Aerodynamic lift- A real problem

[MadisonMPG]

Originally Posted by Christ

Does a brick maintain attached flow? (Joking, of course.)

If I correctly understand what you're saying, the brick is neutral because it's shape is evenly proportioned on all surfaces along the stream, correct?

Completely OT question - Since wind forces can erode a surface, would a sufficient high speed wind eventually turn a clay brick into a teardrop profile?

Wow, I sat and thought about this for a while. I think a sandstorm might round the edges, but I don't think it could ever wrap around to make a tear drop shape. Then again, that is how rain drops are made... but they aren't even tear drop shaped.

I have added nothing to this thread...lol my bad.

[Christ]

A constant airflow (in my head) would eventually make the teardrop shape, but it would take literally forever, because of the equivalent force necessary to maintain attached flow while eroding the structure.

First, the erosion would round the forward corners. After that, if the flow were maintained in the same direction, the flow would be subject to boundary layer adhesion, and the majority of erosion would occur still at the front, but some erosion would now also occur at the sides, although to a much lesser extent, since the blunt force of the flow occurs at the leading face, and the rest is subject to energy expended to maintain attached flow. The next thing to be rounded would be the rear, due to the vortex effect creating a vacuum behind the brick.

This creates an egg shape.

The next effect is still a matter of attached flow, but as you can see (from my thought) the majority of effects after the front erosion allows for attached flow is from vortices drawing on the structure of the object. The eventual erosion of the tail section (beyond the widest point of the tear drop) should actually occur from the back to the front, and should occur at a rate equivalent to the energy being expended in the vortex, which decreases as the vortex gets smaller.

In other words, if you were to divide the events into time sections, the first section would be the erosion of the face into a mostly rounded object which represented a shape that (at the constant wind speed and direction) would maintain attached flow along the surface of the object in question.

The second time period would be the rounding of the trailing edges of the object, as well as surface erosion of the length of the object on all contacted sides.

The third time period would be the eventual erosion of the object into a form that would allow for perfectly attached flow along it's entire surface, with no vortex created at the trailing edge.

The fourth time period would encompass the eventual complete erosion of the object, as it basically went from an originally sized object of optimal shape, to a smaller and smaller object of the same optimal shape, until the object was completely eroded into basic components.

(The linearity of the thought requires that the bonding structure of the object be exactly uniform at any point in it's body.)

Man, I love thinking about dumb stuff...

[winkosmosis]

Originally Posted by Christ

Does a brick maintain attached flow? (Joking, of course.)

If I correctly understand what you're saying, the brick is neutral because it's shape is evenly proportioned on all surfaces along the stream, correct?

Completely OT question - Since wind forces can erode a surface, would a sufficient high speed wind eventually turn a clay brick into a teardrop profile?

The brick (aligned with movement direction) is neutral because the surfaces are all either perpendicular to flow or parallel, so no surface is making lift or downforce. Most importantly, the top/side/left/right are creating no drag except friction, and the back is not creating anything but vacuum. Is it reasonable to say that angling the top side will increase drag by increasing lift? No... you're decreasing drag.


I don't think a brick would erode into a teadrop. The most force and erosion is always going to be on the front face.

[Christ]

Thought so.

[Nerys]

No tear drop is NOT IDEAL. Tear Drop is closest to ideal as a consequence of our needed compromises IE the need to put STUFF inside it.

Ideal is 2 dimensional line. ie zero frontal/base drag This is what erosion will do to an object IE erase it. the SHAPE of things eroded is mostly about density and type of material. ie one piece erodes faster than another portion etc..

EXERTING A FORCE IS NOT THE SAME DAMNED THING AS LIFT.

LIFT is a very SPECIAL SPECIFIC KIND of force exertion. Lower pressure on top via a venturi effect resulting in an effective higher pressure below the wing PUSHING or SUCKING the wing "up" toward the lower pressure.

THIS is lift. without ALL of those conditions ITS NOT LIFT.

its just pressure. its just a "force vector" nothing more.

your tear drop has PRESSURE has FORCE acting on it. It has NO LIFT.

winkosmosis gets it.

FORCE PRESSURE AND LIFT are not the same thing. its like saying a strawberry is red an apple is red so they must both be apples.

"If there is to be lift, there is also to be a change above the object being lifted. That change must be equalized below the object, meaning that higher pressure under an object, or lower pressure over an object, will create lift. Lift, in essence, is generated by a change in pressure... that would make pressure equivalent to lift, since they change linearly with each other, they can be transposed.

The above being the concrete case in physics, a force can only be counteracted by an equivalent force of opposing direction. This is to say, again, that lift still occurs at any tangent on the shape, but is opposed by the same gross lift on the opposing tangent, creating a net zero lift situation."

Christ. there are so many flaws in this statement where to begin.

your ENTIRE foundation is based on this statement.

"that would make pressure equivalent to lift"

without THAT statement nothing you say makes sense.

here is the problem.

ITS WRONG MAN. NOT EVEN CLOSE to being true.

PRESSURE DOES NOT EQUAL LIFT.

SHAKE UP A COKE. TONS of pressure right? any lift?

but "that would make pressure equivalent to lift" right?

NO.

