Can lift occur if air is flowing over a fixed-shape object, but no air is flowing under any part of the object? Maybe the object is the top half of a wing that's glued to the ground, or the object is dome-shaped, or some other shape. Maybe the object doesn’t actually rise or move, but when air flows over it, it exerts less weight on the ground.
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20Hurricanes and roofs! – Charles Bretana Feb 01 '21 at 14:35
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2@Charles Bretana: Though that could easily be wind getting under the edges of shingles &c, and forcing them up. – jamesqf Feb 01 '21 at 18:59
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@CharlesBretana I have seen a "topper" fly off a pickup truck, it went about ten meters up. – Keith McClary Feb 02 '21 at 03:32
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All the answers here, including the one accepted by the OP, suffer from a misconception. Lift comes from underneath because of the pressure differential. On a weighing scale, the wing is not 'glued to the ground' it is on a moveable platform. This platform is pushed upwards by the higher pressure from underneath. No-one has answered the 'stuck to the ground' question, where the base is immovable and not open to the air. I suggest that there is no lift in that case. – chasly - supports Monica Feb 03 '21 at 14:28
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@chasly-supportsMonica How might one test that hypothesis? – Camille Goudeseune Feb 03 '21 at 16:56
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@Camille Goudeseune - Good question. Strain gauges maybe? You cannot use a hollow aerofoil because that will contain air. You would have to use a solid material. – chasly - supports Monica Feb 03 '21 at 17:05
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However one measures "exerts less weight on the ground, or not," it'd have to deal with how almost any material, even a thick steel beam, deflects when force is applied. Just like the millimillimeter that my scale deflected. So, seal my scale in an airtight plastic bag? – Camille Goudeseune Feb 03 '21 at 17:15
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@Camille Goudeseune - Your scale would still have air inside it. Maybe there are solid-state scales? Perhaps I'll ask this on Physics SE and see if anyone knows a method. :-) P.S. I think I might have an idea but I'll have to think about it. PPS - Just thought of another idea. – chasly - supports Monica Feb 03 '21 at 17:26
4 Answers
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Yes. This jar of paint weighs 34.8 g, but only 32.6 g to 33.4 g when a fan blows at it. The reduction of weight persisted when I repeated the experiment with a hodgepodge of baffles made from CD jewel cases and Post-Its to prevent any air leakage under the scale's sensor plate. Others can easily replicate these results.
Per FlanMan's comment, without the fan, I also tried pushing the jar horizontally with a very sharp pencil, like a pool cue to avoid imparting vertical force.
No matter where I put the jar on the scale,
no matter which way I pushed relative to the scale,
no matter how high the pencil tip touched the jar,
by the time the jar moved, the scale's report changed no more than -0.1 g to +0.2 g, and usually increased: it "got heavier" instead of "producing lift."
(The fan wasn't nearly strong enough to move the jar. High volume, low pressure, in paint sprayer parlance.)
So the horizontal component of the fan's force accounts for at most 1/7 to 1/10 of the effect. One could throw some statistics at this, or a milligram-accurate jeweler's scale, but more confidence would come from trying this in a proper wind tunnel whose load cell is quite independent of the air flow.
With baffles, with fan, without jar, the scale reported -0.5 g to +0.5 g, tending towards the negative (i.e., lift), depending on how I placed the baffles and the fan. However one might label that effect, at least some of the time this flat plate also counted as an example that answers yes to the question.
The scale is a DigiWeigh DW-500BS, unsurprisingly out of production, capacity 500 g, claimed repeatability 0.1 g which I've confirmed every few months with the calibration weights from a triple beam balance.
Camille Goudeseune
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10Nice experiment! If you omit the fan and instead press against the jar horizontally with a pencil, can you get the scale to show less weight? If so, then possibly the air is only pushing the weight of the jar away from the scale's sensor, rather than creating lift. – FlanMan Jan 31 '21 at 22:05
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- one. Could be the scale tray is generating lift (as per Coanda). May we just tare the scale to 0.0 g without the paint (although the bottle, like a stone in the river, will also create lower pressure behind it as well)
– Robert DiGiovanni Feb 01 '21 at 00:04 -
1Off topic, but what's the brand/model of that scale? Amd to the question, if you have a tarp laid flat on the ground (or on your roof, or covering a load on a truck), it will flap around in the wind even if you take care to seal the edges down so the wind can't get underneath. – jamesqf Feb 01 '21 at 04:36
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One slight problem here. There is air pressure underneath the pan. Therefore this is not 'glued to the ground'. – chasly - supports Monica Feb 03 '21 at 14:24
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Yes. Bernoulli end effectors are used in industrial applications to pick up items without physical contact using this principle. Example:
IceGlasses
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1Interesting! What's the advantage to a Bernoulli gripper over say a vacuum? Any idea? – curious_cat Feb 01 '21 at 13:57
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14@curious_cat Because a vacuum will pull the item into physical contact with the suction head. For delicate items (like the silicon wafer shown) this might damage them. – Graham Feb 01 '21 at 15:22
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1Kodak had these installed decades ago to assist in moving large bars of silver for film production. Bars weighed in at about 70 lbs; the vacuum assist lifter suctioned to the top/bleed, the tube would compress/shrink, and the net weight was 0 (but still obviously a lot of mass). Pretty amazing to see in operation- and it expanded the pool of people available to do the work, creating more jobs. – J.Hirsch Feb 01 '21 at 15:44
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@J.Hirsch That sounds fascinating! But I'm still trying to understand it from your description. Have any other links or sketches? Did not understand the 0 weight part nor the role of vacuum vs Bernoulli both in the same application. – curious_cat Feb 01 '21 at 16:17
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@curious_cat https://www.youtube.com/watch?v=ZOTE5zU_uzY - quick search that looked like the item in use, but obviously wasn't because, cough 20 years ago... feeling old today – J.Hirsch Feb 02 '21 at 18:18
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1@J.Hirsch: The thing in this answer blows air out, not suction. The video gets to this after about 25 sec. What you're talking about, for heavy sturdy items, is suction that would result in far less uniform forces on the object, with a high-force area right under the vacuum head, and no force elsewhere. That's fine for moving bars of silver, but not silicon wafers or pieces of cloth like shown in the video in this answer. – Peter Cordes Feb 02 '21 at 20:47
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Whoops! You're right- my fault for not studying the item in question fast. I'm assuming they'd use N2 for the wafers then. – J.Hirsch Feb 04 '21 at 18:11
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Yes. A classical example of this often taught in schools is the experiment where you cut a strip of paper and blow across the top of it, causing the paper to lift. Here is a video demonstration:
Kirk Woll
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Yes, because the still air under the object has (by Bernoulli) a higher pressure than the moving air above. That's why, in high winds, entire roofs are sometimes detached from houses...
xxavier
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1I'm sure that's part of it, but the air flowing downward beyond the object also plays a large part. – João Mendes Feb 01 '21 at 16:20
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@Max, exactly right! Total Pressure, (the vector sum of static and dynamic pressure), at least sub-sonically, must remain constant. It has to, because of conservation of energy. – Charles Bretana Nov 16 '22 at 15:24