Pratt & Whitney has completed testing on a proof of concept adaptive bypass variant of the F135 fighter engine. The adaptive three-stream fan test was completed as part of the U.S. Air Force Research Laboratory’s Adaptive Engine Technology Development (AETD) program at Arnold Air Force Base in Tullahoma, Tenn. Modern jet engines utilize two flow streams: one through the combustion section of the engine and a one that bypasses the combustion section around the outside of the engine.
Source: avwebP&W Hits Adaptive Fan Milestone