On Wed, 7 Dec 2005, Rick Monteverde wrote: > >But the incoming air will fill the vacuum chamber, with > >the wave travelling at roughly the speed of sound! > >In human time scale, as soon as you open the valve > >and generate an air jet, significant air pressure > >appears on the OTHER side of the wing. You can't > >just claim that the pressure there is insignificant, > >instead you have to measure it, millisecond by millisecond. > > The pump is large compared to the small jar volume, and once that dense > air in the jet disperses, which it does very quickly, density and > pressure get pretty low pretty fast before much of it swirls around > underneath the foil. To see it and its scale is convincing. Seeing my > writing about it isn't.
Eh. Seeing the demonstration wouldn't convince me, since my brain would insist that "SINCE the airfoil is deflected upwards, THEREFORE the pressure underneath is greater than the pressure above." :) > > >If you can show that air can PULL on a curved wing > >(i.e. create an absolute negative pressure,) > >that's something very interesting. > > Yup. It's been shown too, but not by me. Google should bring it up with > words like van der Waals, airfoil, boundary layer, etc. But that's just lowered pressure, not absolute negative (attraction) pressure. Boundary layer stuff is weird, but I've never seen articles talking about negative gas pressure. It's hard to see how a molecule, by colliding with a surface, could *attract* that surface. And it's hard to see how widely separated molecules could attract each other on average, especially if they're moving fast enough to bounce during collisions (which would create a strong repulsion force which would have to be canceled out by any attraction mechanism.) If they don't bounce during collisions, then that's called condensation. :) > Why else would a > flow stick against a surface and follow it down around a curve like > that? For air jets in air, or for water jets underwater, Coanda Effect explains it: air flows always entrain adjacent air, pulling the adjacent air into the flow. Or said another way, flows always represent lower pressure, so if air is flowing parallel to an object, the perpendicular force between the flow and the object will be reduced, causing the flow and the object to accelerate towards each other as the outer (non-flowing) air exerts its non-reduced pressure. Blow some air parallel to one side of a dangling piece of paper and the paper will be pushed into the flow so it "adheres" to the flow. And the flow will "stick" to the paper, bending away from it's original trajectory. Separate topic: In that old SciAm article about Coanda Effect, they found that tiny structures within the boundary layer could have large effects, so a small step or striation on the surface would make the flow-adhesion effect stronger. I remember one oddity from conventional textbooks: if you put a polished sphere in a wind tunnel, the smoke will curve around the sphere and follow the back of the sphere for quite a ways before "detaching" and becoming turbulent... but if you add a small disk of thin sandpaper (or even roughened paint) to the very front of the sphere, the smoke then detaches right at the circumference of the sphere, and it won't follow the curve around to the back of the sphere at all. Just that tiny change to the front of the sphere will put the entire rear of the sphere into "stall mode." Aircraft designers know all about the effect: just a small bit of rough ice on the leading edge and top of an aircraft wing will trigger early flow-detachment, ruining the lift and leading to crashes on takeoff. That's why they're so paranoid about "de-icing" the tops of airliner wings. The airfoil bottoms are mostly irrelevant (and you can even hang huge fuel tanks and racks of missles down there.) Also there's a whole group of experimental aircraft hobbyists who specialize in high-lift "laminar flow" wings with highly polished upper surfaces. These aren't widely used because their characteristics are seriously altered by a small bit of raindrops clinging to the wing. > > I never finished construction on it, but I started a rig where the > airfoil sat on a membrane with good vacuum under the membrane in a > separate chamnber from the air above the foil. Air jet would hit the top > of the foil as before, but the whole bottom side would be against the > membrane. Pump would keep the air above at as low a pressure as possible > while the jet shot across the foil surface. Now THAT would be more convincing (even more convincing that measuring the pressure under your first airfoil.) > I figure the foil would > still rise into the airflow, pulling up on the membrane with the > certain-to-be-lower pressure below it. > > Maybe simpler to use a split chamber with water instead of air? Or use an oil stream in a vacuum? But then you might get genuinely negative fluid pressure, the same negative pressure that's the source of "surface tension." > > - Rick > > > > (((((((((((((((((( ( ( ( ( (O) ) ) ) ) ))))))))))))))))))) William J. Beaty SCIENCE HOBBYIST website billb at amasci com http://amasci.com EE/programmer/sci-exhibits amateur science, hobby projects, sci fair Seattle, WA 206-789-0775 unusual phenomena, tesla coils, weird sci
