simple. (see my Webernator)

Quote:
So friction drag is not because the fluid is sliding across the surface, but because the moving surface is grabbing on to the closest layer and causing the fluid to slide across itself. So any change in surface must "alter" aerodynamic flow to have any effect... ie. big enough to introduce turbulent into streamlined flow, or separated into turbulent flow.... but since both of these are bad with the primary effect, one must ensure the gain is comparable elsewhere. Again, back to vortex generators.

>>Conventional wisdom says that the smoother the surface
>>the better the vehicle slips through the air (closing of gaps, removal of >>wipers, etc.).

Agreed, but I think you really mean "smoother the shape".

>>In the same vein, an introduction of a surface with less surface
>>area would produce less wind resistance.

The former does not lead to this conclusion, however, and I can't figure out how the math does either. Intuitively it does make sense I agree... but I don't see how it maps to any of the equations... but I am not expert here so let me know what I am missing. It certainly does seem to me that less sufrace area leads to less fluid friction and thus to less engergy dissipation, but the fluid force equation on a body doesn't take this into account.... ie a long cylinder experiences the same force effect as a short cylinder.
The first part of your quote goes to the "waxed or unwaxed" surface argument that was settled (as far as I'm concerned) some time ago. Yes, to some extent the smoother a surface the less boundary layer interference and in return there will be less surface frictional drag coeffecient. But to every rule there is an exception and the aerodynamics field is no exception. You may be interested in this book.

Okay. I'll see if I can draw a verbal image. Say you have a series of mountains. As the wind blows over the mountains the wind blows into the valleys. Now suppose you could shape these mountains so the wind would only intersect the peaks of the mountains. What you've basically done is reduce the surface area of the mountain. What my experiment envisions is modifying a surface so the apparent area of surface available to the fluidic forces is minimized. The end goal (I guess) is to find a way to reduce the wake turbulence to as close to zero as possible without using a bunch of fancy wings.

Remember the Reynolds number which is:

RN =rVc / Viscosity

RN = Reynolds Number
r = Air Density
V = Velocity
c = distance over which the air flows

What I'm trying to test is whether it's possible to change 'c' without actually changing the physical length of the vehicle. I know, I know the big cone/small cone theory says bullocks to that. But I challenge that thinking. All those numbers make sense with a smooth surface. Now, I'm not proposing the old "waxed versus sanded" argument in boating but something that may not have been tried before.

Remember the mountain deal? Well, I look at it this way. If I can break the fluid flow of the air over the vehicle so that it impacts the upper area of the irregular surfaces only in most conditions then the total frictional surface exposed would be less and should require less effort to move through the atmosphere. The question is how to do this. Would we accelerate the air as it intersects the vehicle with a mechanical device or use passive means to proceed over this irregular surface.

Some of them have been solved. I suspect this is an old list.
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Old 06-03-2007, 05:10 PM   #23
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cool... got to run and test my 5-wire o2, but then give me some time to digest your stuff. I am enjoying posting with you... this aero stuff is fun, and like you said, very non-intuitive. I have not looked at things closely yet from the Reynolds perspective so I know I am missing things.... More later thanks
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Old 06-03-2007, 05:22 PM   #24
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Pardon if I'm way off base with this -lots of interesting facts in this thread -Could you possibly take say : 4x8 sheets of plywood/fiber board ,cut to fit the bed top ,drill 2.5 - 3" holes evenly spaced ,tightly cover with a poly film or fabric and somehow get the covering to sink into the holes (yet be semi rigid ) ? This would give a removeable bumpy A-B-A-B test bed...cover .
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Old 06-04-2007, 12:33 PM   #25
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I'm not sure that would achieve results. But. Pegboard. We'd have to figure out a way to get something to drip partway through the holes (fiberglass resin would be too hard to sand) that could be adjusted in height for testing. Have to think about that.
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Old 06-05-2007, 07:15 AM   #26
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sry, haven't forgot about this, just have been in o2 wiring hell.

One question: on the Reynolds and length.... how does the "length" in the relationship vary with respect to the shape of the object? Consider that a smooth plate or cylinder would have a Reynolds characteristic length of the actual measured length. Add some surface roughness and it doesn't change. But start adding some bumps and as the bump size increases, the actual flow would start to alter based on the contours and the Reynolds characteristic length then get's longer???? Is this right?

