Time-dependent flow problems are governed by the following set of ordinary differential equations:
(29–1) |
which is subject to initial conditions of the type
(29–2) |
is the vector of nodal unknowns
such as velocity, pressure, temperature, viscoelastic extra stresses,
and free surface location. The symbol
denotes
the time-derivative of
. The matrices
and
are the
mass and stiffness matrices, which may depend on the unknown vector
. The
vector
corresponds to the volumetric
forcing function and the natural boundary conditions.
The above equations are solved in Ansys Polyflow by means of
a parabolic time-stepping procedure. Instead of trying to satisfy Equation 29–1 at an arbitrary
time , Ansys Polyflow will calculate a solution of Equation 29–1 at a discrete
set of times
, defined by
(29–3) |
(29–4) |
where the subscript refers to the time step.
From Equation 29–1, the time derivative can
be obtained. Consider the function
:
(29–5) |
An approximation of is
created using the formula
(29–6) |
Also, using the first-order discretization of the first derivative
(29–7) |
results in
(29–8) |
Different values of result in different integration methods
with different accuracy and stability attributes.