Uses of the lift force

  • #1
Hello,
I'm trying to find situations in which it is important or useful to look at the force generated by a difference in pressure due to an increase in velocity of a fluid. I.e, pressure changes due to Bernoulli's equation. This should be simple by itself, but I am looking for situations that involve plane surfaces (the two-dimensional analogue of a line, not the thing that flies).
So far, I found out about houses. During storms the difference in pressure because of the strong wind will cause a lift force on the roof of the house, and might rip it away. A roof is more or less two plane surfaces joined together at an angle, so it is a situation involving lift forces and planes.
Searches have been rather fruitless because every source just talks about birds and planes (the flying kind this time) and how airfoils work with the lift force.
Thanks for answers.
 
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  • #2
phinds
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Hello,
I'm trying to find situations in which it is important or useful to look at the force generated by a difference in pressure due to an increase in velocity of a fluid.
Well, bridge designs now make use of inverted air foils to hold the deck down in high winds. Not sure if that is an answer to your question or not since I'm not clear on what you are looking for.
 
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  • #3
FactChecker
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Aerodynamic down-force on a race car?
Supercavitating submarine or torpedo? (I think Bernoulli is involved)
 
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  • #4
The bridge and the downforce are not exactly what I'm looking, but they are close.
Basically I'm looking for examples where on one side the air around a surface is stationary, and on the other side it is moving. In the house example, the air below the roof is stationary, and the air above moves. In these cases you would not need a curved surface such as an airfoil to produce the force.
Is there any other example where there is a lift force without an airfoil?
 
  • #5
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Airfoil just refers to the cross sectional shape of the lifting surface.
An airfoil may be cambered or uncambered. An uncambered airfoil (flat plate or symmetrical) still has a useable lift coefficient.
An appropriately design cambered airfoil will have a higher L/D and max lift coefficient, though.
 
  • #6
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Aerodynamics is well understood and tested; while not perfect it works.
Maglev is the only other way I can think of that can produce a useful lifting force.
No, I guess rockets are also
 
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  • #7
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Recall that in aerodynamics lift means aerodynamic force perpendicular to the relative wind.
Lift may point in any direction, including down, depending on the situation.
In everyday usage, however, lift is the force that counteracts gravity.
 
  • #8
FactChecker
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Airfoil just refers to the cross sectional shape of the lifting surface.
An airfoil may be cambered or uncambered. An uncambered airfoil (flat plate or symmetrical) still has a useable lift coefficient.
An appropriately design cambered airfoil will have a higher L/D and max lift coefficient, though.
But an airfoil with no camber would have to have an angle of attack to create lift. I don't think that is what the OP is looking for.
I don't think there will be an example unless there is some type of venturi choked flow. I guess you could have some type of venturi shape on one side and have the other side flat. That should create a force on the flat side.

If a tilted flat surface is acceptable, there should be a lot of examples, including crude propellers. It is hard to convince some people that a tilted flat surface is showing Bernoulli rather than just air impact on the other side. But there are experiments that prove the point.
 
  • #9
That's exactly what I am looking for I guess. An uncambered airfoil, since it is a flat surface. FactChecker got it right. I am looking for flat surfaces, even if they are tilted. Any examples of where they could be used? Thanks for the terms
 
  • #10
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That's exactly what I am looking for I guess. An uncambered airfoil, since it is a flat surface. FactChecker got it right. I am looking for flat surfaces, even if they are tilted. Any examples of where they could be used? Thanks for the terms
Just to clarify, the camber indicates the lack of symmetry between the two sides, not necessarily how flat the sides are. A circle would have no camber.
 
  • #11
Just to clarify, the camber indicates the lack of symmetry between the two sides, not necessarily how flat the sides are. A circle would have no camber.
Alright, thank you
 
  • #12
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That's exactly what I am looking for I guess. An uncambered airfoil, since it is a flat surface. FactChecker got it right. I am looking for flat surfaces, even if they are tilted. Any examples of where they could be used? Thanks for the terms
Would such items as water skis, snow sled, carriage runners satisfy the criteria?
 
  • #13
Would such items as water skis, snow sled, carriage runners satisfy the criteria?

Looks like they operate on Newton's third law, so that when the medium strikes, they reciprocate the force. and they are also more or less flat, or at the very least need a tilt to work properly. It seems like they do satisfy the criteria.
Thank you, I have an idea of what to look for now.
 

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  • #14
Merlin3189
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I don't know much about this, but some situations occur to me:
A train passing through a tunnel - low pressure of surrounding air -> discomfort for passengers and possible significance for structural design.
Train passing - people on platform, workers at trackside, need to stand back to avoid being "sucked" towards train.
High sided vehicles on motorway passing lighter vehicles such as motorcycles; maybe two such vehicles passing might need to ensure adequate separation.
?Do boats have a problem when running closely along side?
Blocks of flats (or I suppose any building with large flat walls) - inconvenience to residents of strong draughts from open window; possible structural considerations.
Boat cover trying to tear itself off when driving at speed, even though it is securely fastened (almost sealed) at edges.
 
