A short time ago, this fine board managed to dispel some long-held ignorance of my own with regards to the way aeroplanes fly - or Bernoulli Schmernoulli, for the most part.
So what is the straight dope on the trick where you put a vacuum cleaner into reverse mode so that it blows, then you keep a ping pong ball “magically” floating in the air a few inches above the vertically-held nozzle? Is this still pure Bernoulli Principle?
I’ve always assumed it is a Bernoulli phenomenon, as if the ball decides to move sideways into the surrounding still air, it is promptly pushed back into the moving stream by the higher surrounding pressure. However, after having my long-held views on aircraft lift proven wrong, I’m now not so sure.
Here’s a kitchen experiment for you (really - try it). Turn on your sink tap at medium speed, so that the water flow becomes a nice clear stream on its way down (no aerator bubbles in it). Now approach the stream with the side of a finger held horizontally. When your finger touches the water stream, which way will the flow be defleted? It kind of seems like it would “bounce” off the edge of your finger and go to the side opposite it, but what really happens is that your finger slows down the water flow right against your skin, and the water wants to stick together, so the whole stream bends and curves to the side your finger is on. Since you’re deflecting the stream that direction, there is also a small force pulling your finger (heh heh) towards the center of the water stream.
In fact, if you half-fill the sink and float a ping pong ball under the stream, the ping pong ball will stay there, because if it starts to go off on one side, it will deflect the water stream around the ball, pulling the ball back to the middle.
The same thing is happening with the air flow. If the ball gets off to one side, it will have a greater airflow on the side closer to the center of the air flow. The round shape of the ball will deflect that air around the curve of the ball, so the opposite force for that air deflection on the ball pulls it back to the middle.
By the way, this can be explained by Bernoulli, but you’d have to know speeds at each point, and it’s not very practical. But the air deflection explanation works great for giving an intuitive understanding of what’s going on.
Yes, it is, to some extent. Add a bit of the Coanda Effect to fill the picture.
The Coanda effct is the tendency of fluids to follow the contour of objects due to the fluids viscosity, if you direct a stream of water from a hose to a ball, the water will stick to the ball´s surface and surround it instead of bouncing back or in some random direction.
THe same happens with a ping pong ball and an air stream; in this case the ball directs the airflow around it when hovering directly over the vacuum´s nozzle, there´s a regular, symetrical flow around it, and the air molecules hitting the ball from bellow keep it hovering. What gives stability to this arrangement (as in the ball not flying right away) is that in case the ball moves to one side of the nozzle, let´s say to the right, the airflow that surrounded the ball now acts more on one side (left) than the other, this creates an asymetry which brings our friend Coanda back to the rescue, the faster airstream on the left is curved slightle to the right by the ball´s curvature, and is deflected to the righ; now the direct physical explanation is that since a bit of air is deflected to one side, there´s an opposite and equal force applied on the ball to the right side, thus a small “thrust”, or more properly defined, lift, is generated, which incidentally brings the ball back to it´s original position.
Now where´s Bernoulli?, the air is moving faster by the left side than by the right side, so it is diverted to the lesser pressure area (to the right) where air is moving slower; now Bernoulli and the air deflection are two ways to see the same thing, really; you can´t have one without the other. However is woth pointing that the usual explanation for lift involving Bernoulli is ill concived, not only is not accurate in the mechanism, it´s also innecesary complex, on the other hand understanding lift by the reaction of a mass of air diverted down is easier to grasp and more acurate (than the usual teaching).
There’s also the aspect of Bernoulli which we use in the fire service to pump crud out of flooded basements without risking the intake of garbage into an expensive pump. A 1 3/4" or 2" handline is fitted with a smooth bore tip, and is rigged to fit just within the end of a 5" or 6" hard sleeve. Submerge this rig in the basement, pressurize the handline, and it will effectively evacuate the basement of water, owing to the negative pressure area created immediately around the nozzle tip.
