What do you Mean by Fluid Mechanics? 

Forces that occur due to the movement of fluid are known as fluid mechanics. Following are the fluids present: 

  • The first one is water 
  • Second is air 

What Can You Say About Fluid Forces? 

Object’s motion is affected very rarely due to the motion of a fluid (e.g., shot put). Some significant fluid forces cases are – badminton, baseball, swimming, cycling, etc. 

Below are the most determined fluid forces: 

  • Fluid force for Buoyancy. 
  • Drag fluid force. 
  • Lift F. 
Drag Force 

Resistive force movement due to fluid is known as Drag (air or water).The force acting in the opposite direction of the flow velocity is known as drag force.  

  • The drag force is also known by the drag coefficient. 
  • The drag force can be measured directly by simply attaching the body subjected to fluid flow to a calibrated spring and measuring the displacement in the flow direction. 

The factors affecting the drag force are: 

  • Area of cross-section affects drag force  (form drag). 
  • The smoothness of the surface affects drag force (surface drag). 
  • Force perpendicular to the direction of flow is known as lift flow which also affects the drag force. 
  • The lift force is not vertical. 
Types of Drag 

1. Surface drag:  Due to the motion of fluid on the surface depends upon the smoothness. For example, the body during swimming the time period of swimming; the suits wear for skiing and speed skating. 

2. Form drag:  The relation for this type of drag force is between the fluid and the area of cross-section. Certain examples can be considered as the upright posture of bicyclist during v. crouched position for motion. In the case of a swimmer: the relationship will be for buoyancy and what will be the maximum height attained by the body in water. 

The drag force is given by the equation: 


F D = 1 2 ρ v 2 C D A

  • F represents the drag 
  • ρ denotes the density 
  • v is the speed 
  • A is the area of cross-section 
  • CD is the drag coefficient 

With the relative motion of the fluid, the resultant force that is acting perpendicular to it is known as lift which is created due to pressure variation on opposite sides of an object because of the flow of fluid. 

The lift force is determined using the lift equation 

L= 1 2 C D Ar v 2


  • A is the area of cross-section 
  • C is the coefficient of drag 
  • r is the radius 
  • v denotes the velocity. 

For example, the wings of an airplane (hydrofoil). 

Bernoulli’s Principle 

Here the relation comes out to be the inverse proportion exists between the velocity and the pressure. 

  • Lower pressure due to fast relative velocity. 
  • Higher pressure due to slower relative velocity. 

Bernoulli’s equation is given as 

P 1 + 1 2 ρ v 2 1 +ρg h 1 = P 2 + 1 2 ρ v 2 2 +ρg h 2


  • P denotes the pressure 
  • v is the velocity 
  • ρis the density 
  • h is the height 
  • the subscript 1 and 2 denotes the two-point where the principle is being considered. 

For example 

• Baseball: slider, curveball
• Golf: hook, slice
• Tennis: the top-spin forehand.
• Auto racing: downforce.
• Soccer: “bender”.
• Volleyball: top-spin jump serve.

What is the Magnus Effect? 

Due to projectile spinning the path described as the curved segment. The principle of Bernoulli’s and the pressure variation caused by relative differences in inflow velocities. 

The pressure in the flow direction and the shear forces due to the wall have the tendency to move and thus the summation is the lift. Due to the forces by fluids, the moment is created and thus rotation of the body occurs. 

  • Rolling moment:  Flow direction moment. 
  • Yawing moment: Lift direction moment 
  • Pitching moment: Direction the moment for side force 

The dependent parameters for drag force and lift forces are 

  • Fluid density, 
  • Upstream velocity,  
  • Size and shape,  
  • Orientation of the body. 

What is Buoyancy? 

It defines the tendency of a body to float and to have resistance for which the sinking does not occur in the fluid. 

Archimedes’ Principle 

A buoyant force will be experienced in a fluidic medium that will be equal to the total volume of the fluid that gets displaced when submerged in water. 

For example, a boat in the lake. A certain volume of water gets displaced when the boat is submerged. This displaced water tells about the buoyant force Due to the resistance, floating will occur if the weight comes out to less or equal to that of water. Density related concept is Buoyancy. 

Density = mass/volume  

Given by equation, 

ρ= M V

where ρ is the density, M is the mass and V is the volume of the substance.

Solid-body fluid applications are present in phenomena such as.

1. Automobile drag force, underground pipelines.
2. Airplane wings that develop lift.
3. Rainfall for the upward draft, snow, hail, and due to high winds the dust particles.
4. The commute of red blood cells due to the flow of blood.
5. Liquid droplets disbursement due to the spray.

For a rested body on which the fluid is moving (such as the wind blowing over a building), and for a body traveling through a quiescent fluid, this condition is known as flow over bodies or external. It’s very complicated to predict the geometry for the external flow fields. Based on the experimental data the correlations have to be relied upon.

Some Important Terms 

  • Free-stream velocity: For the flow of fluid, the body being approached due to the velocity of this flow. 
  • Two-dimensional flow: For a very long body having an area of cross-section as constant as well as the direction of flow towards the normal of the body 
  • Axisymmetric flow: Rotational dynamics of a body about the axis in the flow direction, this is also considered to be a two-dimensional flow. 
  • Three-dimensional flow: The type of flow that is neither two-dimensional nor asymmetric for the case of flow over a car. 
  • Incompressible flow: Buildings, submarines, and flow over the automobile. 
  • Compressible flows: For the high-speed aircraft and rocket flow. 
  • At the condition of low-velocity compressible effects are negligible (flows with Ma < 0.3). 
  • Streamlined body: The type of the body where the drag force that is due to the friction appears to be lower for the condition of the object moving through a fluid. 
  • Bluff or Blunt body: If a body (such as a building) has the property to resist the flow. Generally, It stands easier for a streamlined body to force through the fluid.

Important Points 

  • Skin friction drag is considered to be a part of the drag force that is due to the direct shear stress on the wall (or just friction drag). 
  • The drag coefficient decreases as the ellipse become slimmer. 
  • Due to the attachment of the boundary layer and the longer surface, there is a reduction in the drag coefficient at the high aspect ratio which results in the recovery of pressure and is also a great measure to reduce noise and vibration.  
  • Only for the blunt bodies having much high velocity for the fluid flow the body type of streamline should be preferred due to lower drag coefficient(and thus high Reynolds numbers) where there arises the possibility of separation of flow. For a body consisting of a lower value of Reynolds, number streamline is not mandatory. 
  • There is a presence of resistance when a body moves through a fluid. 
  • Forces and moments are exerted by the fluid in various directions. 

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

  • Bachelors in Technology (Mechanical Engineering) 
  • Masters in Technology (Mechanical Engineering) 
  • Bachelors in Technology (Civil Engineering) 
  • Masters in Technology (Civil Engineering) 
  • Bachelors in Science (Physics) 
  • Masters in Science (Physics) 

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