Fluid Properties Fluid Classification

Matter is considered to exist in three states

• Solid

• Liquid

• Gaseous

The term fluid applies to both liquids and gases, including liquids and gases containing particulate matter of various sizes.

When a shearing stress is imposed on a solid, deformation occurs, until a point is reached when the internal stresses produced balance the shearing stresses. Provided the elastic limit for the material is not exceeded the solid will return to its original shape when the load is removed.

A fluid, on the other hand, flows under the action of a shearing stress no matter how small this stress is. A fluid at rest has no shearing stresses, and all forces are at right angles to the surrounding surfaces. Materials such as glass and solid bitumen are fluids and, if stressed for a period of time, will tend to flow.

Ideal Fluid

A theoretical ideal fluid situation, “a perfect fluid” having a constant den­sity and no viscosity, is often used in a theoretical analysis.

Real Fluid

A real fluid will have a velocity gradient when flowing due to the viscosity of the fluid.

Incompressible Fluid

An incompressible fluid is a fluid whose density remains constant during flow. Liquids are normally treated as being incompressible, as a gas can be when only slight pressure variation occurs.

Compressible Fluid

A compressible fluid is a fluid in which significant density variations that occur during its flow have to be considered, as is usually the case with vapors and gases.

Flow Classification

Flows may be subdivided into steady and unsteady, uniform and nonuni­form, laminar and turbulent, and rotational and irrotational flows.

Steady Flow

A flow is steady when the conditions at any point remain constant with respect to time.

Unsteady Flow

An unsteady flow is one in which the conditions at any point vary with time; such a flow is also called a transient flow.

Uniform Flow

A flow is uniform when the velocity of flow is the same at any given in­stant at every point in the fluid. This state of affairs can exist only with an ideal fluid. However, steady flow (uniform flow) is assumed to take place in a duct with the velocity constant along a streamline.

Nonuniform Flow

In a nonuniform flow the velocity varies from point to point along a streamline.

Laminar Flow

Laminar flow occurs at low flow rates, in which all particles of a fluid move parallel to the walls of the duct.

Transitional Flow

The flow region between laminar and turbulent flow is called transitional flow. It is three dimensional and varies with time.

Turbulent Flow

Turbulent flow occurs at higher flow rates. The particles of the fluid have velocity components perpendicular to the general direction of flow.

Rotational Flow

Rotational flow occurs in an element of a fluid that rotates about its axis, in addition to having translational motion (e. g., water passing through a pad­dle wheel).

Irrotational Flow

Irrotational flow occurs when the fluid motion rotates about its axis (e. g., water flowing in a bend in a pipe).

Other definitions to consider are:

Path Line

A path line is the path traced by a single particle of fluid over a period of time.


A streamline shows the direction of a number of particles of fluid at the same instant in time. Flow cannot take place across a streamline. Path lines and streamlines will be identical for steady flow.

Stream Tube

A number of streamlines form a stream tube. Flows can enter and leave a stream tube only through the ends.

Stream Surface

A stream surface is the surface of a stream tube.

Streak Line

When a dye is injected into a fluid, the resulting streak lines provide flow visualization of fluid particles that have passed the same density of the fluid.

One-, Two-, or Three-Dimensional Flow

Flow may be steady but have a variation of velocity, pressure, etc., with position. If one optional coordinate is used to describe the flow it is one­dimensional, a typical case being uniform flow in a constant-area duct.

Two-dimensional flow is in the x and y directions, while three-dimen­sional flow is in the x, y, and z directions.

A fluid can be considered as being liquid, which is incompressible, or a gas, which is easily compressible. When a force of sufficient magnitude is ap­plied to a fluid, motion will occur provided the frictional resistance within an open system is overcome.

A gas expands in an enclosure to fill up the entire space, while a liquid presents a free surface in contact with the gas boundary above it.

Once a fluid starts to move in a conduit, shearing forces are set up, the maximum being at the wall of the conduit. At this surface the velocity is at the lowest, while in adjacent layers above this surface the velocity increases as the shearing stresses decrease.

It is the dynamic viscosity p, of the gas/fluid that determines its ability for free flow. Very viscous fluids require a large energy input to overcome the fric­tional forces.