The terminal velocity achievement consists of falling 30,000 feet. There aren't any places in the Aperture Science Enrichment Center where you can fall that far, but your portal gun allows you to fall infinitely if you wish.
In order to get the terminal velocity achievement, you just need to be some place where you can make a portal on the ceiling that faces a portal on the floor. Turn on portal funneling and make a portal above you and then directly below you. Point your portal gun straight at the ground, and you should be able to fall into the portal forever.
This achievement takes a while to be completed, even in the biggest rooms that you can find in the game. So you will have to just leave the game alone for a while as your character continues to fall.