Pressure is just pressure. nothing more nothing less. Pressure is "PART" of the lift equation but thats all it is. PART of it. they are not synonymous.

any more than a TIRE equals CAR. no a tire is simple a COMPONENT of the car. Pressure is simple a COMPONENT of lift.

without the "rest" of the components you don't have lift. Just like if you only have 4 tires sitting in your driveway YOU DO NOT HAVE A CAR.

until you have the rest of the pieces needed you don't have a car

until you have the rest of the pieces needed you don't have LIFT.

[Christ]

You just said openly that Newton's Third Law doesn't make sense.

Basically, you're trying to tell me that negative pressure acting on an object in any plane doesn't cause a change in force direction of that object, correct?

So, if I were to assume you're correct, which, according to ALL the studying I've done on fluid dynamics, you're not, I'm getting the idea from you that airplanes just don't fly.

If negative pressure is exerted on any part of an object, the object will move in that direction. THAT is how lift works. Lowered pressure caused by acceleration of flow adhering to (but not solely becuase of) Bernoulli's principle. Once that has occurred, Newton 3 takes effect, in that all forces must equalize. If the pressure above the object is lower than the pressure below the object, the object will experience an increase in fluid buoyancy.

You can't argue with that.

The only way to counteract the force acting on the object is with an opposite equivalent force, i.e. the equivalent negative pressure on an exactly opposite point on the same shape, or two other exactly equivalent forces (plus the first one) acting on tangents which would cancel each other out, etc and so on.

The NET RESULT is zero lift in any direction, but the gross effect is still lift IN ALL DIRECTIONS due to pressure change IN ALL DIRECTIONS.

Imagine the object is hollow, with no entrance or exit to the interior space. There is dead air inside, at atmospheric pressure. That pressure causes the object to expand when the pressure outside the object is lowered, so the shape itself would expand at it's widest point, or the area on which the volume of gas inside could have the most effect on the surface of the object. Each segment of the object is, in essence, lifting away from the other segments. All segments working against each other in an equilibrium means that the net effect is zero.

[JackMcCornack]

Originally Posted by Nerys

...you are wrong...this is where you guys are getting confused...you guys are unclear...you are mistaken...your not fully grasping...you also do not understand...you think<snip>

Am I included in the "...you guys..." portion of this post? Do you disagree with my opinion that a car with a bit of lift has less drag than a car with zero lift? I think your AE schooling is more current than mine--heck, I don't even think we called anything "base drag" back then--so I'm sure willing to read and learn from you.

Originally Posted by Nerys

while it sounds semantic in discussion mathmatically its a night and day difference.

I want to know because I'm designing a high mileage body for my car, and I can see there are different ways of looking at this (as my dad used to say, "Two plus two equals five, for very large values of two."), so if I concede that...

Originally Posted by Nerys

the produce of lift by the curved aero cover has ABSOLUTELY NOTHING to do with the reduction of drag by not letting it be formed by a blunt rear shape base drag.

they are UNRELATED.

...is a valid view, and that I "...do not understand how drag is effective and added up in the equation of overall drag...", can you tell me if the net effects of these unrelated strategies for minimizing a car's drag adds up to a force with a vector perpendicular to the ground plane, or as us laymen call it, lift? 'Cause right now I'm going by the books, Hoerner to Hucho, and it looks like this body is going to generate lift equal to about half its drag (Cd ~0.30, Cl ~0.15) and everything I'm considering to reduce lift appears it will increase drag. While in theory...

> OPTIMAL is zero drag

...and in that theoretical condition there'd be zero lift too, in the practical world where we're trying to reduce--not eliminate--aerodynamic drag, will we achieve our lowest drag with no lift or with some lift?

[Christ]

Originally Posted by NASA Website

NO MOTION, NO LIFT

Lift is generated by the difference in velocity between the solid object and the fluid. There must be motion between the object and the fluid: no motion, no lift. It makes no difference whether the object moves through a static fluid, or the fluid moves past a static solid object. Lift acts perpendicular to the motion.

What this doesn't say is that lift is also potential, in that just because an object as a whole exhibits zero lift in any direction perpendicular to motion, doesn't mean that individual parts of the same object would not be subject to lift. If each part of a whole would be subject to lift in every possible direction perpendicular to motion, the net effect is zero lift, but the potential for lift is still there.

The principal of zero lift on a tear drop profile also assumes that the tear drop shape is traveling in dead air. Active airspace directly affects the way flow negotiates the surface of a shape, and thus, even a teardrop would act as an airfoil in normal conditions, except in an unpredictable direction based on the low-pressure area it contacted.

Lift doesn't only go up, it can go in any direction perpendicular to the direction of motion. The concept that lift means up is caused by thinking 2 dimensionally.

This forum is making me wish I hadn't sold all my books years ago when I quit high school. I knew it was a bad idea back then.

[thatguitarguy]

So downforce is lift. I've read that Indy cars and Formula 1 cars create enough downforce at 200+mph that they could drive on the walls or ceiling of a tunnel.

[Christ]

Downforce is negative lift, yes.

It wasn't until recently in the history of F1 that the cars were designed in such a way that they could create more downforce than the weight of the car, allowing them to adhere to the walls or roof of a tunnel when operated at proper speeds.

Lift is increased as speed increases, which actually means that any vehicle could do this, as long as it generated a net negative lift over the surface of the vehicle, and could have sufficient speed to create enough negative lift to counteract the weight of the vehicle.