Dynamic Lift
Dynamic lift is the uplift experienced by a body when it is
placed in a fluid in the state of motion. This can be explained basing on the
concept that the total energies of a fluid like pressure energy, potential
energy and the kinetic energy always remains constant. To understand this in
detail we can deal this in 3 different parts.
In case one let us consider a ball moving with a uniform
velocity having only translatory motion in a fluid. This air applies a equal
influence both on the upper surface as well as the lower surface of the fluid
therefore the ball is not going to experience any effective force.
In case two let us consider the same ball having only spin
motion. In this case the body takes the fluid around it in the circular motion
as shown. It is similar to the lot of the earth atmosphere revolving around the
Sun.Because of the influence of the body some portion of this fluid starts
revolving around it. The upper layers of the fluid as well as the lower layers
of the fluid has circular motion.They have the same velocity but in the
opposite direction. Being the velocities are same, their kinetic energies are
also going to be the same and hence the body is not going to experience any
resultant force.
In case three let us consider the body having both
translatory motion as well as the spin. Because of the translatory motion the
fluids upper at layer will get some velocity whereas due to the spin the upper
layer will get some another portion of the velocity in the same direction.Thus
the effective velocity in the upper portion of the fluid is going to be more
which will increase its kinetic energy. As a result its pressure energy
decreases at that point. In the similar way there will be more pressure at the
bottom and hence the body will experience a uplift. This uplift is called
dynamic lift.
In real life generally all the bodies will have both translatory motion and the spin motion due to the air friction therefore this dynamic lift is also common for all the bodies.
In real life generally all the bodies will have both translatory motion and the spin motion due to the air friction therefore this dynamic lift is also common for all the bodies.
Air foil
Even the wings of the aero plane is designed in such a way
that it has to take the advantage of the dynamic lift. Its upper surface is
little bit curved whereas the lower surfaces the flat surface. Because of the
curved surf race the fluid will cross that part with a higher velocity and
hence it will have more kinetic energy. Hence it will have less pressure energy
as the total energy of the system shall always remain constant.
Similarly the lower portion of the wing will have more
pressure energy which will help the body to take off quite easily.
In a cyclone day if someone is in a hut and close the door of
that hut to protect from the cyclone it is going to take the roof of the
hut fly into the air a because of the
uplift.
It is simply because the strong air strikes the upper surface of the hut with higher velocity and hence there kinetic energy will be more and pressure energy will be less. As the a flow into the hut is small its kinetic energy is going to be less and the pressure energy is going to be more. This creates a unwanted up thrust on the top of the hut therefore it can fly in the air in a cyclone day.
It is simply because the strong air strikes the upper surface of the hut with higher velocity and hence there kinetic energy will be more and pressure energy will be less. As the a flow into the hut is small its kinetic energy is going to be less and the pressure energy is going to be more. This creates a unwanted up thrust on the top of the hut therefore it can fly in the air in a cyclone day.
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