An inquiry on lift... Pressure?

  1. Hello,

    I wish to begin by stating I appreciate all feedback and the time taken to answer my inquiry. I have read high and low. I have a general understanding of lift (not well developed), and have read many articles on Bernoulli vs. Newton, circulation theory, Coandă effect, Magnus effect, etc.

    To start my inquiry off, I have found a patent that discusses what I am talking about, using 'recycled' moving air to create a pressure difference on one side of a shroud ( The patent described is for vacuum cleaning, multi-surface suction, etc. The basic idea is to connect the intake and outtake of a centrifugal fan to create a pseudo vortex, with one part of a shroud open (the side you wish to have a pressure difference). I have also played with some Navier-Stokes simulations and confirmed that there is a lower pressure on the open end.

    My inquiry is, if on one side the device has a lower pressure, and atmospheric pressure is on the opposite side, is there a force (of the entire 'machine') in the direction of the open part of the shroud? With the simulators, it shows the device sucking particles as expected (with a low pressure on the surface in question), but how does Newton come into play here? Could someone explain how there could be a force (if any) with the lower pressure in that region if the device was not near a surface (such as a force in the direction of the opening)? Would rotating the round surfaces produce a noticeable difference in pressure (as per Magnus effect (or does this only apply to a free stream))?

    I've tried looking at the problem as a sum of all forces acting on the surfaces, but it's quite difficult.

    Once again, I am ignorant on these matters, but very interested and will do the work to understand. Thank you VERY much for your answers.

    Attached Files:

    Last edited: Jun 22, 2014
  2. jcsd
  3. sophiecentaur

    sophiecentaur 13,289
    Science Advisor
    Gold Member

    Which aspect of "Newton" were you having difficulty with? Momentum is conserved and this implies that the fan machine will stay still when connected to the Earth ('infinite mass') and could move significantly is it were not fixed (mass is comparable with that of the ejected air + dust) due to Newton 3. This particular issue always applies for situations with immoveable parts.
    1 person likes this.
  4. A few questions?

    Is there any port through which the air is expelled outside the chamber (except of course for the bottom hole)?
    If yes then there would be a low pressure zone and air would be continually sucked in.
    If not then a small amount of air will be sucked in creating a higher density inside and stabilize the air flow (no more suction).
    1 person likes this.
  5. Thank you very much for your replies!

    The difficulty I had understanding was the air is ejected and fed back into the fan, so no outside air is really pushed off of or pulled towards the machine, implying that it can't move. The machine does have a low pressure zone on one side compared with the other, so I would assume it could move. I can understand the problem from the 'Bernoulli' frame of thinking, with the device moving in the direction of the low pressure area, but when thinking in the 'Newton' frame, I don't see any deflection of air in the opposite direction of the machine's movement.

    The port in which it is expelled is fed directly back into itself. The air exits the sides of the centrifugal fan, gets redirected, which satisfies the air being continually sucked back in condition. I agree with your thoughts, but I'm just confused on how the machine could move. The Navier-Stokes simulation shows the device always creating the low pressure zone for the duration of the simulation. I'll try to get a screenshot soon.

    Edit: I've added another picture that includes what the simulator shows just for additional information. The orange is high pressure, green low. So I suppose my main question is: How can this device move in the direction of the low pressure zone (there is the fast moving air over the surfaces), without expelling air in the opposite direction or sucking in a lot of 'new' air towards itself?

    Attached Files:

    Last edited: Jun 22, 2014
  6. To consolidate my inquiry: Does the higher atmospheric pressure on the side opposite the low pressure zone create a force in the direction of the zone, and if so, how without pushing off of air or sucking air towards itself?

    It seems like it would violate Newton's Third Law as no air is being expelled, but the higher relative pressure on one side seems like it would in fact move...

    If this question could be answered for me, I could finally get some sleep tonight!

  7. Found something close to similar. Maybe!?!
    The patent itself does not mention much motion of the whole thing just an attraction to the surface. So it would be similar to down-force of F1 cars.

    You should look into Bernoulli's and the low pressure created by moving the air.
    Last edited by a moderator: Sep 25, 2014
  8. Excellent video, thanks for sharing! I've looked into a similar setup, and if the exhaust isn't fed back into the fan, it does indeed produce a force (image provided). I believe I understand how Bernoulli's principle works, and how the momentum is exchanged in this set up. If the suction end is free and the machine is allowed to move, the air moves towards the machine and expelled radially, allowing an exchange of momentum.

    For the machine where the air is recycled and no net downward movement of air is seen, I don't know how the the same force could move the machine similarly, even with the same low pressure zone. I wish I could create a setup just to see what is going on, as the simulations I'm using just show pressure differences.

    Once again, great video!

    Attached Files:

    • cart.png
      File size:
      613 bytes
    Last edited by a moderator: Sep 25, 2014
  9. I took the liberty of reading the vortex attraction patent. It would have an area of lower pressure, but I do not think the item will be able to move if it is not near a surface. I think it acts as suction cup does to wall. Interesting machine though.
    Last edited: Jun 27, 2014
  10. A suction cup requires a surface to operate though. It can't work if it's not near a surface, as atmospheric pressure would be acting on both sides of the suction cup.

