Page 79 - Industrial Ventilation Design Guidebook

44 CHAPTER 4 PHYSICAL FUNDAMENTALS

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 pad-
dle 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.

Streamline
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 one-
dimensional, a typical case being uniform flow in a constant-area duct.
Two-dimensional flow is in the x and y directions, while three-dimen-
sional 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 ap-
plied 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.