Basic Concepts of Fluids | Introduction to Fluids | NPTEL Video | Fluid Mechanics | By Ashutosh Nautiyal

Basic Concepts of Fluid 

Fluid

Introduction and Basic Concepts

• Mechanics:- It is a branch of physical science that deals with energy and forces and their effect on both stationary and moving bodies.

• Statics:- It is a branch of mechanics that deals with bodies at rest.

• Dynamics:- It is a branch of mechanics that deals with bodies in motion.

• Fluid mechanics:- It deals with the behaviour of fluids at rest or in motion, and the interaction of fluids with solids or other fluids at the boundaries.

• Fluid statics:- It is a branch of fluid mechanics that deals with fluids at rest.

• Hydrodynamics:- It deals with the motion of fluids that can be approximated as incompressible.

• Hydraulics:- It is a sub-category of hydrodynamics which deals with the flow of fluids in pipes and open channels.

• Gas dynamics:- It deals with the flow of fluids that undergo significant density change.

• Aerodynamics:- It deals with the flow of gases over bodies such as aircraft and automobiles.

• Meteorology, Oceanography and Hydrology:- It deals with the naturally occurring flows.

• Stress:- Force per unit area is called stress.

• Normal stress:- Normal component of force acting on a surface per unit area is called normal stress.

• Shear stress:- Tangential component of force acting on a surface per unit area is called shear stress.

• Pressure:- Normal stress in a fluid at rest is called pressure.

• Fluid:- A fluid is a substance which deforms continuously under the influence of shear stress. In solids, stress is proportional to strain but in fluids, stress is proportional to strain rate.

Explanatory Video

Virtual Fluid Balls (VFB) Concept

• Fluid balls differ in size, volume and mass.

• Number of VFB depends on the complexity of fluid flow problems.

• Fluid balls may disintegrate for representing the flow complexity.

No-slip condition

• A fluid in direct contact with a solid surface sticks to the surface and there is no slip. This condition is called a No-slip condition.

Applications of Fluid Mechanics

• Its applications can be seen in natural flows, weather, pipe systems, cars, aircraft, spacecraft, human body, power plant, boats, ships, wind turbines, etc.

Classification of Fluid flows

• Viscous flow:- The flow in which viscous forces are significant is called viscous flow.

• Inviscid flow:- The flow in regions where viscous forces are negligibly small in comparison to inertial or pressure force is called inviscid flow.

• External flow:- The flow of an unbounded fluid over a surface is called external flow.

• Internal flow:- The flow of a fluid which is completely bounded by a solid surface is called internal flow.

• Steady flow:- The flow in which there is no change at any point with time is called steady flow.

• Unsteady flow:- The flow in which there is a change at any point with time is called unsteady flow.

• Periodic flow:- A kind of unsteady flow in which flow oscillates about a steady mean is called a periodic flow.

• Forced flow:- When a fluid is forced to flow over a surface or in a pipe by external means such as a pump, this type of flow is called a forced flow.

• Natural flow:- The fluid motion due to natural means such as buoyancy effect is called natural flow. Ex:- Rise of warmer fluid and fall of cooler fluid.

• Laminar flow:- The flow in which ordered fluid motion is characterized by smooth layers of the fluids is called laminar flow. Ex:- Oils at low velocities.

• Turbulent flow:- The flow in which highly disordered fluid motion typically occurs at higher velocities and is characterized by velocity fluctuations is called turbulent flow. Ex:- Low viscosity fluids such as air at high velocities.

• Transitional flow:- The flow alternating between laminar and turbulent flow is called transitional flow.

• Compressible flow:- The flow in which the density of the fluid changes is called compressible flow. Ex:- High-speed gas flow.

• Incompressible flow:- The flow in which the density of the fluid remains nearly constant throughout. Ex:- Fluid flow when Ma<0.3.

Mach number

• When analyzing rockets, aircraft which involve high-speed gas flows, the flow speed is often expressed as Mach number.

                                                        Speed of flow

• Mach number  =      _____________

                                                       Speed of sound

• Ma=1 (Sonic flow)

• Ma>1 (Supersonic flow)

• Ma<1 (Sub-sonic flow)

• Ma>>1 (Hypersonic flow)

One, Two, Three- Dimensional Flows

• The flow field is characterized by velocity distribution.

          V= V (x, y, z, t) or V= V (r, θ, z, t)

• The flow is said to be 1D, 2D or 3D if the flow velocity varies in 1D, 2D or 3D directions.

• Variation of velocity in certain directions is small relative to other directions and is ignored.

*Self Typed
*Source- Internet, Books, Self-Analysis