VIT, PUNE - Department Of Mechanical Engg.

“DRAG AND LIFT FORCE ”

Introduction:

Drag Force:

The drag force is present everywhere around us. We thrive in a ball of fluids (air and water). Drag forces appear whenever there is motion in air or water or in any other fluid.When objects travel through fluids (a gas or a liquid), they will undoubtedly encounter resistive forces called drag forces.The drag force always acts in the opposite direction to fluid flow. If the body’s motion exists in the fluid-like air, it is called aerodynamic drag. And, if the fluid is water, it is called hydrodynamic drag.

Direction of motion

Lift Force:

Lift is a mechanical force. It is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of a gravitational field,or an electromagnetic field, where one object can affect another object without being in physical contact. For lift to be generated, the solid body must be in contact with the fluid: no fluid, no lift.

Drag Force Types:

Parasite Drag:

Parasite drag is a drag produced due to the motion of an object through a fluid. With respect to aviation, the object is an aircraft and the fluid is the atmospheric air. Parasite drag occurs due to air molecules. Parasite drag is classified as form drag or pressure drag, skin friction drag and interference drag.

Skin Drag Force:

Skin friction drag is a drag produced due to friction between an object (aircraft) & fluid (atmospheric air). The rough surface will have high skin friction drag and conversely a smooth surface will have less skin friction drag.

Form drag or pressure drag:

Form drag is produced due to the shape of the object moving through the fluid. It depends on the cross section of an object. An object with a larger cross section and blunt shape will have a larger form drag whereas an object with a smaller cross section area and a sharper shape will have a lesser form drag.

Interference Drag:

Interference drag is created due to the interference of multiple airflows, which have varyingspeeds. The interference of various aircraft components generates this drag force. It is precisely due to non-similar airflow around the fuselage and the wing.

Wave Drag:

This unique drag force is limited to supersonic scenarios. It is a type of induced drag force produced from non-cancelling static pressure variables to either side of a shock wave striking on the outer surface of the object, from which the wave is generated.

Drag Force Equation:

Drag Equation is used to find the force of drag on an object due to motion through an enclosed fluid system.

Where,

Fd = the drag force

ρ = the mass density of the fluid

υ = the flow velocity relative to the object

A = the reference area

Cd = the drag coefficient

where C is the drag coefficient, A is the area of the object facing the fluid, and ρ is the density of the fluid.

Lift force Equation:

Lift force is basically defined as the force exerted on the body in a direction perpendicular to the direction of motion when fluid is moving and body is stationary or body is moving and fluid is stationary.

Lift force will be represented by the symbol F_L.

Where,

q = ½ * pv²

C_L = Co-efficient of lift

A = Area of the body which is projected area of the body perpendicular to the

direction of flow

ρ = Density of the fluid

V = Flow velocity relative to the object

q = Dynamic Pressure

Demonstration with Bernoulli’s Principle:

In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli, a swiss mathemetician, who published it in 1738 in his book Hydrodynamics.

A practical application of Bernoulli’s Principle is the venturi tube. The venturi tube has an air inlet that narrows to a throat (constricted point) and an outlet section that increases in diameter toward the rear. The diameter of the outlet is the same as that of the inlet. The mass of air entering the tube must exactly equal the mass exiting the tube. At the constriction, the speed must increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube. When the air speeds up, the pressure also decreases. Past the constriction, the airflow slows and the pressure increases.

Bernoulli’s equation formula is a relation between pressure, kinetic energy, and gravitational potential energy of a fluid in a container.

Where p is the pressure exerted by the fluid, v is the velocity of the fluid, ρ is the density of the fluid and h is the height of the container.

Bernoulli’s equation gives great insight into the balance between pressure, velocity and elevation.

Application of Lift and Drag force:

We all know that gravity is a force that pulls everything towards the Earth’s surface. This pull is called the weight force. Planes and birds have to be able to provide enough lift force to oppose the weight force. Lift is caused by the variation in air pressure when air flows under and over an airplane’s wings. It acts upwards against weight and must be greater in order for the aircraft to fly.