Back to the mountain analogy. Isn't this similar to the bed of a pickup truck? ... Where dropping the tailgate can hurt things because it changes the "effective" shape and alters the flow? (with the tailgate up, a static cushion of air makes the tailgate look more like it has a tonnaue cover on it, then with the tailgate down the flow starts going downward into the bed and outward.... ie, effectively longer...).

I guess what I am thinking is that the best possible "shape" is a smooth shape... as variations in the surface are added, intially they are not relavent, but eventually can significantly alter the flow path, making it "longer".

If this reasoning is correct (comments pls!), then surface alterations can indeed alter Reynolds characteristic length, but only by increasing the values... the minimum can never be less than the overall "macroscopic" shape length. In other words, I can't shorten the car, but I can certainly make it longer.
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Old 06-07-2007, 05:52 PM   #27
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Originally Posted by lca13 View Post
sry, haven't forgot about this, just have been in o2 wiring hell.

One question: on the Reynolds and length.... how does the "length" in the relationship vary with respect to the shape of the object? Consider that a smooth plate or cylinder would have a Reynolds characteristic length of the actual measured length. Add some surface roughness and it doesn't change. But start adding some bumps and as the bump size increases, the actual flow would start to alter based on the contours and the Reynolds characteristic length then get's longer???? Is this right?

Back to the mountain analogy. Isn't this similar to the bed of a pickup truck? ... Where dropping the tailgate can hurt things because it changes the "effective" shape and alters the flow? (with the tailgate up, a static cushion of air makes the tailgate look more like it has a tonnaue cover on it, then with the tailgate down the flow starts going downward into the bed and outward.... ie, effectively longer...).

I guess what I am thinking is that the best possible "shape" is a smooth shape... as variations in the surface are added, intially they are not relavent, but eventually can significantly alter the flow path, making it "longer".

If this reasoning is correct (comments pls!), then surface alterations can indeed alter Reynolds characteristic length, but only by increasing the values... the minimum can never be less than the overall "macroscopic" shape length. In other words, I can't shorten the car, but I can certainly make it longer.
Sorry about your O2hell. I've been there. Nothing like crawling under a vehicle. 50 times! . BTW, I'm going to the shack to film some filler this weekend so I won't see any of this until next week.

Now bear with me. I know I sound arrogant. After all I play the most arrogant man in the world on "This Old Shack".

Where to start..... Dropping the tailgate of a truck depends on the shape of the front end, the Cd, the length of the box, the ride height and the length of the tailgate. This is where the infamous "Mythbusters" episodes gets tripped up. Where mileage gets hurt is in the relationship of the airflow over the top and the "buffeting" of the flow as in it's relationship to the airflow from under the vehicle. In simplified prose, when the gate is down, the high pressure over the top of the truck is increased at a lower height which CAN have an effect on the aero drag on the under body of the vehicle depending on the air flow under said vehicle.

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One question: on the Reynolds and length.... how does the "length" in the relationship vary with respect to the shape of the object? Consider that a smooth plate or cylinder would have a Reynolds characteristic length of the actual measured length. Add some surface roughness and it doesn't change. But start adding some bumps and as the bump size increases, the actual flow would start to alter based on the contours and the Reynolds characteristic length then get's longer???? Is this right?
It doesn't and that's the beauty of the Reynolds number. It's designed to be quick and dirty without a lot of other aspects getting in the way. Remember, you assume this that and the other thing and plug in basic numbers that you know. It's a great starting point for aero work. Emphasis on starting. All it is is the ratio of inertial forces to viscus forces. What we're looking at is reducing the inertial forces via the experiment, aka raising portion of the vehicle in a macroscopic format (bumps) in order to smooth the air flow over a region of the body of the vehicle at lower speeds. We know this works at mach numbers but are attempting to translate it to lower speeds. Remember, I've ran my hands over these MiG-29s. They're like garbage cans. Used garbage cans. The panels don't even fit. But they made them fly at 2,000 mph. They claimed it was via the bumpy paint. I believe them; I've seen this crap. They're junk and shouldn't fly over mach 1. Seriously. Good God, if it was a car it would get 2 mpg with a 10 hp engine!

What I'm thinking with the bumps is shortening the apparent length of the vehicle. Not the physical length but the length the fluid flow sees as it's length. I know it's an esoteric concept but I see mathematics visually (which can be a real pain when trying to do the math in writing) and what I see makes sense. Although, it may not.

