## Monday, 26 November 2018

We were discussing the basics of Boundary layer theorylaminar boundary layer, turbulent boundary layer, boundary layer thickness, displacement thickness and momentum thickness and energy thickness, in the subject of fluid mechanics, in our recent posts.

After understanding the fundamentals of boundary layer, we will see here the concept of drag force, lift force and finally we will also find the expression for drag force and lift force with the help of this post.

In fluid mechanics problems, it is very important to determine the forces that the fluid exerts on the body. Engineers usually apply the basics of such forces in order to quantify the efficiency and aerodynamic performance of the body.

Before understanding the basics of lift force and drag force, we must understand here the forces acting on submerged bodies.

There are basically two cases that we will see here.

### Case 1: Fluid is moving and body is stationary

If a body is stationary and fluid is moving with a certain velocity, fluid will exert the force over the stationary submerged body. Force exerted by the moving air over the surface of a tree could be considered as one example of above case.

Air is also a fluid and tree is submerged inside the air, now there will be one force which will be applied over the surface of tree by the moving fluid.

### Case 2: Fluid is stationary and body is moving

If fluid is stationary and a body is moving with a certain velocity, there will be some force induced by the fluid on the body because moving body will cut the various layers of the fluid.

Swimming through the water could be considered as one example of above case. Forces acting on submerged bodies will be divided in to drag force and lift force.

Now we will go directly to the definition part of drag and lift forces.

### Drag Force

Force exerted on the body in a direction parallel to the direction of motion when fluid is moving and body is stationary or body is moving and fluid is stationary.

In simple we can say that force, which will be applied by the fluid over the body, in the direction of motion will be termed as drag force.

Drag force will be represented by the symbol FD

Drag is basically an undesirable effect and we want to reduce the effect of drag because it takes power to overcome it.

But in some cases, drag effect is good and beneficial such as in case of automobile brakes, parachutes.

### Lift force

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 FL.
Where,
CD = Co-efficient of drag
CL = Co-efficient of lift
A = Area of the body which is projected area of the body perpendicular to the direction of flow
FR = Resultant force on the body
ρ = Density of the fluid
V = Flow velocity relative to the object

Further we will go ahead to start a new topic i.e. Drag and lift coefficient, in the subject of fluid mechanics, with the help of our next post.

Do you have any suggestions? Please write in comment box.

## Reference:

Fluid mechanics, By R. K. Bansal