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Bernoulli-pressure differential airfoil

  1. Jun 23, 2010 #1
    bernoullis theromen -

    1)when a fluid flows through narrower cross section it should move at higher velocity to maintain mass flow rate.

    2)for a fluid to move at a higher velocity as in the above statement the pressure difference between the higher cross section and lower cross section should be more. That means the pressure
    decreases in the direction of flow


    my question is

    1)whether the fluid moves with high velocity because of higher pressure differential or

    2)whether the higher pressure differential caused the fluid to move at high velocity

    3)in the above discussions we are discussing about pressure differences along fluid flow direction but how this effect creates a pressure differential perpendicular to fluid flow direction in the top and
    bottom of the airfoil. in the top and bottom of airfoil the fluid flow should have created pressure differential only along fluid flow.
     
  2. jcsd
  3. Jun 23, 2010 #2
    bernoullis theroem -

    sorry
    rewording my first question

    1)when a fluid flows through narrower cross section it should move at higher velocity to maintain mass flow rate.

    2)for a fluid to move at a higher velocity as in the above statement the pressure difference between the higher cross section and lower cross section should be more. That means the pressure
    decreases in the direction of flow


    my question is

    1)whether the fluid moves with high velocity and then creates a pressure differentiall or

    2)whether the higher pressure differential caused the fluid to move at high velocity

    3)in the above discussions we are discussing about pressure differences along fluid flow direction but how this effect creates a pressure differential perpendicular to fluid flow direction in the top and
    bottom of the airfoil. in the top and bottom of airfoil the fluid flow should have created pressure differential only along fluid flow.
     
  4. Jun 24, 2010 #3

    1)

    http://en.wikipedia.org/wiki/Dynamic_pressure" [Broken]
     
    Last edited by a moderator: May 4, 2017
  5. Jun 25, 2010 #4

    minger

    User Avatar
    Science Advisor

    Fluids, like anything else move because there is a net force on them. Let's look a diffuser. The fluid comes in from the left at high velocity, and through the diffuser, leaves at a much higher pressure, over a much larger area. In the diffuser, there is clearly a net force in the opposite direction of flow. So, why is the fluid not flowing backwards?

    The answer is because you're merely looking at one component of the overall system. If this were water coming out of your faucet, then you're forgetting about the high pressure water tanks that feed your house.

    Also recall that we have this pesky thing called friction. That will always decrease total pressure along a pipe run.
     
  6. Jun 29, 2010 #5
    Not sure if this will help, but it's always been a chicken and the egg kind of problem.

    Let's look at a nozzle. We know from conservation of mass that if the area is decreasing, the velocity must be increasing for an incompressible flow.

    But in order for the velocity of the water particles to accelerate, there must be a force acting on them in the direction of motion. This force acting on them is the pressure gradient across any given particle.

    So as the nozzle converges, the pressure must continually be dropping to maintain a pressure gradient on the water particles in order to speed them up.

    So I always found it more intuitive to look it as "pressure gradients accelerate flows, and cause the velocity to change". You could however look it as "You have so much energy in the fluid, so as the velocity changes, the pressure must also changes because in a frictionless incompressible flow, their summation is constant."

    Hope that helped.
     
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