Cohesion, Adhesion, Surface Tension and Capillarity

Cohesion means inter-molecular attraction between molecules of the same liquid. 

Adhesion means attraction between the molecules of a liquid and the molecules of a solid boundary surface in contact with the liquid.

Surface tension is caused by the force of cohesion at the free surface.

Capillarity is defined as the phenomenon of rise or fall of a liquid surface in a small tube relative to the adjacent general level of liquid when the tube is held vertically in the liquid. Capillarity rise or fall is due to the combined effect of adhesion and cohesion.

Convective and Local Acceleration

Convective acceleration is defined as the rate of change of velocity due to the change of position of fluid particles in a fluid flow.

Local acceleration or Temporal acceleration is defined as the rate of change of velocity with respect to time at a given point in a flow field.

Fluid Flows

A flow is considered steady if the fluid characteristics like velocity, pressure, density, etc., at a point do not change with time.

Unsteady flow is defined as that type of flow in which the fluid characteristics like velocity, pressure, density change with respect to time. Eg. Flow of water in river.

A flow is said to be laminar when the various fluid particles move in layers (or laminae) with one layer of fluid sliding smoothly over an adjacent layer.

In a turbulent flow, the fluid particles move in an entirely haphazard or disorderly manner, that results in a rapid and continuous mixing of the fluid.

Compressible fluid is one in which the density changes from point to point.

Incompressible fluid is one in which the density is constant for the fluid flow.

When the velocity profile shape no longer changes with increasing distance, the flow is said to be fully developed.

Control Volume

A control volume is an arbitrary volume in space through which fluid flows. The concept of control volume is used in fluid mechanics studies to understand the changes that take place in the fluid characteristics.

Viscosity

Viscosity is a measure of the internal fluid friction that causes resistance to flow. It is defined as the shear stress required to produce unit rate of shear strain.

In SI units, viscosity is expressed in Ns/m².

Effect of Temperature on Viscosity of Water and Air
The viscosity of water decreases with the increase of temperature.
The viscosity of air increases with the increase of temperature.