Neknek: Scalar solve in only one session feasible?

[Chinthaka Jacob]

Dear Nek Community,

  I would appreciate your thoughts on a slightly non-standard neknek configuration. The objective is to restrict scalar transport to a subdomain while retaining a globally consistent velocity field via overlapping Schwarz iterations.

  My goal is a two-session neknek setup where:

• velocity is solved in both sessions (standard coupling), but
• a passive scalar is solved only in one session (say Session 1),
• while Session 2 carries velocity only (i.e. no scalar evolution).

  From both the documentation and my tests, it appears that neknek assumes field symmetry across sessions. In particular, scalar arrays are expected to exist (and potentially be exchanged) in both sessions.

Am I correct in understanding that there is no straightforward way to:

• disable scalar evolution in one session while preserving velocity coupling, or
• define a “one-sided” scalar transport problem within the current neknek framework?

More specifically, has anyone successfully implemented a setup where:

• velocity is coupled across sessions, but
• the scalar is solved in only one session?

As an alternative, I also explored a “nested mesh” approach (slave mesh as a strict subset, with scalar-only solve using interpolated velocity). However, this led to robustness issues, particularly related to gs_setup, which is why I am revisiting neknek.

Any insights, or confirmation that this is fundamentally unsupported, would be very helpful.

Chinthaka.