After a couple phone interviews I was brought in to the main office (near Chicago) for a full day interview. The day started with breakfast with the hiring manager. I gave a technical presentation of previous work, followed by a Q&A. I had series of 1-on-1 interviews the rest of the day. Some of these covered technical engineering issues, others were higher level discussions with more senior people. There was a group lunch at a restaurant in which I was asked a couple fluid mechanics problems.