Is circulation real?

Discussion in 'Hydrodynamics and Aerodynamics' started by Mikko Brummer, Jan 25, 2013.

  1. Sailor Al
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    Sailor Al Senior Member

    It is not clear which part you are refuting, could you elaborate please?
     
  2. Sailor Al
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    Sailor Al Senior Member

    Your post has prompted me to consider the wider implications of thermodynamics in my theory and, whilst in the relatively low velocity environment of yacht racing and even subsonic aircraft, temperature is not an issue, I realise that its consideration is an omission that has now been rectified.
    But in answer to your question, the answer is that, on an aircraft wing, there will be a temperature increase due to the pressure increase at the leading edge and lower surface and, due to the pressure reduction, a temperature decrease over the upper surface of the wing. On a yacht the increase and decrease in temperature will occur on the windward side and leeward side of the sail respectively.
    However, whilst considering the velocities achieved in sailing and low speed aircraft, the temperature changes will be small (I haven't yet done the ΔT= ΔPx ΔV/R estimates for air) and difficult to measure due to the rapid heat transfer to the surrounding air through conduction and convection.
    I have updated the theory to include these considerations.
     
  3. Sailor Al
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    Sailor Al Senior Member

    Absolutely true.
    However, we're not talking absolutes here. The discussion relates to the environment of sailing boats and subsonic aircraft. The pressures involved in this environment are measured in fractions of an atmosphere. Basing an objection on phenomena occurring at 200 atmospheres and Mach50 is hardly appropriate.
    As I have pointed out ad nauseam, air does not change from being incompressible to compressible at Mach 0.3, nor anywhere from Mach 0.0 to Mach 1.0. (I'm also led to believe that over Mach 1, its behaviour is entirely due to its compressibility, but that's definitely out of scope of this Forum)
    It's time to put that canard to bed.
    From the sailing perspective, the ORC VPPs are based on a table of "Aero coefficients" (Pages 48-52).
    I have a strong suspicion that aircraft wings have also been calculated using L/D ratios determined from 100+ years of experimentation.
    If not, please provide a link to an algorithm which will take the geometry of an aerofoil, the dimensions of a wing, airspeed and altitude and, without a lookup table of L/D ratios, generate a value for the lift generated in Newtons.
     
    Last edited: Aug 14, 2022
  4. Barry
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    Barry Senior Member

    Bernoulli's Theorem applies to compressible fluids as well. A basic flaw in your analysis
     
  5. Sailor Al
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    Sailor Al Senior Member

    And that is the nub of the issue.
    According to Princeton University
    "The Bernoulli equation states that,
    [​IMG]
    where

    1. points 1 and 2 lie on a streamline,
    2. the fluid has constant density,
    3. the flow is steady, and
    4. there is no friction."
    Point 2 excludes gases which, unlike liquids, do NOT have constant density. Their density is dependent upon their pressure, i.e they are compressible.
    Encyclopedia Britannica also makes it clear:
    "Bernoulli’s theorem, in fluid dynamics, relation among the pressure, velocity, and elevation in a moving fluid (liquid or gas), the compressibility and viscosity (internal friction) of which are negligible and the flow of which is steady, or laminar."
    Please, please do your own research.
     
    Last edited: Aug 15, 2022
  6. Mikko Brummer
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    Mikko Brummer Senior Member

    Here's one among hundreds I have:

    https://www.3ds.com/fileadmin/PRODU...ight_test_manoeuvers_on_the_Diamond_D-JET.pdf
     
  7. Sailor Al
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    Sailor Al Senior Member

    I have speed-read the paper a couple of times, but couldn't see any detail of the geometry of the aerofoil being tested. The only reference to inputs was in Section 3:
    "The required inputs to run the simulation are:
    • - D-JET model geometry (actual loft) with flow through inlet
    • - D-JET mass, centre of gravity and full inertia tensor at the test point time
    • - Test point airspeed, air density, temperature and dynamic viscosity
    • - Flight controls deflections corresponding to the test point, slightly reduced
    by a factor determined from static wind tunnel data validation where applicable."

