GENIUS FORUMS

When I was a young boy I thought the force of wind resistance upon a symmetrical object at the front of it would be roughly equal to the wind resistance at the back of it.

But is that a universal rule?

If an object that looks like two equal cones joined at the broad base is the projectile, will its drag coefficient be the same at the leading edge half, as the trailing edge half, through all fluids at all speeds?

And what about just one cone?

If leading and trailing are the same in the above example, would it not matter whether the blunt end, or the sharp end of the cone leads, traveling through a fluid?
Whether or not at wind speeds (zero to a few hundred MPH), what about at reentry speeds of many thousands of MPH?
What about through water?
What about through calibrated ordinance gelatin?

30 character limit on sigline?

Yep, but you can still get a URL in there if you use something like url.ie. But it won't be an active link because they have disabled that, too.

sear wrote:When I was a young boy I thought the force of wind resistance upon a symmetrical object at the front of it would be roughly equal to the wind resistance at the back of it.

Back and front of symetrical object is relative ie. depends on what you call back and front.

sear wrote:But is that a universal rule?

If an object that looks like two equal cones joined at the broad base is the projectile, will its drag coefficient be the same at the leading edge half, as the trailing edge half, through all fluids at all speeds?

It is not equal, but it is a good aerodynamic design as it reduces the drag from trailing edge.
This graph shows the average amount of drag that was produced by the spring scale in each trial. It gives a quick analysis of the data. Lower bars are better.
http://www.selah.k12.wa.us/SOAR/SciProj ... raphs.html

sear wrote:If leading and trailing are the same in the above example, would it not matter whether the blunt end, or the sharp end of the cone leads, traveling through a fluid?

It would. There is a reason why knife is sharp.

sear wrote:Whether or not at wind speeds (zero to a few hundred MPH), what about at reentry speeds of many thousands of MPH?
What about through water?
What about through calibrated ordinance gelatin?

Just more resistance to overcome. Did you know that glass is a very viscous liquid?

http://en.wikipedia.org/wiki/Aerodynamics

"Unidian"

Nat?
Is that you in there buddy?
Hello, and welcome to the Genius Forum.

"Back and front of symetrical object is relative ie. depends on what you call back and front." hs

Excellent point hs.
I should have said "geometrically symmetrical" to distinguish it from being aerodynamically symmetrical (assuming there is a substantive difference in this context).

"This graph shows ..." hs

Please pardon my nerdism here, but:
Boss!

I like it hs. Thanks for the link, the graphs, and the research. College project?

I infer that, regardless of 3 dimensional profile (shape), the area of maximum cross section in the plane perpendicular to the flight path is equal in the 4 shapes.

I'm a bit surprised that the teardrop would win. I thought water droplet's more or less mushroom, resulting in poorer aerodynamics. It's obviously relative.

At what relative speed did these objects pass through the fluid? What was the fluid?

"Just more resistance to overcome." hs

But I'm not sure it's that simple.
For one thing, I'm pretty sure it's not linear (you didn't suggest otherwise).

Compounding that, there are two different kinds of drag.
There's parasitic drag; probably mostly what you measured and graphed.
But there's also induced drag, such as wing tip vortices.

And though I don't know the details, things seem to go a little nutty aerodynamically at and or near the sound barrier.
Early test pilots experienced severe control issues at speeds around mach 0.95 or so (or whatever the number was).

"Did you know that glass is a very viscous liquid?" hs

We just did that debate a month or so ago at another board.

Supporting that notion is the appearance of some old panes of glass, which can plainly be seen to be sagging.

But I believe that appearance results from the method of manufacture; and that the glass was like that the day it was made.

The reason I disbelieve the cold, viscous liquid glass theory is because if it were true, glass would have to have TWO melting points; because around 1300C or so, glass becomes easily malleable; enough so to pour, blow into bubbles (bottles), etc.

The notion that a liquid could shatter seems counterintuitive to me.

If an object that looks like two equal cones joined at the broad base is the projectile, will its drag coefficient be the same at the leading edge half, as the trailing edge half, through all fluids at all speeds?

I don't see how it could be the same. There would be less air pressure at any given point on the trailing cone's surface compared to the corresponding point on the leading cone. The leading cone temporarily displaces air from a volume through which the trailing cone immediately passes before the air can rush back in to fill it entirely.

Yep, it's me.

And glass is an amorphous solid.

sear wrote: I like it hs. Thanks for the link, the graphs, and the research. College project?

