Engineering Fluid
Mechanics (UME 307)
Dr. S.S.MALLICK, TIET
Lecture 12: Fluid Flow Basics
Fluid Flow: Fluid is in motion – also broadly
known as “Fluid Dynamics”
• A streamline is defined as a line drawn
through the flow field in such a
manner that the local velocity vector is
tangent to the streamline at every
point along the line at that instant.
Thus the tangent of the streamline at a
given time gives the direction of the
velocity vector.
• A streamline, however, does not
indicate the magnitude of the velocity.
The flow pattern provided by the
streamlines is an instantaneous
visualization of the flow field.
Steady versus unsteady flow
Steady: Fluid flow properties do not change with time at a particular
location
Unsteady: Fluid flow properties change with time at a particular
location
Steady: ∂P/∂t = 0; ∂V/∂t = 0; ∂ρ/∂t = 0 …..at a location
Unsteady: ∂P/∂t ≠ 0; ∂V/∂t ≠ 0; ∂ρ/∂t ≠ 0 …… at a location
Uniform versus non-uniform flow
Uniform: Fluid flow properties do not change with location (or space) at
a particular time
Non-uniform: Fluid flow properties change with location (or space) at a
particular time
Uniform: ∂P/∂s = 0; ∂V/∂s = 0; ∂ρ/∂s = 0 …..at a time
Non-Uniform: ∂P/∂s ≠ 0; ∂V/∂s ≠ 0; ∂ρ/∂s ≠ 0 …… at a time
Laminar versus turbulent flow
Laminar: Fluid flow happens in an
orderly manner, layer by layer, in
streamlines
Turbulent: Fluid flow happens in a
random manner, no orderly
movement, mixing of streams
Compressible versus incompressible flow
Compressible flow: Appreciable changes
in fluid density along the flow (due to
change in location) – typically gas flow
Incompressible flow: No appreciable
changes in fluid density along the flow
(due to change in location) – typically
liquid flow
Internal and external flow
Internal flow: Such as flow
through a pipe, where the
fluid flow surfaces are
bound
External flow: Such as flow
over a pipe or over a flat
plat, where the fluid flow
surfaces are not-bound
1-2-3 dimensional flow
A flow field is best
characterized by the velocity
distribution, and thus a flow
is said to be 1, 2, or 3
dimensional if flow velocity
varies in 1, 2 or 3 primary
dimensions, respectively.
• It is good enough to consider 1
dimensional flow in most of the
mechanical engineering application,
although practically there could be
some velocity component in ‘y’ and
‘z’ directions, but the predominant
flow direction would be in ‘x’
direction (and hence, 1-D flow
assumption becomes valid) – this
just makes the flow analysis much
simple (yet providing practically
acceptable results)
• People’s definition about 1 and 2
dimensional flow and 2 and 3
dimensional flow can be somewhat
overlapping
Rotational and Irrotational flow
The difference between
rotational and irrotational flow:
fluid elements in a rotational
region of the flow rotate, but
those in an irrotational region
of the flow do not
GUESS
FLOW
TYPES
Declaration
Certain figures, texts, excerpts taken for the preparation of this content are
from the following references and from various internet sources and
constitute only insignificant amount of the original work. The intent of this
content is purely educational (for teaching purpose only to TIET registered
students under regular mode) and not for any commercial use or onward
reproduction.
References:
1. Fluid Mechanics, Yunus A. Cengel and John M. Cimbala, Tata McGraw-
Hill Education, 2010
2. Engineering Fluid Mechanics, Clayton T. Crowe, Donald F. Elger, John A.
Roberson, Wiley, 2004
3. Fluid Mechanics, Frank M. White, Tata McGraw-Hill Education, 2011
