3 ms·

Causing a pressure difference is the same as causing a force vector. There's no gotcha here, it's just two ways of looking at the same thing.

lowkey 5y ago
My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.
- jjk166 5y ago
  The upper flow does move faster, and must for the sake of vortex production.
  - lowkey 5y ago
    Please explain what you mean here in simple language. I don’t understand how you conclude that the upper flow must move faster and I don’t understand how it relates to vortex production.
    - jjk166 5y ago
      To get a force acting upwards on the wing, there must be a downward reaction on the airstream. This downward force on the airstream bust change its direction. This change of direction is a rotation about the airfoil. Specifically a downwards rotation. The airstream moving above the centerline of this rotation is moving in the same direction, and thus will be accelerated faster. The airstream moving below the centerline of this rotation is moving in the opposite direction, and so will be decelerated to a slower speed. The center of this rotation happens to be along the camber line of the airfoil, so all the air above the camber line (ie over the top of the airfoil) must move faster, while all the air below the airfoil must move slower. The vortex is just the bulk rotational movement of the airflow. In fact, the airfoil can be replaced by anything that will generate the same vortex, like a rotating cylinder.
lowkey 5y ago
See my sister comment. The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. The problem with this theory is that there is no physical reason why both streamlines must arrive at the back of the wing at the same time - and per experimental verification, in fact they don’t.
- jjk166 5y ago
  > The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. That's not how the Bernoulli effect explains the pressure differential. The bernoulli explanation is that air builds up in front of the airfoil, creating a high pressure region, while there is a low pressure region created in the wake of the wing. This pressure differential forces accelerates air over the wing. For an asymmetric airfoil, more of this flow is over the top than the bottom, so the airflow over the top is faster, and thus lower pressure, than the airflow under the wing. The "equal time" thing is a pop-science misunderstanding.
  - boublepop 5y ago
    That is sadly literally how it is explained both in classes and in textbooks. It’s incorrect but it’s how it’s taught which is the root of a lot of misconceptions and why this “explanation” which is not correct keeps living on.

lowkey 5y ago

My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.

jjk166 5y ago
The upper flow does move faster, and must for the sake of vortex production.
- lowkey 5y ago
  Please explain what you mean here in simple language. I don’t understand how you conclude that the upper flow must move faster and I don’t understand how it relates to vortex production.
  - jjk166 5y ago
    To get a force acting upwards on the wing, there must be a downward reaction on the airstream. This downward force on the airstream bust change its direction. This change of direction is a rotation about the airfoil. Specifically a downwards rotation. The airstream moving above the centerline of this rotation is moving in the same direction, and thus will be accelerated faster. The airstream moving below the centerline of this rotation is moving in the opposite direction, and so will be decelerated to a slower speed. The center of this rotation happens to be along the camber line of the airfoil, so all the air above the camber line (ie over the top of the airfoil) must move faster, while all the air below the airfoil must move slower. The vortex is just the bulk rotational movement of the airflow. In fact, the airfoil can be replaced by anything that will generate the same vortex, like a rotating cylinder.

lowkey 5y ago

See my sister comment. The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. The problem with this theory is that there is no physical reason why both streamlines must arrive at the back of the wing at the same time - and per experimental verification, in fact they don’t.

jjk166 5y ago
> The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. That's not how the Bernoulli effect explains the pressure differential. The bernoulli explanation is that air builds up in front of the airfoil, creating a high pressure region, while there is a low pressure region created in the wake of the wing. This pressure differential forces accelerates air over the wing. For an asymmetric airfoil, more of this flow is over the top than the bottom, so the airflow over the top is faster, and thus lower pressure, than the airflow under the wing. The "equal time" thing is a pop-science misunderstanding.
- boublepop 5y ago
  That is sadly literally how it is explained both in classes and in textbooks. It’s incorrect but it’s how it’s taught which is the root of a lot of misconceptions and why this “explanation” which is not correct keeps living on.