That would have been too much of a coincidence. :-P I just dug it up on the net. Sorry for confusion.

sear wrote: I infer that, regardless of 3 dimensional profile (shape), the area of maximum cross section in the plane perpendicular to the flight path is equal in the 4 shapes.

It is the surface area and shape that counts...but you lost me there... what is maximum cross section in this case?

sear wrote:I'm a bit surprised that the teardrop would win. I thought water droplet's more or less mushroom, resulting in poorer aerodynamics. It's obviously relative.

I think he had all the shapes made out of same hard material most likely wood. ie. Teardrop shaped object.
http://www.selah.k12.wa.us/SOAR/SciProj ... ml#helpame

sear wrote:At what relative speed did these objects pass through the fluid? What was the fluid?

I think it was air.

sear wrote:But I'm not sure it's that simple.
For one thing, I'm pretty sure it's not linear (you didn't suggest otherwise).

Compounding that, there are two different kinds of drag.
There's parasitic drag; probably mostly what you measured and graphed.
But there's also induced drag, such as wing tip vortices.

I think he graphed the total drag on the shapes.

14. Hang the spring scale above the wind tunnel.
15. Take out the styrofoam vent piece.
16. Attach the shape you are going to use to the spring scale using the string.
17. Make sure the wind tunnel is air tight.
18. Do necessary adjustments.
19. Hook up vacuum cleaner to the bottom of the wind tunnel, making sure no air can get out.
20. Measure the weight of the shape you are going to use.
21. Turn the vacuum cleaner on.
22. Measure the spring scale.
23. Subtract the first measurement from the second.

Both also can be minimized to almost insignificant effect. Parasitic drag = by creating micro pockets to trap air/water and thus creating a air cushion on the surface area in contact with flowing air over shape, it is strange that only golf balls have that, I wonder what is the reason. Tip vortices can be broken up also.

sear wrote:And though I don't know the details, things seem to go a little nutty aerodynamically at and or near the sound barrier.

Punching hole through your own sound wave .. yes, i can imagine :).

sear wrote:(Liquid glass.) Supporting that notion is the appearance of some old panes of glass, which can plainly be seen to be sagging.

Yes, that's where I got it from.

sear wrote: The reason I disbelieve the cold, viscous liquid glass theory is because if it were true, glass would have to have TWO melting points; because around 1300C or so, glass becomes easily malleable; enough so to pour, blow into bubbles (bottles), etc.

Good point..

sear wrote:The notion that a liquid could shatter seems counterintuitive to me

Solid can still be fluid, but good point.

A supersolid is a spatially ordered material (that is, a solid or crystal) with superfluid properties. When quantum fluids, like helium-4, are cooled below a certain characteristic temperature, they undergo the superfluid transition and enter a state of zero viscosity, that is, there can be flow without friction. This transition is thought to be associated with a Bose-Einstein condensation.

In helium-4, it has been conjectured since 1970 that it might be possible to create a supersolid, which means the solid would show a fluid-like frictionless flow of particles.
-----
The experiment of Kim and Chan looked for superflow by means of a “torsional oscillator.” Picture a record turntable that is attached tightly to a springy spindle in the center. Instead of rotating at constant speed, the turntable is started off by a small twist clockwise and then let go. The spring causes it to spin back counterclockwise for a small angle, then clockwise, and then counterclockwise, and so on, a bit analogous to a swinging pendulum. Now glue a thin hollow donut centered on the turntable with solid helium-4 inside. The rate of oscillation of the turntable and donut depend on the amount of solid moving with it. If there is frictionless superfluid inside, then the mass moving with the donut is less and the twisting motion will occur at a faster rate. In this way one can measure the amount of superfluid existing at various temperatures. Kim and Chan found that up to about 2% of the material in the donut was superfluid. Similar experiments in other laboratories have confirmed these results.[citations needed] A mysterious feature, not in agreement with the old theories, is that the transition continues to occur at high pressures. (Source http://www.answers.com/topic/supersolid)

"I don't see how it could be the same. There would be less air pressure at any given point on the trailing cone's surface compared to the corresponding point on the leading cone." broke

I'm not asserting it is the same in this example (though that was my boyhood suspicion).
But it could be, if it is as follows:

Imagine the ambient air pressure is 10 PSI.
Imagine that as the leading cone (of the dual-cone object) passes through, the pressure builds to 12 PSI on it.
Then, the pressure on the trailing cone is 8 PSI.
Then wouldn't it be the same? A 2 PSI net pressure increase on leading surface, and a 2 PSI net pressure decrease on the trailing surface.
Whether it does that under any condition of speed, pressure, fluid viscosity I don't know.

