Ram Air a Myth...?
09-08-2010, 08:06 PM
#81
That is a good question. To address the part about the airfoil, the popular description is that air moves faster over the top than the bottom due to the shape, and thus if you use Bernoulli's equation you can calculate the pressure differential and then the lift. This assumes that the air going over the top of the wing and the bottom get from the leading edge to the trailing edge at the same time. In fact, this is not the case. The wing does however alter the momentum of the air creating a downwash, with the opposite reaction resulting in the lift (it also imparts some forward momentum on the surrounding air, and the opposite reaction manifests as drag). It may be simpler to imagine that the wing is actually pulling air from above and pushing it down at an angle corresponding to the angle of attack, and at a rate depending on velocity. This explains why a perfectly flat wing can generate lift, or even a wing that is flat on the top yet curving down on the bottom (which does not work in the popular Bernoulli explanation of lift). It also explains why a fan, propeller, or rotor work. You would have to google something like modern explanation of lift newton or something like that to find this information.
In the aerodynamics of a car, I would ask if the high and low pressure areas are due to the air being compressed or decompressed in those areas, or if what is being called pressure is really an effect of the collisions with more or less air molecules depending on the shape of the streamlines? In addition, there should be a boundary layer around the car for give or take an inch or so that moves along with the car, riding along with all the surfaces of the vehicle. The air outside the boundary layer is affected as the car moves through it. In fact by running into the air you impart energy to it and it also moves forward, but not at the same velocity as you are moving. Just like the wing above, this imparts drag on the car, and also creates a pocket behind the vehicle that can be utilized for drafting. To get to the point, air colliding with the windshield does not compress the air; it cannot because air cannot be compressed at this speed. Cowl induction isn't a bad way to get air into the motor though. It is a lot cooler air than drawing engine bay air, which is probably why the vents for the interior are in the cowl. And if the Bernoulli equation was 100% right, then when you drive through air that is sitting still and disturb it by imparting velocity, then all the air around the car would be low pressure.
In the aerodynamics of a car, I would ask if the high and low pressure areas are due to the air being compressed or decompressed in those areas, or if what is being called pressure is really an effect of the collisions with more or less air molecules depending on the shape of the streamlines? In addition, there should be a boundary layer around the car for give or take an inch or so that moves along with the car, riding along with all the surfaces of the vehicle. The air outside the boundary layer is affected as the car moves through it. In fact by running into the air you impart energy to it and it also moves forward, but not at the same velocity as you are moving. Just like the wing above, this imparts drag on the car, and also creates a pocket behind the vehicle that can be utilized for drafting. To get to the point, air colliding with the windshield does not compress the air; it cannot because air cannot be compressed at this speed. Cowl induction isn't a bad way to get air into the motor though. It is a lot cooler air than drawing engine bay air, which is probably why the vents for the interior are in the cowl. And if the Bernoulli equation was 100% right, then when you drive through air that is sitting still and disturb it by imparting velocity, then all the air around the car would be low pressure.
