Terminal velocity

In the game Portal — which needs no introduction — there is an achievement entitled “Terminal Velocity”. Under Portal's physics engine, actual terminal velocity is reached much more quickly than the not-quite-stratospheric 30,000 feet (9144 m (yes, exactly)) that Chell must fall to be awarded this achievement. This is one of the easier achievements to unlock; all it requires is a flat ceiling, a flat floor, and a little patience.

The physics engine doesn't appear to discriminate — for a number of reasons — between in-portal and out-of-portal terminal velocity; `v_t` is a constant for Chell, regardless of where she's falling either from or to. Given the observed effects of the portals, though, this isn't actually what would happen: there would be a small but measurable — perhaps even noticeable — increase in terminal velocity in the case where she is falling from one portal directly down into another.

As can be observed in several locations throughout the game, objects above (or below) portals are affected by gravity exactly as though the portals were not present. Relevantly, this necessarily includes a) Chell and b) air (especially the air Chell is pushing). Thus there will be a significant downward component to the velocity of the air in the column of space between the two portals. This would be true even if only air were present in the column: the entire column of air would simply undergo free-fall.

There are limits to this, of course: the air in the column will not necessarily stay in the column due to Brownian motion and interaction with the air (or walls) adjacent to the column. Still, there would be at least a standing downward breeze. (The actual force of the breeze in the equilibrium state would, ceteris paribus, be roughly inversely proportional to the surface area of the air-column, where interaction takes place; intuitively it seems that the shape and size of the surrounding room should affect it, but I can't think how to state that at the moment.)

At any rate, the air in the column would exert less backwards force on a falling Chell, since it would be falling with her; her actual terminal velocity would be her normal `v_t` plus the speed of the equilibrium-state breeze. (Whether or not her presence would affect the latter value I am not at all sure.)