Spoon - Airfoil Challenge

First off, I haven't actually tested this, but I assume the result would be that the flow actually sucks the spoon further into it, coming to some kind of equilibrium position such that the spoon impedes the flow of water moreso that at initial contact. Depending on what reason you subscribe to for the generation of lift on an airfoil, there could be a number of solutions.

One would be that, as the water flows over the back of the spoon, it must curve to follow the curvature of the spoon. Thanks to Newton's first law, we know that there must be some kind of force that causes the water to bend - otherwise it would continue in a straight path. This could be due to surface tension keeping the water attached to the spoon, but, as is the case with airfoils, the force that causes the most curvature is due to a pressure gradient. If you have a high pressure on one side of a streamline through the flow and a low pressure on the other, the fluid will curve towards the low pressure region. It is this that causes the water to stay attached to the spoon. Now, as to why this causes the spoon to get sucked into the flow, well once the water leaves your faucet, it is effectively at atmospheric pressure. For the water to curve and follow the contours of the spoon, this requires a zone where the pressure is lower than that of the water, on average, which would then be lower than atmospheric pressure (which also happens to be the pressure on the dry side of the spoon). Since there is a higher pressure on the dry side than the wet side, the spoon is forced further into the flow until the gravitational force on the spoon and the spoon's impulse force on the water match the force caused by the pressure differences.

(I realize that some might point out that the force that would tend to curve the water would be that of the water molecules further upstream. Consider, though, a continuum point of view, rather than a particle based one, and you see that this force is equivalent to an increased relative pressure. But, of course, since the water must be at atmospheric pressure - else it expand into the air surrounding it - this means not that the pressure above is larger in an absolute sense, but that the pressure below is decreased.)

Another solution would be due to the fact that, assuming the flow of water is largely isentropic, the total pressure of a flow is constant (total pressure being the sum of the static pressure as well as the dynamic pressure). As the water curves around the spoon, it speeds up, thus increasing the dynamic pressure, in turn decreasing the static pressure, and we again arrive at the pressure differential across the spoon causing it to enter further into the flow.

Of course, if the spoon happens to be pushed out of the flow, well then I just tossed $7000 in tuition down the drain.

Cheers

The term that describes the sticking of the water or any fluid to a curved surface is the Coanda Effect. The movement of the spoon is caused partially by the draw of the water's surface tension and the redirect of the mass of the water in a different direction. (For every action there is an equal and opposite reaction. The redirected water is kinda acting like a jet pushing the spoon into the impinging stream.

Unless the flow is on both sides of the spoon, you will not create any lift. Flow striking the bottom side of the spoon will just deflect the spoon away again and again in a repetitive half-pendulum motion.

Actually, flow on one side alone works just fine. Try it. The spoon gets "sucked" into the water stream, even with the convave side of the spoon dry.

In some ways this is similar to the experiment in which one blows over the top surface of a sheet of paper, and the paper lifts (a classic aerodynamics demo). Of course, here there are two different fluids in play. If you actually do the experiment and move the spoon around in the stream, you'll find that the force causing the spoon to move into the stream is directly related to the amount of deflection of that stream. Even if the flow is directed across both the "low pressure" and "high pressure" surfaces of the spoon (the spoon has to be off-center in the flow to do this, avoiding the handle) the spoon still moves in proportion to the amount of stream deflection. Coanda effect is the name given to the tendency of a stream to follow a curveforce ing just one side of the spoon. The effect is present in both airflow and water flow, and probably not explained entirely by surface tension. Look at this link: http://jnaudin.free.fr/html/coanda.htm I like to think of fluid foils as simple deflectors, with the lift created being due to Newtons equal and opposite thing.

I have a faucet, we call it a 'tap' here in the U.K. But more importantly I have sticky toffee, nice and maleable when warm. I can adapt the shape where the water leaves the surface, One problem remains, "How do I remove the toffee from the spoon?"

Okay! Thanks for the great replies!!! I thought that Talented Fool was going to get it when he brought up Newton, but then he got lost when he started a dissertation on pressure. Close, but not quite there.

However, Dave Meador pretty much nailed it when he spoke of action and reaction. So hats off to Dave!

Yes, the spoon will get drawn into the water stream pretty aggressively (try it and see). Water follows the curve of the back of the spoon and is physically redirected. The vector at which the water leaves the spoon has a component that is opposite of the direction perpendicular and away from the tap water stream (before striking the spoon). That force of redirection easily overpowers any force that tends to push the spoon away from the water and the spoon soon spends it time inside the waterfall.

Now, ask yourself how this effect may actually play an important role in the airfoil of an aircraft wing. Could this same property cause lift on the wing and would it be significant?

Not intending to be an ungratious runner-up, but I must point out that there must be a cause for the water to follow the back of the spoon, which would be the pressure differential (of course there is a viscous component at the boundaries of the flow keeping it attached to the spoon, but the pressure keeps the 'inner' flow following the spoon's contour). Yes, of course, Newton's third law comes in to play as there is definately a momentum component of the flow as it leaves the trailing edge of the spoon that must be compensated for, but there most definately is a suction effect there too or else the flow would not stay attached. It is the low pressure along the back of the spoon (which creates a lift force on the spoon and on an airfoil) that causes the change in momentum of the water (or vice versa, if you wish) that further drives the water out to the side and the spoon into the flow. Really, forces, pressures and velocities, and momentum are all facets of the same phenomena insofar as the physics of the situation causes immediate and measurable effects in all these areas.

Anyway, good question to begin with.

As for its use on a wing, it is being put to use, but in the way that I mentioned earlier that it is more or less a by-product of the lift phenomenon. Unless you are going supersonic or high angles of attack, the actual downwash in of itself off the trailing edge of a wing constitutes a fairly minimal contribution to the overall lift of the wing. Most airfoils are designed to maximize the pressure differential between upper and lower surfaces during level flight, but an extra boost helps during takeoff and landing when extra emphasis is put on directing the flow of air downwards.

The real point of the puzzle is that the water's direction is redirected and the vector change of the water's trail requires force to get there. For every action there is an equal, but opposite reaction is the real answer that I was looking for.

The force that drives the spoon into the stream is that redirection. What makes the water bend around the spoon is the Coanda Effect. At least that is my understanding of it. You may be correct that there are pressure differences that drive that bending, I am not going to discount that.

So both you and Dave did speak of the redirection of water and that is what is key! I am happy give credit to both of you for that!

Thanks for your contributions!