CM From , joined Dec 1969, posts, RR: Reply 1, posted (5 months 3 days 2 hours ago) and read 1313 times:
The VFN is a device to reduce bypass area at low thrust settings. You can think of it as a very small/simple thrust reverser which partially blocks the bypass air flow... effectively reducing the bypass ratio of the engine. The GTF architecture (and other very high bypass ratio engines) are designed for a very slow fan speed relative to the core. As a result, when the core is at very low speeds, the fan is at increased risk of stalling the LPT. Reducing the fan flow area and keeping some back side pressure on the fan helps prevent this.