DRAG

 

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DRAG

Drag is a force of resistance when a body is moving in water or air. In air, it is called aerodynamic drag. It is expressed in Newton’s. It is always against the thrust force. It opposes forward motion. It is a force opposing the relative motion of the moving object in a fluid. It is in the direction of the moving fluid but it is in opposite direction to the moving object.  It is directly proportional to the velocity of the fluid in a laminar flow and square of the velocity in a turbulent flow. At high speeds, almost 50 % power is spent in overcoming drag. The ideal speed for minimum drag is 70 to 80 km/h.  Aero-foil design has been found the best to reduce drag. It is used in cars, aero planes, space vehicles, ships, submarines, boats and in a pipe flow. If there is no drag force, a plane will never come to a halt.

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

 TABLE : TYPES OF DRAG

Sr. No. Name of drag Cause of drag Ho-w 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 (imperfections or surface irregularities) or surface smoothness of the body moving n a fluid. More drag on a rough surface 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

FD = CDρV2 A/2

Where FD is the drag force

CD is the drag coefficient, depends on shape size and contour of the moving object and the Reynolds number and it 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 by adopting an aerofoil 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.

FD = mg

mg = CDρV2 A/2

Where

m is the mass of the moving object

CD 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