"The leading cone temporarily displaces air from a volume through which the trailing cone immediately passes before the air can rush back in to fill it entirely." broke

At low speed and high pressure, perhaps.

But there are two things to consider.

a) At extremely low pressure (such as 1.0 X 10 E -6 Torr) we don't measure pressure, as in PSI. At that high vacuum state, gas molecules are so rare, it makes more sense to measure "mean free-path", the distance the molecules travel before they collide. A gas (& a plasma for that matter) act different at very low pressure.*

b) But at more normal pressure, gas flows at a rate (for that pressure, & other factors).
If the projectile passing through it is traveling faster than that rate, then I think the tailing cone will travel in a relative vacuum that loses symmetry with the leading cone's pressure.

Maybe this is called "cavitation" (which is something outboard motor propellers can do).

"glass is an amorphous solid." Unidian

That's what I figured, amorphous crystaline solid.

"... what is maximum cross section in this case?" hs

If the cone is one foot tall (and the base is circular, not eliptical) and the cone is traveling along its axis of radial symmetry, then its "maximum cross section" is at its base.

For convenience let's say that area is 6.000... square inches.
If all the objects (cube, cone, teardrop, etc) have a maximum cross sectional area that is not also 6.000... square inches, then you're not really testing relative "shape".

Why not?

Because if the maximum cross-sectional area of 3 of the four shapes is 6.000... square inches, but the 4th shape has a maximum cross-sectional area of 20,000. square inches, it would be reasonable to assume the vastly, vastly, vastly larger object would encounter more wind resistance, virtually regardless of shape.
Attributing that to "shape" would be preposterous.

Even four objects of the same material, and each of equal mass in those same four shapes could have different maximum cross-sectional areas (m c-s a).

example:
3 of the 4 could have m c-s a of 6.000... square inches, but the teardrop could be extremely elongated, and therefore have a m c-s a of 0.00400... square inches.

Attributing the lowest drag in that case to "shape" would be silly.

Doctor the m c-s a to any of the four objects to those values (three at 6.000... and the fourth at 0.00400... and that fourth, regardless of shape, is going to be the one with lowest drag.

So for the test to genuinely test the affect of shape on drag, all four objects should have the same m c-s a; in the plane perpendicular to the axis of travel.

"golf balls" hs

Right. The last article I read about that called them "turbulator pits".

"Bose-Einstein condensation" hs' post

Live & learn.

* So different in fact that the physics of pumping them changes.
When the gas can flow (in the PSI range) it can be pumped mechanically.
When in the mean free-path range, diffusion pumps are used. Basically there's a cryogenic head on it. The molecules chug along until they slam into the cryo-head, their temperature drops, and they're so cold they don't have the Brownian motion to leave, and they just sit there stuck.

sear wrote: "glass is an amorphous solid." Unidian
That's what I figured, amorphous crystaline solid.

Curiosity got the upper hand -_^

crystalline solids: molecules are ordered in a regular lattice
fluids: molecules are disordered and are not rigidly bound.
glasses: molecules are disordered but are rigidly bound.

For crystalline substances the distinction between the solid and liquid states is very clear, but what about glasses? Indeed, where do polymers, gels, foams, liquid crystals, powders and colloids fit into this picture? Some people say that there is no clear distinction between a solid and a liquid in general. A solid, they claim, should just be defined as a liquid with a very high viscosity. They set an arbitrary limit of 1013 poises above which they say it's a solid and below which it's a liquid.

According to another point of view, this ignores a distinction between viscosity of liquids and plasticity of solids. An ideal Newtonian liquid deforms at a rate which is proportional to stresses applied and its viscosity. For arbitrarily small stresses a viscous liquid will flow. Molasses, pine pitch and Silly Putty are examples of liquids with very high viscosity which flow very slowly under only the force of their own weight. On the other hand, plastics can be very soft but are still considered solid because they have rigidity and do not flow.

Solids are elastic when small stresses are applied. They deform but return to their original shape when the stress is removed. When higher stresses are applied some solids break while others exhibit plasticity. Plasticity means that they deform and don't return to their original shape when the stress is removed. Many substances including metals such as copper have plasticity. The resistance to flow under plastic deformation is called its viscoplasticity. This is like viscosity, except that there's a minimum stress known as the elastic limit below which there is no plasticity. Materials with plasticity do not flow, but they may creep, meaning they deform slowly but only when held under constant stress.

