Flagship of the Pentagon's Versatile Affordable Advanced Turbine Engines (Vaate) research effort, Advent is a technology demonstration under which Rolls and GE will ground-test engines that combine high supersonic thrust with subsonic fuel efficiency. Advent engines feature adaptive fans that can vary their mass flow and pressure ratio, matching airflow to the flight regime, from takeoff to supersonic cruise. And where conventional turbofans have hot core and cooler bypass flowpaths, Advent introduces a cool “third stream” for power extraction and thermal management—and noise reduction.
Both the GE and Rolls variable-cycle engine designs use the third-stream capability to reduce jet noise by modifying the velocity profile of the nozzle exhaust. Where other sources of noise can be tackled within the engine itself, by controlling its generation and attenuation, jet noise is produced outside the engine, by the violent shearing between the high-velocity exhaust stream and the surrounding air. Modifying the jet plume before it leaves the exhaust is one of the only ways to reduce noise.
“The high-speed flow typical of jet exhaust produces noise from the velocity shear, so we have to modify the velocity profile in the exhaust to reduce shear, detune the frequency of the noise and focus the noise in different directions,” says John Kusnierek, business development director for the Liberty Works.
Rolls's solution is the mixer-ejector nozzle, which first mixes the core and bypass flows using a lobed mixer, then brings in slower freestream air through ejector doors to mix with the higher-velocity jet exhaust. “The concept of the ejector goes back a long way, but previously it was used for thrust augmentation and never the modulation and management of noise,” he says.
Under its Phase 1 task, Liberty Works defined an integrated inlet/engine/nozzle propulsion system meeting Lockheed's aerodynamic and acoustic performance goals. The advantages of variable cycle include more flexible airflow scheduling across the flight regime and the potential to better match aircraft thrust requirement at the takeoff, transonic and supersonic cruise points.
The nozzle has two ejector doors on the outside and two diverter flaps inside. The external doors pivot 5-15 deg., creating openings through which freestream air is drawn into the nozzle. The internal flaps pivot 8-12 deg., creating crescent openings during takeoff and allowing third-stream air to enter the main flow. The doors and flaps rotate proportionately using the same actuator, but with different settings from takeoff to cruise. Both doors and flaps are closed during cruise.
A 15% scale model of the variable mixer-ejector nozzle was tested in the anechoic-dome wind tunnel of NASA Glenn's Aero-Acoustic Propulsion Laboratory. Tests were run at Mach 0.3 rolling-takeoff conditions, with five ejector-door angles, three divergent-flap angles, three third-stream throat areas and three nozzle clocking angles.
The nozzle is designed for a 36,400-lb.-gross-thrust engine in which mass flow is split between the core (almost 15%), bypass (more than 58%), outer third stream (18%) and ejector flow (9%). Of the third-stream flow, 65% is used to pump the ejector to entrain freestream air, and 15% flows through the inner flap slots to enhance mixing of the main, third-stream and freestream flows.
Preliminary noise estimates for a four-engine aircraft were a cumulative 18.3 EPNdb below Stage 3 noise limits and 8.3 db below Stage 4. Lockheed Martin's predictions for the final three-engine concept developed for Phase 1 are a cumulative 22 EPNdb below Stage 3, short of its goal of 25-30 db, but still 12 db below today's Stage 4 limits.
“It's fairly complicated, but the concept performed as advertised,” says Coen. The tests uncovered tones that were dependent on door and flap settings. “There was an issue with the small passages in the model,” he says. Covering gaps, bumps and cavities created by movement of the doors reduced or eliminated the tones. “If high-frequency tones due to third-stream flow can be removed, noise levels may be reduced 1-1.5 decibels,” Rolls says.