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.
$\begingroup$
$\endgroup$
8
-
20$\begingroup$ Hurricanes and roofs! $\endgroup$– Charles BretanaFeb 1, 2021 at 14:35
-
2$\begingroup$ @Charles Bretana: Though that could easily be wind getting under the edges of shingles &c, and forcing them up. $\endgroup$– jamesqfFeb 1, 2021 at 18:59
-
$\begingroup$ @CharlesBretana I have seen a "topper" fly off a pickup truck, it went about ten meters up. $\endgroup$– Keith McClaryFeb 2, 2021 at 3:32
-
$\begingroup$ 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. $\endgroup$– chasly - supports MonicaFeb 3, 2021 at 14:28
-
$\begingroup$ @chasly-supportsMonica How might one test that hypothesis? $\endgroup$– Camille GoudeseuneFeb 3, 2021 at 16:56
|
Show 3 more comments
4 Answers
$\begingroup$
$\endgroup$
4
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.
answered Jan 31, 2021 at 21:16
-
10$\begingroup$ Nice 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. $\endgroup$– FlanManJan 31, 2021 at 22:05
-
5$\begingroup$ + 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) $\endgroup$ Feb 1, 2021 at 0:04
-
1$\begingroup$ Off 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. $\endgroup$– jamesqfFeb 1, 2021 at 4:36
-
$\begingroup$ One slight problem here. There is air pressure underneath the pan. Therefore this is not 'glued to the ground'. $\endgroup$ Feb 3, 2021 at 14:24
$\begingroup$
$\endgroup$
7
Yes. Bernoulli end effectors are used in industrial applications to pick up items without physical contact using this principle. Example:
-
1$\begingroup$ Interesting! What's the advantage to a Bernoulli gripper over say a vacuum? Any idea? $\endgroup$ Feb 1, 2021 at 13:57
-
14$\begingroup$ @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. $\endgroup$– GrahamFeb 1, 2021 at 15:22
-
1$\begingroup$ Kodak 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. $\endgroup$– J.HirschFeb 1, 2021 at 15:44
-
$\begingroup$ @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. $\endgroup$ Feb 1, 2021 at 16:17
-
1$\begingroup$ @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. $\endgroup$ Feb 2, 2021 at 20:47
$\begingroup$
$\endgroup$
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:
$\begingroup$
$\endgroup$
3
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...
-
-
1$\begingroup$ I'm sure that's part of it, but the air flowing downward beyond the object also plays a large part. $\endgroup$ Feb 1, 2021 at 16:20
-
$\begingroup$ @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. $\endgroup$ Nov 16, 2022 at 15:24