DRAG CLASS NOTES FOR MECHANICAL ENGINEERING

DRAG CLASS NOTES FOR

MECHANICAL ENGINEERING

Drag is a force of resistance when a body is

moving in water or air. In air, it is

aerodynamic drag. It opposes the thrust

force. It opposes forward motion.

Direction of drag is in opposite direction to

the moving object.  Firstly drag is directly

proportional to the velocity of the fluid in

a laminar flow. Secondly drag is

proportional to the square of the velocity

in a turbulent flow. At high speeds, almost

50 % power is spent in overcoming drag.

Aero-foil design has been found the best

to reduce drag. It is used in various vehicles.

Factors on Which Drag Force Depends

1. Viscosity of the fluid (water or air)

2. Upstream velocity

3. Density of the fluid

4. Shape, size and contour of the moving object in the fluid

 TYPES OF DRAG

Sr. No.	Name of drag	Cause of drag	How to reduce the drag?
1.	Parasitic drag—It is of two types Form drag or pressure drag	Drag depends on the shape of the body moving in a fluid.  For example, A body having larger cross section and blunt shape will have larger form drag and vice versa.	Make the shape like an aerofoil to reduce drag
2.	Skin friction drag	It is due to the surface roughness of the body moving n a fluid.	Make the surface more smooth
3.	Profile drag	Sum of skin friction drag and form drag is total drag.
4.	Interference drag	Drag is due mixing different stream lines between aero-plane components such as for wings and fuselage at the wing.	Install the different parts to have minimum interference
5.	Lift Induced drag	There is a lift force in a flying aircraft. It keeps the aircraft in air. During the stable flight lift is equal to the weight of the aircraft. Drag produced due to lift is called lift induced drag.
6.	Wave drag	The drag produced at sonic and super- sonic speeds. These high speeds produce shock waves. These shock waves produce the wave drag.	Try to keep speed below sonic speed

MAGNITUDE OF THE DRAG FORCE

Drag force is proportional to the square of the velocity and given by the equation

F_D = C_DρV² A/2

Where F_D is the drag force

C_D is the drag coefficient

It depends on shape size and contour of the moving object. This also depends on Reynolds number. Drag coefficient is a dimensionless quantity

ρ is the density of the fluid

V is the relative velocity of the fluid

A is the projected surface area of the object perpendicular to the motion of the fluid

Drag force always opposes motion. It consumes power which is actually a waste. This power consumed is proportional to the cube of the relative velocity. Thus drag is very high at high speeds. It should be reduced to a minimum. It is achieved by adopting an aero foil design of objects moving in water or air.

 TERMINAL VELOCITY

The velocity at which drag force becomes equal to the weight is called a terminal velocity.

F_D = mg

mg = C_DρV² A/2

Where

m is the mass of the moving object

C_{D is the drag coefficient}

ρ is the density of the fluid

V is the relative velocity

A is the surface area of the object normal to the fluid.

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