  • #15
phinds
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WINDOWS ... forgot all about that one until @Merlin3189 reminded me. There have been plenty of instances where poor design allowed windows in high rises to pop out in high winds and this is now a major consideration in designing high rises.
 
  • #16
Nidum
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Is there any other example where there is a lift force without an airfoil ?

(1) Hydrodynamic bearings .

(2) Some types of turbine blade .
 
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  • #17
Ibix
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Actually, the inlet valve to my toilet cistern works on this principle.
 
  • #18
Merlin3189
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I don't know whether a chem lab filter pump (aspirator pump, venturi pump) counts? Hardly plane surfaces, especially in glass, but could be conical and cylindrical surfaces for a metal one.
Which reminds me of carburettors, then air-brush, paint spray and, in the dim& distant days before aerosol cans with HC propellants, Hand-pump operated DDT fly sprays. All use Bernouilli effect, though the surfaces don't move and only figure to direct the fluid flow.
 
  • #19
A.T.
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I am looking for flat surfaces, even if they are tilted.
Many toy plane models have flat wings.
 
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  • #20
rcgldr
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train passing by people, truck near motorcycle on highway
The main issue here is transition. At the front of a train or truck the air is diverted outward, impacting on any surrounding objects like people at a train station or a motorcycle near the front of a truck. The pressure at the side of a train or truck is not that much different than ambient. The pressure behind a train or truck is lower than ambient. The main issue would be transition into and then out of the diverted flow at the front, due to overcompensation that leads to inward motion after transitioning out of the outward flow. From my experience on a motorcycle, other than reduced drag if following behind a truck, there's not a lot of sideways force when transitioning into or out of the low pressure zone behind a truck.

camber
As posted by FactChecker, camber refers to asymmetry between upper and lower surface of a wing. There's also camber line, which is a line at the midpoint between upper and lower surfaces. Sometimes a wing with a curved camber line is referred to as cambered, even though that usage conflicts with the definition of camber.

skis, ...
These operate at the boundary between two media. In the case of skis, water below, air above. Fully submerged hydrofoils operate similar to wings, but partially submerged hydrofoils act like skis.

hand pumped ddt sprays
In the case of a flit gun, a stream of air is blown over the open end of a tube. This results in vortice flow about the end of the open tube, reducing pressure, drawing and atomizing fluid into the stream of air. The stream of air from the pump has above ambient pressure, but the vortice flow reduces that pressure to below ambient. Some atomizers divert some of the pump pressure into the chamber that contains the fluid to pressurize the chamber. Carburetors often use the same open ended tube vortice effect in addition to reduced pressure venturi ports to increase the fuel flow.

flat wings
As A.T. posted, toy balsa models use flat wing. A nearly flat wing is good enough for a small model glider such as the one in the article below. Note that not all of the aerodynamic aspects mentioned in the article are correct, but the model itself does glide reasonably well.

http://www.4p8.com/eric.brasseur/glider2.html [Broken]
 
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  • #21
Thanks for all the answers. They have been of great use in finding examples of the lift force, and useful ones at that.
 
  • #22
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But an airfoil with no camber would have to have an angle of attack to create lift.
Angle of attack is the angle between the relative wind and the zero lift line. All airfoils need an angle of attack to create lift.

The tail surfaces of fabric covered airplanes (Taylorcraft, Piper Cub, Aeronca Champ) use flat plate airfoils.
 
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  • #23
FactChecker
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Angle of attack is the angle between the relative wind and the zero lift line. All airfoils need an angle of attack to create lift.

The tail surfaces of fabric covered airplanes (Taylorcraft, Piper Cub, Aeronca Champ) use flat plate airfoils.
In any case, the airflow is unsymmetric to get lift. It's not clear to me why the OP desires a flat surface, but I think he expects that to be a simple case. It is not.
 
  • #24
In any case, the airflow is unsymmetric to get lift. It's not clear to me why the OP desires a flat surface, but I think he expects that to be a simple case. It is not.
I wanted a flat surface because I was conducting an experiment on how the lift force changes with the angle of attack of the flat surface. I needed uses of the lift force to justify my choice of experiment, so I came here after finding none. Well, anyway thanks to that I got a ton of examples, so thank you to all contributors.
After conducting the experiment I can say that in my case at least, the lift force vs the (sine*cosine) of the angle of attack is linear, and there is a non-zero constant because I used a box of air with a flat lid. I also found a paper on a similar experiment if anyone is interested, and the results did agree with what I found. Though, I must admit that my conditions were not always exactly controlled, so there are some fluctuations in my data. The relationship is clear anyway.
 
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