Viscosity
In a Nut Shell: Clearly water
flows more freely than molasses. The
fluid property exhibited in this
case is viscosity. For Newtonian fluids
the shear stress, τ, is directly
proportional to the time rate
of shearing strain, du/dy, developed in the fluid. The proportionality constant, μ, is
called the “dynamic” or “absolute”, viscosity. The
governing relation relating shear stress to time rate of shear strain for a
Newtonian fluid takes the
following form:
An
important characteristic of viscous fluids is the so-called, “no-slip”
condition at solid boundaries. For
viscous fluids the fluid velocity increases from zero, at the boundary, to
some value at the edge of
the “boundary layer” beyond which the velocity can be approximated as
uniform. Kinematic
viscosity, ν, is the ratio of the dynamic viscosity to the density of
the fluid as given below.
The
table below list dimensions and common units related to viscosity. Quantity Dimensions Common Units
Dynamic
(or kinematic) viscosity may be temperature dependent, such as exhibited in multi-grade
oil (10W30 as an example). See
appendices in text. Click here for
examples. |
Copyright © 2019 Richard C. Coddington
All rights reserved.