Not just new-technology engines. Concorde's Olympuses (Olympii?) required a "de-bow" procedure for re-starting (using very low rpms) between 15 min. and several hours after shutdown (time period when the engine was cooler than operational, but not yet cool
enough). The debow start allowed the temperature to increase slowly to operational, while minimizing vibrations while it was still bowed.
That was a bowing of the shaft(s), however, not the rotor (turbine/compressor disk).
The PW1100G engine with problems on the NEO also appears to be an issue with differential or asymmetrical
shaft cooling and bowing, that in turn "tilts" the rotor slightly out of alignment.
The bowing is due to differential expansion and contraction forces on the relatively long skinny shaft. Like pushing on both ends of a drinking straw - it will bend in the middle.
EDIT - PW CEO mentions that the bowing is less pronounced when the engine is "fan mounted" rather than "core mounted" on the pylon. The engine was tested on a 747, with room for the whole engine UNDER the wing - the smaller A320 mounts the engine higher and cantilevered out ahead of the wing for ground clearance. I'm guessing that is what he meant.
Their temporary fix is a long, slow spool-up to warm up the engine parts - like the Concorde de-bow procedure.
http://aviationweek.com/commercial-a...starting-issue