Recap: Introduction & Fundamental Concepts of Fluid Properties | Fluid Mechanics for Mechanical Engineering PDF Download

A fluid is a substance that deforms continuously when subjected to even an infinitesimal shear stress. 

• Solids can resist tangential stress at static conditions undergoing a definite deformation while a fluid can do it only at dynamic conditions undergoing a continuous deformation as long as the shear stress is applied.
• The concept of continuum assumes a continuous distribution of mass within the matter or system with no empty space. 
• In the continuum approach, properties of a system can be expressed as continuous functions of space and time. A dimensionless parameter known as Knudsen number K_n = λ/L where λ is the mean free path and L is the characteristic length, aptly describes the degree of departure from continuum.
  The concept of continuum usually holds good when K_n< 0.01.
• Viscosity is a property of a fluid by virtue of which it offers resistance to flow. The shear stress at a point in a moving fluid is directly proportional to the rate of shear strain. For a one dimensional flow,   The constant of proportionality μ is known as coefficient of viscosity or simply the viscosity. The relationship is known as the Newton's law of viscosity and the fluids which obey this law are known as Newtonian fluids. 
• The relationship between the shear stress and the rate of shear strain is known as the constitutive equation. The fluids whose constitutive equations are not linear through origin (do not obey the Newton's law of viscosity) are known as non-Newtonian fluids. 
• For a Newtonian fluid, viscosity is a function of temperature only. With an increase in temperature, the viscosity of a liquid decreases, while that of a gas increases. 
• For non-Newtonian fluid, the viscosity depends not only on temperature but also on the deformation rate of the fluid. Kinematic viscosity v is defined as μ/p. 
• Compressibility of a substance is the measure of its change in volume or density under the action of external forces. It is usually characterized by the bulk modulus of elasticity
  
• A flow is said to be incompressible when the change in its density due to the change in pressure brought about by the fluid motion is negligibly small. When the flow velocity is equal to or less than 0.33 times of the local acoustic speed, the relative change in density of the fluid, due to flow, becomes equal to or less than 5 per cent respectively, and hence the flow is considered to be incompressible. 
• The force of attraction between the molecules of a fluid is known as cohesion, while the force of attraction between the molecules of a fluid and of a solid is known as adhesion. The interplay of these two intermolecular forces explains the phenomena of surface tension and capillary rise or depression. 
• A free surface of the liquid is always under stretched condition implying the existence of a tensile force on the surface. The magnitude of this force per unit length of an imaginary line drawn along the liquid surface is known as the surface tension coefficient or simply the surface tension.