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.