    For the device, it creates the low pressure zone actively, which leads to the question of if atmospheric pressure would 'push' the device, as lower-than-atmospheric pressure is on the other side of the device.
  11. I've been trying to build a model to test this vortex attractor to see if a machine on a stand will produce a force when not near a surface, but it's taking some time as I am trying to get useful parts together (using PVC, an electric leaf blower and various parts). Does anyone have any ideas or thoughts about the device and how it may produce a force when not near a surface?

    Any information on this in addition to what's been discussed above would be GREATLY appreciated... the only real question I have is about a force in the direction of the low pressure zone without a downward reaction force.
    Last edited: Jun 30, 2014
  12. I have a friend that build something similar to the described fan, which is used for surfaces that are not flat. He said it does have force in direction of the open side if the fan is not fixed to the surface. I believe it is due to pressure Difference. For reaction, I am not sure. I think possibly it displaces normal pressure air like hot air baloon.
  13. Thanks for the answer Preet.

    I appreciate your confirmation that the device would indeed produce a force, but is it there any reaction to the air to produce the force? I still do not understand how it could move without pulling air towards itself or pushing air away from itself. I can understand it from a 'Bernoulli' frame of mind, but not from a 'Newton' frame of mind.

    I still think I should build the test for myself, to see what exactly is going on. Does anyone else have any ideas on this or that can confirm this?
  14. Simple answer: it couldn't. If it is actually generating a net force, it must involve some reaction mass.
  15. sophiecentaur

    sophiecentaur 13,289
    Science Advisor
    Gold Member

    Newton's Laws must come into this but it doesn't mean that it's necessarily in an obvious way. Looking at the whole machine and the surrounding air, there has to be a force moving air away from the surface which will provide the force to hold the machine against the surface (sufficient pressure on the top times area). There will be a sum of elemental momentum changes of air, to account for the normal forces. This is the same for heavier than air flying machines - a net downward flow of air must exist in the vicinity of an aircraft but no total movement. The wing 'takes up' much more effective space than just the metal bits and so does this machine.
    There will be no build up of air in the larger sense - there will be circulation and dissipation of energy overall into the atmosphere.
  16. rcgldr

    rcgldr 7,398
    Homework Helper

    At any moment in time, the blades inside the device exert a tangental force onto the air, but as the blades turn, what was once a tangental force results in an radially outwards acceleration and velocity. It's similar to an object inside a spinning hollow tube being thrown outwards (assuming the end of the tube is open).

    During the initial start up, a small amount of air is expelled due to the outwards force, until a pressure gradient develops that is equal to and opposing the outwards force, resulting in a reduced pressure within the chamber. After equilibrium is reached, there is no net inwards or outwards flow of air from the chamber.
  17. Very interesting way to think about it, thanks! Looking at the whole machine and the surrounding air, I took another look at the Navier-Stokes simulation with more detail (the simulator lets you place 'fans' and 'walls', which helps to visualize the flow and density). It appears that the low pressure region sucks in the fluid from above the machine, and 'adds' it to the existing circulating flow inside the machine (this is only true if not near a surface, it does precisely what you've described near a surface), so it may in fact be 'pulling' the air towards itself in a similar fashion as the cart drawing above to explain the reaction of the machine. This would explain how there is a force in that direction without pushing air down.

    If it is sucking air into itself, I would think this would compress the air a bit until reaching an equilibrium point (as stated above at some point), but the simulator doesn't seem to show any signs of reaching that point, so I'm at a loss again on how this force may be maintained over a period of time.

    I really appreciate the feedback, it is VERY informative and helpful.

    I just saw this post after posting my response. Very interesting... I think I can see what you're describing. Once equilibrium is reached (if not near a surface and on a 'cart'), would it still produce a force in the direction of the open end, even with not net inwards or outwards flow?
    Last edited: Jul 2, 2014
  18. rcgldr

    rcgldr 7,398
    Homework Helper

    As a wing travels through a volume of air it exerts a downwards and somewhat forwards impulse to the air. The downwards component of that impulse eventually reaches the surface of the earth, and the pressure at the surface of the air is related to the weight of the air and all objects supported by the air (for objects not experiencing a vertical component of acceleration). The forwards component of the impulse would eventually end up opposed by viscosity in the air, and friction between the air and objects on the earth, but the air ends up displaced.

    If the flow is not restricted, then the fan will tend to draw air near the center, and blow air outwards, sort of a poorly implemented centrifugal pump. The chamber and/or a nearby surface are required in order to prevent any net air flow.
  19. sophiecentaur

    sophiecentaur 13,289
    Science Advisor
    Gold Member

    Yes and the total force on the Earth's surface is the same as the weight of the plane (in level flight). This seems to be neglected and even sort of denied by people who favour the exclusive use of the Bernouli explanation of flight. The explanation for flight is like the actual process - i.e. it's in layers.
  20. Thank you all for your insightful answers. I appreciate it very much! I may do some experimenting and post later, comparing this design with the design in the video posted earlier. It would be interesting to play around with this. Does anyone recommend a good simulator to view Navier-Stokes flow with custom 'fans' and 'walls'? The one I have is very low resolution. it would be great to play around with these ideas in more detail.
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Draft saved Draft deleted