Rheology
RAMISWAMY, 1985 Made user friendly by Kapila 1990
HELLO. In this tutorial we shall learn about the different types of fluids
and their behavior. At the end of each part, there will be a quiz; you
are advised to take notes as you go through the program.
A definition first: Rheology is the science of deformation and flow.
One common factor between solids, liquids, and all materials whose behavior
is intermediate between solids and liquid is that if we apply a stress
or
load on any of them they will deform or strain
.
Models for material behavior based on stress-strain relationships are
called rheological equations of state. In a flowing fluid a measure of
strain is the velocity gradient perpendicular to the direction of flow.
Fluids are broadly divided into two categories: NEWTONIAN and NON-NEWTONIAN.
NEWTONIAN FLUIDS:
This is the classical idealized fluid. in this case, the stress-strain
( strain is expresses as the rate of shear ) relationship is linear. The
equation for a NEWTONIAN FLUID is:
SS = V * SN
where SS = Shear stress, SN = Shear strain, and V = A constant of proportionality
This constant v is called the viscosity of the fluid. In common dimensionless
numbers such as the Reynolds number, it is assigned the symbol µ .
On a plot, the equation appears as follows:
Note that for a newtonian fluid the viscosity is constant irrespective
of the shear stresses and independent of time. Newtonian behaviour is approximated
by gases and some liquids,notably water. Non-newtonian fluids fall into
three broad classes
TIME-INDEPENDENT FLUIDS: Here the shear rate (i.e., the strain) is a function
of shear stress and nothing else. i.e., SN is a function of (SS) ONLY.
Note that this applies to newtonian fluids as well; thus the newtonian
fluid may be considered as the simplest case of this class.
TIME-DEPENDENT FLUIDS: Here the SS - SN relationship depends on how the
fluid has been sheared and on previous history.
ELASTOVISCOUS FLUIDS: These fluids are predominantly viscous, but exhibit
partial elastic recovery after deformation. This class could be considered
as a special subclass of (b), but is normally treated separately.
More on Time-Independent fluids
Quit
HTML conversion 31-May-96