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If this reasoning is correct (comments pls!), then surface alterations can indeed alter Reynolds characteristic length, but only by increasing the values... the minimum can never be less than the overall "macroscopic" shape length. In other words, I can't shorten the car, but I can certainly make it longer.
Ah, you're halfway there. Think the opposite. You can decrease the values by a method of decreasing the exposure to the fluid flow. Let me put it this way. Say you have a vehicle that is 20 feet long and is exposed to the fluid (air) flow over it's entire length. Now say the same vehicle is (for giggles) a formula one car that has various canards and such that direct the air right over the body at the 5 foot mark to 10 feet behind it. The air is only exposed to 5 feet of the car. Now how long does the fluid "think" it is? 20 feet? Why? The fluid is exposed to 5 feet of length. The mass? No, traveling through air has nothing to do with mass - accelerating and keeping the mass at speed has to do with the mass, not the aerodynamics. Aerodynamics is how the vehicle splits the molecules of the fluid that it travels through in the most efficient manner possible.

Honestly, I haven't had this good of a discussion with anyone in years that hasn't degenerated into name calling. I thank everyone for reading and inputting various bits of information. I'm honestly enjoying myself.
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Old 06-07-2007, 08:39 PM   #28
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Ok, I'm not really following the whole Reynolds length thing, but that's ok. Conceptually, I view the bumpy paint and dimpling issue in a more literal sense of how the fluid or air actually moves over a surface. Specifically, bumps create additional vortex motion into the medium on a small scale. The vorticies that the bumps generate could be thought of like ball bearings between the surface and surrounding medium. And instead of having to maintain a smooth flow through the medium alone, the bearings act as a buffer over imperfections in the object. The real trick is in generating the smallest bearing layer that will work, as anything larger is accelerating more mass than necessary, and anything smaller will slip into irregularities in the object surface and generate much larger turbulence.

So in a nutshell, the bumpy paint and golf ball behaviors make sense to me on that level at least. The Reynolds length however just adds another dimension to the concept, but unless I'm misunderstanding it (likely), I feel it obscures what is really happening.

Essentially, the virtual bearings created also slow the speed of the medium relative to the contact surface (and accelerate it relative to the surrounding medium), mimicking the effect of something that is actually moving much more slowly.

Perhaps that is what has really already been said though. :P
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Old 06-07-2007, 10:00 PM   #29
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The Reynolds # is more about how to change variables and still duplicate flow patterns. In of itself, it is just another constant describing relationships that we don't really fundamentally understand, just know how they correlate.

>>The air is only exposed to 5 feet of the car.
>>Now how long does the fluid "think" it is? 20 feet? Why?
>>The fluid is exposed to 5 feet of length

This is where you lose me. What is happening behind the 5 feet? If indeed you can direct the flow away from the remainder of the surface (like as you describe, redirecting with canards)... you would end up with a very turbulent area right behind the transition point....?

Maybe what you are trying to describe is having the rear part of the vehicle carry along a cushion of air at zero velocity with respect to the suface. so once you get it moving and at constant speed, it would appear that there is no fluid resistance with respect to the surface....??? But that is nothing more than pure laminar flow.

Remember that an important aspect that is in play is that, regardless of shape, you are always displacing a volume of air. Purely laminar flow displaces the least. In contrast, a poorer shapes have displacement in addition to the volume of the object.... in the form of turbulences.

I am still thinking.... this is fun.
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Old 06-09-2007, 07:29 AM   #30
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Some of those guys also had an amazing amount of fuel lines in their cars. Enough to hold a few gallons of gas. The Petty baby powder story is amusing.
That was Smokey Yunick that had the 5 gallon gasoline line, was discovered when he went to get his car and drove a couple of laps without realizing the tech inspectors had pulled his fuel tank due to complaints about his "superior" mileage. Other Smokey tricks were the windmill operated alternator (actually put a fan blade on his alternator to provide electrical power with no engine drag), and the 67 Chevelle he hand-built at 7/8 scale for racing, was blowing everyone away until someone had to park a real 67 Chevelle beside his, and the size difference was noticed. A lot of the crap Yunick pulled is what resulted in the huge book of rules NAScar now runs, where the vehicle is regulated right down to vehicle height off the ground.
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