Similar artifacts can be observed by using pump instead of fan walls (i.e. it is not a fan wall issue):

It occurs at the top edge of any enclosed space (i.e. it is not an issue specific to the edge of the field):

(Also note that top and bottom compartments do not behave identically as expected - they are identical in size and layout).

I have noticed the presence of this artifact in most all projects that involve air and pressure; it generally manifests itself as some sort of asymmetry in the pressure map in situations that should be symmetrical, or turbulence/buffeting along walls when there should be smooth airflow.

I did not construct test cases to show it, but I also think I recall seeing these same artifacts on the left edge when air is moving vertically. Don't quote me on that.

Maybe not related, but another airflow issue is here: Fan walls pull air through walls.

Build 137, Windows 7 Pro 64-bit.

-C