"Where the Plumes Go," Feature Article, Oct. 2002

where the plumes go

Controlling hot exhaust—with the aid of computational fluid dynamics—is one of the critical design elements in airplane construction.

by Chen Chuck and Douglas R. McCarthy

It is by now standard practice at Boeing to design basic aerodynamic surfaces, such as the wings, engine nacelles, and fuselage, using computational fluid dynamics.

It is less common and often more difficult to use CFD to analyze more complex parts of the airplane, such as high lift systems, engine compartments, auxiliary power units, or the behavior of the efflux from engine thrust reversers.

After a commercial airplane touches down, a piece of engine cowling translates aft and blocker doors drop down, directing the engine airflow into a honeycomb structure called a cascade. The cascade directs the flow forward, to slow the aircraft and decrease lift for more effective braking.

The thrust reverser is used precisely at the time when high lift devices—flaps and slats—are fully deployed. The plumes of hot exhaust must be kept away from these devices.

The plumes should not hit the fuselage or other parts of the aircraft, where repeated heating and cooling could cause premature fatigue. Plumes must not re-enter the engine inlet, or blow debris from the runway in front of the engine, or envelop the vertical tail.

Designers must know exactly where the exhaust plumes go, and they must know it early in the design cycle, because it affects such basic decisions as the placement of the engine on the wing. The enabling technology and research group at Boeing's commercial aircraft unit has developed a computer simulation that provides an early look at thrust reverser exhaust plumes.

Boeing maintains a large number of CFD computer codes. Many were developed by the company or NASA. However, some of the fastest and most accurate CFD codes can be difficult to apply to complicated geometry because they require a highly structured grid of cells.

CFD model shows where a thrust reverser directs hot exhaust.

Not only is the thrust reverser geometrically complex, but the design objective is a general look at where the plumes go, and the analysis will not be run a great many times. Those conditions permit the use of an unstructured mesh, which subdivides space in a more general way and can significantly reduce the time required to set up a problem, sometimes from weeks to days. It requires a compatible CFD program.

For this analysis, we chose the Fluent package from Fluent Inc. of Lebanon, N.H. Boeing often uses commercial off-the-shelf software when it is applicable, and reserves its in-house development efforts for specialized software. Fluent was already widely used at Boeing for other geometrically complex problems, such as cooling flows in engine compartments and dispersion of fire suppression chemicals.

The CFD process begins with a model of the aircraft, created in Catia, from Dassault Systemes of Paris. Preparation includes removing small features of the model that are not necessary for the analysis.

An unstructured mesh is then built around the CAD model. For compatibility with other CFD processes at Boeing, we use another commercial package: ICEM mesh generation software from PTC in Needham, Mass. The meshes typically contain from 3 million to 8 million cells and are partitioned for parallel processing.

The entire CFD analysis, from geometry definition to solution, can be completed in about three days. Tests that involve geometry changes, such as the repositioning of the cascades or the nacelle relative to the wing, or variation of the cascade angles, can be accomplished with minimal remeshing and analysis.

Wind tunnel testing and expense are reduced, but the key benefits are time and risk mitigation. Our process increases early confidence in the design and enables us to shorten the development cycle and deliver a quality product on schedule.

Chen Chuck and Douglas R. McCarthy are scientists and research engineers at The Boeing Co. in Seattle.