Test: Drag Force & Lift Force - Civil Engineering (CE) MCQ

When a fluid flows over a stationary body, a force is exerted by the fluid on the body. Similarly, when the body is moving in a stationary fluid, a force is exerted by the fluid on the body.
This total force can be resolved into two components, one in direction of motion and another perpendicular to the direction of motion.
The perpendicular component of this force to the oncoming flow direction (or relative velocity of fluid flow) is called lift and the horizontal component of this force parallel to the flow direction is called drag force.

• Drag is defined as a forceful pull experienced by the flat plate while the fluid flows over it.
• Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to the Reynolds number than the frictional drag. It depends on the shape of the body.
• Frictional drag comes from friction between the fluid and the surfaces over which it is flowing.

Concept:
Skin Friction Drag 

• The drag on a body resulting from viscous shearing stresses (i.e., friction) over its contact surface (i.e., skin).
• The drag of a very streamlined shape such as a thin, flat plate is frequently expressed in terms of a skin friction drag.
• This drag is a function of Reynolds number.
• An airfoil mainly experiences skin friction drag.

Form Drag (Pressure Drag)

• The drag on a body resulting from the integrated effect of the static pressure acting normal to its surface resolved in the drag direction.
• Unlike the skin friction drag that results from viscous shearing forces tangential to a body’s surface, form drag results from the distribution of pressure normal to the body’s surface.
• In an extreme case of a flat plate normal to the flow, the drag is totally the result of an imbalance in the pressure distribution.

• A body moving through fluid experiences a drag force, which is usually divided into two components: frictional drag, and pressure drag.
• Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers, and it scales with Reynolds number.
• Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag. It depends on the shape of the body.
• Frictional drag is important for attached flows (that is, there is no separation), and it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body.
• For streamlined bodies (like a fish, or an airfoil at small angles of attack), frictional drag is the dominant source of air resistance. For a bluff body (like a brick, a cylinder, or an airfoil at large angles of attack), the dominant source of drag is pressure drag.

Concept: -
When a body is immersed in fluid under motion, it subjected to Drag and lift forces.
Drag Force: - It is the horizontal component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It has two components pressure drag and friction drag.  It is given by –

Lift force: - It is the vertical component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It is given by –

Here,
CL and CD – Coefficient of lift and drag force respectively.
A – maximum projected area of the body (m2)
ρ – density of fluid (kg/m3)
V – average velocity of stream (m/sec)
Note: - If fluid is assumed to be ideal and body is symmetrical such as sphere or cylinder, Both drag and lift force will be zero.

Concept:
Forces on submerged bodies:

• When a fluid flowing over a stationary body, a force is exerted by the fluid on the body. Similarly, when body is moving in a stationary fluid, a force is exerted by the fluid on the body.
• The total force exerted by the fluid on the body is perpendicular to the surface of the body. Thus total force is inclined to the direction of motion. This total force can be resolved into two components, one in direction of motion and other perpendicular to direction of motion.

Drag Force: The component of total force in direction of motion is called “Drag”. Thus drag is the force exerted by the fluid in direction of motion.
Lift Force: the component of total force in direction perpendicular to motion is called “Lift”. Thus lift force is the force exerted by the fluid in direction perpendicular to the motion.
Examples of immersed bodies having drag and/or lift forces:

1. A tall chimney exposed to wind;
2. Flow of water past a bridge pier;
3. Flow of fluids past blades in fans, blowers, compressors, turbines etc.;
4. Motion of aeroplanes, submarines, torpedoes etc.

Examples of bodies where both drag and lift forces are produced:

1. Propeller blades;
2. Aerofoils;
3. Hydrofiles;
4. Rotating cylindrical bodies;
5. Kites etc.

Following points are worth noting:

1. In contrast to drag, the lift forces may exist even in ideal fluids by the presence of circulation.
2. Real fluids also require vortices or circulation around the body for producing lift.
3. In motion of arifoils with finite spans, there is another kind of drag force associated with the lift force, called the "induced drag".

Concept:
Forces on submerged bodies:

• When a fluid flows over a stationary body, a force is exerted by the fluid on the body. Similarly, when the body is moving in a stationary fluid, a force is exerted by the fluid on the body.
• The total force exerted by the fluid on the body is perpendicular to the surface of the body. Thus total force is inclined to the direction of motion. This total force can be resolved into two components, one in direction of motion and the other perpendiculars to the direction of motion.

Drag Force: The component of the total force in direction of motion is called “Drag”. Thus drag is the force exerted by the fluid in direction of motion.
Lift Force: the component of the total force in the direction perpendicular to motion is called “Lift”. Thus lift force is the force exerted by the fluid in the direction perpendicular to the motion.

Form and deformation drag:

• When a fluid flows around an object, exerts a force on it.
• The component of force parallel to the flow direction is Drag force.
• The component of force perpendicular to the flow direction is Lift force.
• Total drag is the sum of Deformation Drag and Form Drag.
• Deformation drag exists at very small velocities i.e. when  Reynolds number is very less preferably less than 2.
• Deformation drag consists of friction drag or surface drag at the surface/boundary.
• Form drag or Pressure drag is due to variation in pressure caused by widespread deformation.
        

Concept:
Drag is the component of force acting parallel to the direction of motion. When fluid flows on the flat plate a forceful pull experienced by the flat plate is known as a drag force.
Coefficient of drag as the ratio of drag to dynamic pressure:

For a flat plate of length L, in laminar flow

In turbulent flow in the range 5 × 10⁵ > Re < 10⁷

Concept: -
When a body is immersed in the fluid under motion, it subjected to Drag and lift forces, this is due to pressure and viscosity.
Drag Force: - It is the horizontal component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It has two components pressure drag and friction drag.  It is given by –

• Friction Drag is also known as Skin Friction Drag.
• The drag on a body resulting from viscous shearing stresses over its contact surface.
• It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity
• The drag of a very streamlined shape such as a thin, flat plate is frequently expressed in terms of a skin friction drag.

Lift force: - It is the vertical component of the force exerted by the fluid around the body when it is immersed in a fluid under motion. It is given by –

• Lift forces acts due to the pressure difference between the top and bottom surface of the plate or body.
• If the axis of the body is parallel to the direction of fluid flow the lift force is zero and only drag force acts.

Here,
C_L and C_D – Coefficient of lift and drag force respectively.
A – a maximum projected area of the body (m²)
ρ – density of fluid (kg/m³)
V – average velocity of stream (m/sec)
Note: - If fluid is assumed to be ideal (Non viscous) and body is symmetrical such as sphere or cylinder, Both drag and lift force will be zero.