So an arbitrary measure of viscosity or viscoplasticity is not a good way to distinguish solids from liquids. Another way to define the distinction between solid and liquid is to say that, if there is a minimum shear stress required to produce a permanent deformation then it is a solid. This is just a precise way of saying it has some rigidity. A liquid can then be defined as a material that will flow. If it is placed in a container it will eventually flow to fill the lower reaches until its own surface is flat. The difficulty is that these two definitions do not cover all cases. There are materials that have some limited flow known as viscoelasticity. The material will deform elastically under stress. If the stress is held for a long time, the deformation becomes permanent even if the stress was small. Materials with viscoelasticity may seem to flow slowly for a while but then stop. It is futile to try to make a clear cut distinction between liquids and solids in cases of such behavior.
Source http://math.ucr.edu/home/baez/physics/G ... glass.html

Just as I thought.. semantics.


what is maximum cross section in this case?" hs

sear wrote: If the cone is one foot tall (and the base is circular, not eliptical) and the cone is traveling along its axis of radial symmetry, then its "maximum cross section" is at its base.

sear wrote:For convenience let's say that area is 6.000... square inches.
If all the objects (cube, cone, teardrop, etc) have a maximum cross sectional area that is not also 6.000... square inches, then you're not really testing relative "shape".

sear wrote:I infer that, regardless of 3 dimensional profile (shape), the area of maximum cross section in the plane perpendicular to the flight path is equal in the 4 shapes.

Wouldn’t you use internal volume ratio to make the tests measurement valid instead of maximum cross section?

ie: internal volume that is same for both a cube and teardrop or cylinder will not have the same size maximum cross sections. If they did the surface area of teardrop and cylinder will be much greater and hence more drag from friction.

sear wrote:Because if the maximum cross-sectional area of 3 of the four shapes is 6.000... square inches, but the 4th shape has a maximum cross-sectional area of 20,000. square inches, it would be reasonable to assume the vastly, vastly, vastly larger object would encounter more wind resistance, virtually regardless of shape.
Attributing that to "shape" would be preposterous.

I think you are working from flawed assumption there ->

sear wrote:I infer that, regardless of 3 dimensional profile (shape), the area of maximum cross section in the plane perpendicular to the flight path is equal in the 4 shapes.

".. in the test..

sear wrote:Even four objects of the same material, and each of equal mass in those same four shapes could have different maximum cross-sectional areas (m c-s a).

example:
3 of the 4 could have m c-s a of 6.000... square inches, but the teardrop could be extremely elongated, and therefore have a m c-s a of 0.00400... square inches.

Attributing the lowest drag in that case to "shape" would be silly.

Hmm... I thought the test was to measure the same surface area of 4 objects vs drag and how different shapes affect it. I don't find that silly. :-P

sear wrote:Doctor the m c-s a to any of the four objects to those values (three at 6.000... and the fourth at 0.00400... and that fourth, regardless of shape, is going to be the one with lowest drag.

I disagree :D

sear wrote:So for the test to genuinely test the affect of shape on drag, all four objects should have the same m c-s a; in the plane perpendicular to the axis of travel.

:D

sear wrote:

"golf balls" hs

Right. The last article I read about that called them "turbulator pits".

"I call them speed holes" H.J.Simpson :-P

Wouldn’t you use internal volume ratio to make the tests measurement valid instead of maximum cross section?

ie: internal volume that is same for both a cube and teardrop or cylinder will not have the same size maximum cross sections. If they did the surface area of teardrop and cylinder will be much greater and hence more drag from friction.

Understood.
That introduces aspect ratio.
My problem with using 4 equal volumes of 4 different shapes (I'm not sure that's precisely what you're suggesting), without controlling for uniform maximum cross-sectional area also is that the most spaghettified one, that is, the one with the smallest maximum cross-section would likely be the one with the least drag, regardless of other shape characteristics.
Use reductio ad absurdum to make the case.
3 of the objects fit inside a lunch box.
The 4th object is 20 feet long, essentially a rod.

Perhaps we have different test objectives in mind here.
But to me, insuring that all maximum cross-sectional areas are equal, and each of the 4 are also of equal volume, and have the same surface area is the best objective test of the affect of shape on drag.

"I think you are working from flawed assumption there" hs

Oh?
Perhaps.
Intuitively, and by experience, larger objects tend to have more drag.
Single-surface hang gliders fly substantially differently than double-surface hang gliders. The latter may be heavier, but fly better.