    This must be an over-simplification. Maybe you can direct me to where the geometry was provided.
     
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    Not an oversimplification... a validation against wind tunnel tests. There's also a validation against actual flight test data of a F-16 fighter, showing an astonihing accuracy.
     

    Attached Files:

    Last edited: Aug 15, 2022
  9. Barry
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    Barry Senior Member


    Bernoulli's Equation https://www.grc.nasa.gov/www/k-12/airplane/bern.html
    Bernoulli Principle - an overview | ScienceDirect Topics https://www.sciencedirect.com/topics/engineering/bernoulli-principle

    The first shows the derivation, the second from Nasa.

    While the the equation that you provided applies to liquids, the other Bernoulli equations apply to compressible Adiabatic or Isothermal conditions. The conditions surrounding a sail qualifies as Isothermal. And probably Adiabatic as per DCockeys post below #327

    The basic principal of Bernoulli's Theorem is that "with an increase of speed of a fluid there is a corresponding decrease in pressure"

    This is not to suggest that a sail provides thrust only due to Bernoulli. The upwind side of the sail adds lift/thrust to the sail due to the change in the velocity of the air between the inlet and outlet side of the profile. Obviously, this increases pressure at the interface between the fabric and air. The Newtonian portion of what is providing lift/thrust to the sail, upwind side

    Velocity and speed, unless in a technical group, are often applied/mentioned/stated/ used incorrectly.

    Velocity has two components Speed and direction. Of course, in most technical discussions, the direction component is often irrelevant for computational purposes . Ie if you are computing flow rates in a pipe, direction is ignored in any straight horizontal section.

    So the purpose of the above discussion is that a mass flow rate impacts the sail at a certain direction and leaves at another. This change in direction is a change in velocity. A change in velocity is an acceleration. An acceleration of a mass
    produces a force and hence provides the upwind (side of the sails) thrust. The leeward side of the sail low pressure side would be explained by Bernoulli at close to the leading edge, and more than likely momentum equations providing draft. The net resultant
    force would be then the difference of the upwind pressure and the and a slightly negative psig pressure behind.


    Bernoulli by itself does not explain the pressure gradient on both sides but it is an element of the leeward surface.
    Bernoulli does not have to explain the entire pressure gradient. Only that his theorem provides that if there is an increase in velocity, there is a decrease in pressure.

    Not dissimilar than a planing hull is supported by a combination of a displacement lift and a dynamic lift to produce a net lift. ( ignoring a few less important contributors to lift)
     
    Last edited: Aug 15, 2022
  10. DCockey
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    DCockey Senior Member

    Based on ????
     
  11. Barry
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    Barry Senior Member

    Based on opinion as there is not enough speed at the leading edge of the sail to produce a change in temperature
     
    Last edited: Aug 15, 2022
  12. DCockey
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    DCockey Senior Member

    Adiabatic is usual assumption/approximation for external low speed flow outside of boundary layers. However the temperature differences may be small enough that the difference between isothermal and adiabatic is very small and irrelevant.
     
  13. Sailor Al
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    Sailor Al Senior Member

    What does the " D-JET model geometry (actual loft) with flow through inlet" contain ?
     
  14. Sailor Al
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    Sailor Al Senior Member

    Any change in the volume or pressure of a gas will involve a temperature change (Charles' Law PV=RT). This just cannot be discounted by an opinion that there's not enough speed to produce a change in temperature. We're talking thermodynamics, not friction.
    It's the movement of the sail relative to the air that generates the pressure change.
     

  15. Sailor Al
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    Sailor Al Senior Member

    What "other Bernoulli equations"? It's the Bernoulli Principle (published in Hydrodynamica in 1738) which defines the relationship between speed and pressure in a fluid that is the topic. It applies to incompressible fluids. Air is a gas, gases are not incompressible. By definition, that's how they differ from liquids.
    And yes, it's a scary proposition to claim that on this topic NASA is wrong, but the evidence would appear to support that argument. Princeton, Encyclopedia Britannica and Bernoulli himself tend to agree they're wrong.
     
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