Page 78 - Industrial Ventilation Design Guidebook

4.1 FLUID FLOW 43

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 nonuni-
form, 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 in-
stant 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.