5 ms·

>Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument

by Rury 7y ago

>Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question Bernoulli's principle is often misunderstood. The principle itself doesn't say anything as to why different speed effects are observed around airfoils. It only states that within a steady state of fluid flow, increases in speed are associated with a drop in static pressure or a decrease in potential energy. That's it. Again, as to why air flows faster over on one side of an airfoil, or similarly a sail isn't explained by the principle. However we can measure the airflow around airfoils and see that it is flowing faster over one side. Hence, we can accurately model forces exhibited on a wing using Bernoulli's principle under the appropriate conditions...

glofish 7y ago
my understanding is that Bernoulli's principle also involves equal transit times. Meaning that the same two positions need to rejoin later. When the sail has no thickness the outer flow cannot go faster and still be a valid Bernoulli effect, because the outer and inner paths have the same distance. That being said, there might still be another similar principle at work, though technically speaking it should not be called Bernoulli.
- starpilot 7y ago
  It isn't at all. Bernoulli never asserted that particles are bound in any way, he just discovered a relationship between static and dynamic pressure. I don't know why people apply his name to the equal transit time theory of lift; there is no real association.
  - munificent 7y ago
    If you don't assume equal transit, then there's no reason to expect the particles above the wing to move faster than those below. And without that, there's no reason Bernoulli's principle would come into play at all.
    - starpilot 7y ago
      Yes there is. Suction on upper surface to maintain attached flow -> lower pressure -> increased speed to conserve momentum. Bernoulli's relationship holds true along a streamline, equal transit time doesn't.
    - antod 7y ago
      There are other ways to expect differences in velocity. A bit like the venturi effect in a pipe constriction, a constant flow rate will have to involve faster velocity when the area of flow reduces. If you trace a line on the wing cross section between the stagnation point and the trailing edge you will see the upper surface restricts/squeezes the flow more than the lower surface. So with a constant flow rate, the upper velocity will be higher even without equal transit.
  - glofish 7y ago
    Perhaps because it is always drawn that way, that the same streamline gets separated then goes over the wing, then goes back together. Then in big letters it says Bernoulli principle. Now that I read about it more, it looks like Bernoulli's principle is just conservation of energy - and as you say, there are no other requirements.
- Dansvidania 7y ago
  I disagree that bernoulli's principle demands equal transit times, but I think it is an irrelevant point. I believe the key fact is instead that the outer flow is forced into a curve, even though the straight path is technically free (no sail to stop it), because a vacuum (low pressure zone, anyone?) would be generated between the outer flow and the sail. Hence, the sail needs no thickness to generate its lift, but only to separate the inner and outer flow. Moreover, due to the viscosity of air, the area of influence of the sails goes well past the immediate layer of outer flow, but rather the layers of outer air closest to the sail have a similar "sail" effect, to a lesser degree, to adjacent layers of air.
- p_l 7y ago
  The "Equal transit time" is one of the "lie to children" explanations of lift. In fact, classic wings depend on inequal speed caused by a vortex behind the wing - which is also an oversimplified model. Such "bootstrap" vortex causes an opposite vortex to form around the wing, giving difference in speed of air on both sides. Once you have the airflow in the right way, Bernoulli's principle gives you lift. That's the very simplified version of a simplified explanation used in aviation teaching materials :)

by Rury 7y ago

glofish 7y ago

my understanding is that Bernoulli's principle also involves equal transit times. Meaning that the same two positions need to rejoin later. When the sail has no thickness the outer flow cannot go faster and still be a valid Bernoulli effect, because the outer and inner paths have the same distance. That being said, there might still be another similar principle at work, though technically speaking it should not be called Bernoulli.

starpilot 7y ago
It isn't at all. Bernoulli never asserted that particles are bound in any way, he just discovered a relationship between static and dynamic pressure. I don't know why people apply his name to the equal transit time theory of lift; there is no real association.
- munificent 7y ago
  If you don't assume equal transit, then there's no reason to expect the particles above the wing to move faster than those below. And without that, there's no reason Bernoulli's principle would come into play at all.
  - starpilot 7y ago
    Yes there is. Suction on upper surface to maintain attached flow -> lower pressure -> increased speed to conserve momentum. Bernoulli's relationship holds true along a streamline, equal transit time doesn't.
  - antod 7y ago
    There are other ways to expect differences in velocity. A bit like the venturi effect in a pipe constriction, a constant flow rate will have to involve faster velocity when the area of flow reduces. If you trace a line on the wing cross section between the stagnation point and the trailing edge you will see the upper surface restricts/squeezes the flow more than the lower surface. So with a constant flow rate, the upper velocity will be higher even without equal transit.
- glofish 7y ago
  Perhaps because it is always drawn that way, that the same streamline gets separated then goes over the wing, then goes back together. Then in big letters it says Bernoulli principle. Now that I read about it more, it looks like Bernoulli's principle is just conservation of energy - and as you say, there are no other requirements.
Dansvidania 7y ago
I disagree that bernoulli's principle demands equal transit times, but I think it is an irrelevant point. I believe the key fact is instead that the outer flow is forced into a curve, even though the straight path is technically free (no sail to stop it), because a vacuum (low pressure zone, anyone?) would be generated between the outer flow and the sail. Hence, the sail needs no thickness to generate its lift, but only to separate the inner and outer flow. Moreover, due to the viscosity of air, the area of influence of the sails goes well past the immediate layer of outer flow, but rather the layers of outer air closest to the sail have a similar "sail" effect, to a lesser degree, to adjacent layers of air.
p_l 7y ago
The "Equal transit time" is one of the "lie to children" explanations of lift. In fact, classic wings depend on inequal speed caused by a vortex behind the wing - which is also an oversimplified model. Such "bootstrap" vortex causes an opposite vortex to form around the wing, giving difference in speed of air on both sides. Once you have the airflow in the right way, Bernoulli's principle gives you lift. That's the very simplified version of a simplified explanation used in aviation teaching materials :)