There I am, using one of those honkin’ big leaf blowers. The ones that look like a jet engine with wheels and a handle. As I’m moving around and blowing them into deeper and deeper piles, I begin to wonder.
The leaves move as though affected by a riptide if I blow too low in the huge soft piles. They curl under and pull away at the upper layers of leaves, just like a riptide in water. If I tip the blower up so the whole pile gets blown air, it moves with a more even movement away from me. The entire operation feels like I’m watching water move.
Are there analogies here? Is air a fluid in it’s physics of flow ? I realize that one can fly because of the differential in air pressure above and below a wing; I doubt one can achieve same under water and so I belive it’s not a pure analogy.
Still…the way the leaves danced and poured felt so fluid to me.
Correct me if I’m wrong, but aren’t gasses and liquids all fluids? The thing with gasses (air, hydrogen, etc.) is that they expand to fill their containers and are compressible, whereas water, mercury, liquid helium, etc. are not compressible.
If you want to get really picky, air is a good example of a newtonian fluid. Water is another, which makes fluid mechanics experiements convenient.
In lay terms, that just means that it’s a “normal” fluid - constant viscosity, and a few other things. Quite a number of fluids we see are non-newtonian - their viscosity changes with rate-of-strain or some other issue.
Air can also be treated in many cases as a perfect gas when compressibility is an issue (i.e., it obeys pv=RT, where R is the specific gas constant, R[sub]universal[/sub] / M). This seems a bit odd at first, since air is actually a hodge-podge of a number of different gases. However, simply using a molecular weight of 28.966 and treating it as any other p.g. works just fine almost all the time. (Until you have to start worrying about chemical reactions at really high temps and pressures).
BTW, wings don’t require a compressible fluid in order to work - they’d function just fine under water. Until the speed in a given flow begins to approach the local sound speed (a good rule of thumb is a Mach number of 0.3 or more), the compressibility of the gas can generally be completely neglected because in the flow it is negligible.
Many low-speed flows can be studied in either air or water, with no change in the results (after accounting for their different densities and viscosities - but not, significantly, compressibility).
Welcome to The Straight Dope, gportela ! And thank you, that is a nifty analogy.
brad_d, I am not sure I understand what you wrote near the bottom. I mentioned wings because an airplane is lifted due to specific properties of moving a curved surface through air. If you fly a plane through water, do you really mean that a differential in pressure will result because of the curve of the wing, and the craft will be elevated upwards through the water?
I don’t mean to hijack my own thread, but you went someplace here that makes no sense to me. It’s all about fighting ignorance.
Of course wings work underwater. How else does a submarine change depths? True, they do have floodable ballast tanks, but they only function up to a point.
However, I feel compelled to point out that wings don’t have to be curved in order to generate a pressure differential. The popular analogy here is the “flying barn door in a tornado” one. All it takes to generate lift is a strong enough pressure differential; it’s just that curved wings are more efficient than barn doors.
brad_d is correct. In fluid mechanics gasses are treated as incompressible at speeds that are a small fraction of the speed of sound. So the only difference between a gas and a liquid at low speeds is viscosity, density and so forth.
Yes, pressures in liquids vary when they within a pipe. Think of pressure and velocity as being two different aspects, potential and kinetic, of the energy of the flow. Energy is conserved and if the velocity goes up the pressure has to go down. So if the flow over the top surface of the wing is faster than the flow over the bottom surface then, energy not being gained or lost, the pressure on top has to be less than the pressure on the bottom.
This is my stupid post posting “preview.” Why doesn’t everybody just ignore this sentence? It doesn’t make much sense, is unnecessary, and the paragraph is much better without it.
Nope. The rule is that at any time, within a body of fluid that is undergoing streamline flow, a certain expression relating the density, pressure, speed, and height has a constant value at all points. If you change conditions the value can change. You can raise the pressure in an entire system without slowing down the flowing fluid: but after you do so it will ramein true that in that system the fast-flowing and high-elevation sections will have lower pressure than where the fluid is slow and low.
Say you are pumping water through a pipe that has thin sections and wide sections. In the thin sections the velocity will be high and the pressure will be low. In the wide sections the velocity will be low and the pressure will be high. If the pipe also undulates, the pressure will be higher (for given velocity) in low sections and lower in high sections.
This is basically a version of the conservation of energy. Fast water is travelling along with kinetic energy: it can only slow down if it does work against pressure (or against gravity). Moving into a zone of high pressure (or moving uphill) it accumulates potential energy in the form of pressure (or gravitational potential), but it slows down. If it then moves toward an area of low pressure (or downhill) the pressure (or gravity) will do work on it, and that will result in it speeding up.
As has been discussed her pretty extensively, looking at differential pressures is not the way to understand how a wing creates lift. It creates lift because it deflects air downwards. A submarine wing deflects water (either up or down, depending on whether it’s raising or lowering the sub). The old ideas that people have, where a wing passes through a fluid without deflecting it, and using some mysterious pressure differentials, is not correct. If the fluid is not deflected, Newton says that no lift can be generated.
Well, of course when you increase the pressure you are increasing the system energy, aren’t you? And that added potential energy input results in an increase in the kinetic energy of the flow. Lets take the case where the flow exhausts into atmospheric pressure, like a garden hose. Now if you open the faucet wider you have increased the pressure at the input of the hose. The output pressure remains the same, atmospheric, so there is a greater pressure drop between the ends of the hose and so there is more energy and increased flow velocity.
However, if you are running water through a pipe at a constant energy level and the flow comes to a place where the pipe diameter increases the velocity will go down and the pressure will go up. Conversely, if you have a smaller diameter section of pipe the velocity will go up and the pressure will go down. This is the venturi effect.
OK… could we have that cite please? I’m interesting in hearing if other people back this one up.
::sigh:: These aren’t “old ideas”, using “mysterious pressure differentials”, it’s how they work. Bernoulli and Newton work with each other, not against. Yes, the airfoil deflects air/water both upwards and downwards. However, Newton’s action/reaction laws don’t explain how certain airfoils at zero angle of attack can still generate lift. Or how a strong enough wind can pull the roof or door off a building.
