Zienkiewicz O.C., Taylor R.L. Vol. 3. The finite - tiera.ru

As we proved earlier, the Galerkin spatial approximation is justi®ed when the
characteristic±Galerkin procedure is used. We can thus write the approximation
ˆ N ~ f …2:92†
and use the weighting N T in the integrated residual expression. Thus we obtain
M… ~ f n ‡ 1 ÿ ~ f n †ˆÿ t‰…C ~ f n ‡ K ~ f n ‡ f n †ÿ t…K u ~ f n ‡ f n s †Š …2:93†
in explicit form without higher-order derivatives and source terms. In the above
equation
…
M ˆ N T …
N d C ˆ N T @
…U
@x
iN† d
i
… T
@N
K ˆ k
@xi @N
…
d f ˆ N
@xi T …2:94†
Q d ‡ b:t:
and Ku and f n s come from the new term introduced by the discretization along the
characteristics. After integration by parts, the expression of Ku and fs is
Ku ˆÿ 1
…
2
fs ˆÿ 1
…
2
@
@x i
@
@x i
…UiN T † @
…U
@x
iN† d …2:95†
i
…U iN T †Q d ‡ b:t: …2:96†
where b.t. stands for integrals along region boundaries. Note that the higher-order
derivatives are not included in the above equation.
The approximation is valid for any scalar convected quantity even if that is the
velocity component Ui itself, as is the case with momentum-conservation equations.
For this reason we have elaborated above the full details of the spatial approximation
as the matrices will be repeatedly used.
It is of interest that the explicit form of Eq. (2.93) is only conditionally stable. For
one-dimensional problems, the stability condition is given as (neglecting the e€ect of
sources)
t 4 tcrit ˆ h
…2:97†
jUj
for linear elements.
In two-dimensional problems the criteria time step may be computed as 62;63
t crit ˆ
t t
t ‡ t
Characteristic-based methods 41
…2:98†
where t is given by Eq. (2.97) and t ˆ h 2 =2k is the di€usive limit for the critical
one-dimensional time step.
Further, with t ˆ t crit the steady-state solution results in an (almost) identical
di€usion change to that obtained by using the optimal streamline upwinding
procedures discussed in Part I of this chapter. Thus if steady-state solutions are the
main objective of the computation such a value of t should be used in connection
with the K u term.