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Fluid Mechanics:
Glossary
Glossary
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| barotropic |
A barotropic fluid is one whose pressure and
density are related by an equation of state that does not contain
the temperature as a dependent variable. Mathematically, the
equation of state can be expressed as p = p(r) or r = r(p). |
| compressible |
A fluid flow is compressible if its density
r changes appreciably (typically by a few
percent) within the domain of interest. Typically, this will occur
when the fluid velocity exceeds Mach 0.3. Hence, low velocity flows
(both gas and liquids) behave incompressibly. |
| density,
r |
The mass of fluid per unit volume. For a
compressible fluid flow, the density can vary from place to
place. |
| incompressible |
An incompressible fluid is one whose density is
constant everywhere. All fluids behave incompressibly (to within 5%)
when their maximum velocities are below Mach 0.3. |
| inviscid |
Not viscous. |
| irrotational |
An irrotational fluid flow is one whose
streamlines never loop back on themselves. Typically, only inviscid
fluids can be irrotational. Of course, a uniform viscid fluid flow
without boundaries is also irrotational, but this is a special (and
boring!) case. |
laminar
(non-turbulent) |
An organized flow field that can be described
with streamlines. In order for laminar flow to be permissible, the
viscous stresses must dominate over the fluid inertia
stresses. |
| Mach |
Mach number is the relative velocity of a fluid
compared to its sonic velocity. Mach numbers less than 1 correspond
to sub-sonic velocities, and Mach numbers > 1 correspond to
super-sonic velocities. |
| Newtonian |
A Newtonian fluid is a viscous fluid whose shear
stresses are a linear function of the fluid strain rate.
Mathematically, this can be expressed as: tij =
Kijqp*Dpq, where tij is the shear stress
component, and Dpq are fluid strain rate
components. |
| perfect |
A perfect fluid is defined as a fluid with zero
viscosity (i.e. inviscid). |
| rotational |
A rotational fluid flow can contain streamlines
that loop back on themselves. Hence, fluid particles following such
streamlines will travel along closed paths. Bounded (and hence
nonuniform) viscous fluids exhibit rotational flow, typically within
their boundary layers. Since all real fluids are viscous to some
amount, all real fluids exhibit a level of rotational flow somewhere
in their domain. Regions of rotational flow correspond to the
regions of viscous losses in a fluid. Inviscid fluid flows can also
be rotational, but these are special nonphysical cases. For an
inviscid fluid flow to be rotational, it must be set up that way by
initial conditions. The amount of rotation (called the velocity
circulation) in an inviscid fluid flow is conserved, provided
that the fluid is also barotropic and subject only to conservative
body forces. This conservation is known as Kelvin's Theorem
of constant circulation. |
| Stokesian |
A Stokesian (or non-Newtonian) fluid is a
viscous fluid whose shear stresses are a non-linear function of the
fluid strain rate. |
| streamline |
A path in a steady flow field along which a
given fluid particle travels. |
| turbulent |
A flow field that cannot be described with
streamlines in the absolute sense. However, time-averaged
streamlines can be defined to describe the average behavior of the
flow. In turbulent flow, the inertia stresses dominate over the
viscous stresses, leading to small-scale chaotic behavior in the
fluid motion. |
| viscosity,
m |
A fluid property that relates the magnitude of
fluid shear stresses to the fluid strain rate, or more simply, to
the spatial rate of change in the fluid velocity field.
Mathematically, this is expressed as: t =
m*(dV/dy), where t is the shear stress in the same direction as
the fluid velocity V, and y is a direction
perpendicular to the fluid velocity direction. |
