Dynamic Lift and Air Foil

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.

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.