The GE36 was (as far as I know) the only attempt to make an ungeared UDF. Only one engine was made as a concept demonstrator on a contract with NASA.
In principle it was a separate unit added on the back of an off the shelf GE
F-404 engine from an F/A-18 Super Hornet fighter. It was used as gas generator to power the aft UDF unit.
The back unit first split the gas into a wide annular gas flow so the UDF turbines could get a proper moment arm to pull the fan blades at the rather low speed, not much over 1500 rpm.
The UDF unit had two turbines each with 7 stages. One was a quite normal turbine pulling one fan in one direction.
The other 7 stages turbine "replaced" the stator vanes. The turbine blades were mounted on their outer ends inside a "barrel" which rotated in the opposite direction and pulled the other fan.
Mechanically it was an immensely complicated thing, and just the lubricating system was a nightmare to develop. How they also made variable pitch on all 16 fan blades is beyond me. But they did.
It had the potential to produce a more lightweight unit with high efficiency by eliminating the gearbox, and by eliminating the drag from separate stator vanes.
The single GE36 was optimized for speeds up to Mach 0.72, and a reviced version good for Mach 0.80 was planned.
The project was discontinued due to noise related problems.
The front fan on the Mach 0.72 version would be largely subsonic, but due to the accelerated air by the front fan the aft fan tips were supersonic. On a Mach 0.80 version the supersonic noise would have been somewhat more of a problem.