Release Notes OVERFLOW 2.4

2.4 20 July 2022

The big change for OVERFLOW Version 2.4 is the addition of the Parallel SSOR
linear solver, essentially a restricted additive Schwarz option, as described
in AIAA Paper # 2021-2747, which can be accessed through a new logical
variable GLOBAL_LINEAR_SOLVE in &GLOBAL. The basic idea is to make the
overset boundaries more implicit by communicating delta Q updates during the
linear solver process, doing a better job of solving the linear problem and
hopefully resulting in better nonlinear convergence, at the cost of more MPI
communication and higher memory usage. More details on how to use this new
option are discussed in doc/readmes/README.gls. The 2.4 release should
maintain backwards compatibility for most users, with slight changes to wall
functions, but for those users who have their own modifications to the code,
be aware that there were fairly significant changes at the OVERFL level to
accommodate inter-processor communication during the linear solve.

Further notes:

1: For startup and for higher speed flows, the new GLOBAL_LINEAR_SOLVE option
can be used in conjunction with ILHS = 6/7/8 to use more dissipative
Steger-Warming flux Jacobians as well as implicit physical and overset
boundary conditions; this should help users who have had difficulty getting
the Improved SSOR (ILHS = 26/27/28) to start. Be aware that starting
simulations using a constant DTPHYS may result in very high CFL numbers near
the wall and might destabilize the simulation.

2: The Improved and Parallel SSOR schemes have been adjusted to allow for
two-point boundary condition linearizations, improving convergence when using
symmetry planes.