"I thought the test was to measure the same surface area of 4 objects vs drag and how different shapes affect it." hs

Perhaps it was.
I'm not demanding a refund. He / they can do any science they like.
But it seems to me maximum cross-sectional area should be controlled for; particularly in my single cone / dual cone original inquiry. And I believe that control can be obtained while also controlling for uniform surface area, and uniform volume. Just vary the aspect ratio.

"I disagree" hs

I had in mind to exclude absurd corkscrew shapes, etc.

sear wrote:My problem with using 4 equal volumes of 4 different shapes (I'm not sure that's precisely what you're suggesting),

That was what I was suggesting.

sear wrote:without controlling for uniform maximum cross-sectional area also is that the most spaghettified one, that is, the one with the smallest maximum cross-section would likely be the one with the least drag, regardless of other shape characteristics.

Makes sense in a way, but lets say you have cylindrical object with flat ends with diameter of 5cm maximum cross section and a teardrop shaped object with maximum cross section of 20 cm. Are you saying that you belive that there will be less air resistance due to drag on the cylinder than on the teardrop shape in a flight? Or did I misunderstand you?

There are great vids on the net of army testing aerodynamics of high velocity projectiles such as bunker buster missiles etc. I am curious about what you are trying to achive with this line of inquiry.

sear wrote:Perhaps we have different test objectives in mind here.
But to me, insuring that all maximum cross-sectional areas are equal, and each of the 4 are also of equal volume, and have the same surface area is the best objective test of the affect of shape on drag.

I have no objectives, just trying to help and keep the conversation going untill someone with a clue pipes up :D BTW. What happened to our foulmouthed zonist jew know it all Vic? Did Dan ban him?

sear wrote:Intuitively, and by experience, larger objects tend to have more drag.

I think it has to do with larger surface area in contact with air the large objects tend to have.

sear wrote:When I was a young boy I thought the force of wind resistance upon a symmetrical object at the front of it would be roughly equal to the wind resistance at the back of it.

But is that a universal rule?

It's not a universal rule. I never had those thoughts as a young boy.

Jason wrote:

sear wrote:When I was a young boy I thought the force of wind resistance upon a symmetrical object at the front of it would be roughly equal to the wind resistance at the back of it.

But is that a universal rule?

It's not a universal rule. I never had those thoughts as a young boy.

lolz

"Are you saying that you belive that there will be less air resistance due to drag on the cylinder than on the teardrop shape in a flight? Or did I misunderstand you?" hs

I'd be more inclined to call the "cylinder" a rod at that point. But yes, I suspect so.

But hs, it's almost 9:AM/ET here now.

I'm re-thinking my earlier posts.

I indicated I believed 4 objects all of the same material could be made with the same:
- surface area
- mass
- max. cross-section

Then I think I drifted into varying aspect ratio.

a) I haven't done the math.
b) I haven't neglected that due to sloth. The math of that is simply beyond me.
c) I assume / believe varing aspect ratio can bring those 3 criteria into equality among the 4 objects, but I have no proof of it.

"I am curious about what you are trying to achive with this line of inquiry." hs

What I'm trying to figure out is the 3 cones shapes.
1) is the dual cone; one pointing toward direction of travel, the other pointing away, object mated at the cone bases.
2) a single cone, pointed toward direction of travel.
3) a single cone, pointed away from direction of travel.

On a cone (with circular base) the circular base (which contains virtually no material) has the same cross-sectional area as the entire rest of the cone.

Specifically I'm trying to figure out the relation between drag induced by pressure build up at the front, vs pressure drop at the back.

Many (if not most)(but not all) bullets tend to be pointy at the front, less so at the back.
Subtleties ensue.
A boat-tail bullet is alleged to travel farther (less drag) than a bullet of same caliber and mass, with less refined trailing edge.

"I think it has to do with larger surface area in contact with air the large objects tend to have." hs

That's perfectly plausible; and in my opinion, most likely.

"just trying to help and keep the conversation going untill someone with a clue pipes up" hs

Well done, IMAO.

"I never had those thoughts as a young boy." Jason

Elementary (Newtonian) physics has fascinated me from a very early age. By the time I got to college I had to contend with the reality that I grasp concepts pretty well, but am very poor at math; especially memorizing complex formulae. I didn't do well in college statics & physics.
In fact, I've made horribly little progress in my 50+ years. Still in a childhood rut here.

"It's not a universal rule." Jason

Perhaps simplifying my search from cones to a sphere might cut to the chase on this one.

But if there is a case when the drag on the front of the cone is greater than the drag from the back,
and if there's a case where it's the opposite,
that suggests there's a middle case where it's equal.

I'd be curious to know what the conditions are of that middle case; fluid density, speed, etc.