Hydrodynamics: Fluid dynamics or hydrodynamics is that branch of science which is concerned with the study of the motion of the fluids or that of bodies in contact with fluids.
Or, Hydrodynamics is the study of liquid flows specially, it looks at the way different forces affect the movement.
Or, the study of the motion of fluids that are partially incompressible (such as liquids, especially water, and gasses at low speeds) is usually referred to as hydrodynamics.
Stress: Stress is defined as force per unit area and is determined by dividing the force by the area upon which it acts.
Normal stress: The normal component of the force acting on a surface per unit area is called the normal stress.
Shear stress: The tangential component of force acting on a surface per unit area is called shear stress.
Viscous fluid/Imperfect fluid: `Viscous flow is not a continuous fluid substance. There is a frictional force with the layer and within the fluid with itself.
Viscosity: Viscosity is a measure of a fluids resistance to flow.
Viscous flow: A fluid is said to be viscous when normal as well as shearing stress exists.
Due to shearing stress a viscous fluid produces resistance to the body moving through it as well as between the particles of the fluid itself. Syrup and heavy oil and honey are traced as viscous fluids.
In compressible fluid: A flow is said to incompressible if the density remains nearly constant throughout.
Therefore, the volume of every portion of fluid remains unchanged over the course of its motion when the flow is incompressible. For example, the liquids are usually referred to as incompressible substance.
Kinematics: Kinematics is the description of the motion; it concerns only the accurate description of the position of the objects and the change in their position. It does not deal with the sources of their motion. Kinematics involves position velocity and acceleration not force.
Kinematics is defined as the ratio of the absolute velocity to the fluid density,
Uniform flow: A flow in which the fluid particle passes equal velocities at each section of the channel or pipe is called uniform flow.
Non uniform flow: A flow in which the fluid particle passes different velocities at each section of the channel or pipe is called non uniform flow.
Steady flow: A flow in which properties and condition (say, ρ may be velocity, pressure, density, temperature etc) associated with the motion of the fluid are independent of the time so that the flow pattern remains unchanged with the time, is said to be steady flow.
Example: Laminar flow over a sphere.
Unsteady flow: A flow in which properties and condition (velocity, pressure, density, temperature etc) associated with the motion of the fluid are depend on the time so that the flow pattern varies with time, is said to be unsteady flow.