Helping testers who don't have specialized domain knowledge

I am a team lead on a project that develops specialized engineering software. My developers generally have advanced degrees and the domain knowledge needed to work with the software we develop, but my testers don't have the necessary background knowledge. I don't have much flexibility in determining the composition of my team, so I can't hire new testers with an engineering background. What steps can I take to help my test team understand the software better -- or alternatively, how can I make it easier for them to do "black box" testing for which they won't need that knowledge?