Bulk Modulus is a measure of the resistance of a fluid to compression. It is defined as the ratio of pressure stress to volumetric strain.
The value of bulk modulus of a fluid is required to determine Mach number. Mach number (M or Ma) is a dimensionless quantity. It represents the flow of velocity beyond the boundary when compared to the local speed of sound. It is used to determine the resistance of fluid or gas in its compressed state.

In hydraulic engineering, a nappe is a sheet or curtain of water that flows over a weir or dam. There are three types of nappe that form over the crest of a weir, depending on the air ventilation structure of a weir: free nappes, depressed nappes, and clinging nappes. A free nappe, which is ventilated to maintain atmospheric pressure below, does not come into contact with the underside of the weir. A depressed nappe is partially ventilated, which creates negative pressure beneath the nappe. In a depressed nappe the pressure below the nappe is negative.

A flow, in which the streamlines of its moving particles are represented by a straight line, is called an one-dimensional flow.

Absolute viscosity or dynamic viscosity is defined as the property of a liquid which offers resistance to the movement of one layer of liquid over another adjacent layer of liquid. The viscosity of a liquid is due to cohesion and interaction between particles.
Kinematic viscosity is defined as the ratio of absolute viscosity to the density of the liquid.