Viscous solution acceleration

The execution of a viscous case requires the solution of a large linear system every Newton iteration. The coefficient matrix of this system is 1/3 full, although most of its entries are very small. Substantial savings in CPU time (factor of 4 or more) result when these small entries are neglected. SUBROUTINE BLSOLV which solves the large Newton system ignores any off-diagonal element whose magnitude is smaller than the variable VACCEL, which is initialized in SUBROUTINE INIT, and which can be changed at runtime from the VPAR menu with the VACC command.

A nonzero VACCEL parameter should in principle degrade the convergence rate of the viscous solution and thus result in more Newton iterations, although the effect is usually too small to notice. For very low Reynolds number cases (less than 100000), it MAY adversely affect the convergence rate or stability, and one should try reducing VACCEL or even setting it to zero if all other efforts at convergence are unsuccessful.

The value of VACCEL has absolutely no effect on the final converged viscous solution (if attained).