By Dr. Chen Chuck, Research Engineer, and Dr. Douglas R. McCarthy, Research Engineer, The Boeing Company, Seattle, WA

It is, by now, standard practice at Boeing to design aerodynamic surfaces such as the wings, engine nacelles (enclosures), and fuselage using CFD instead of relying on expensive wind tunnel and flight tests. It is less common, and often more difficult, to use CFD to analyze the more geometrically complex parts of the airplane, such as high lift systems (flaps and slats), engine compartments, and auxiliary power units. To perform such an analysis, engineers need to compute airflows around and through systems that are distinguished by very complex geometry and flow patterns. A prime example involves predicting the behavior of the efflux from engine thrust reversers.

Efflux pattern on the airplane for a Mach 0.15 case

A typical commercial airplane deploys its thrust reversers briefly after touch down. A piece of engine cowling moves rearward and blocker doors drop down, directing the engine airflow into a honeycomb structure called a cascade. The cascade directs the flow forward, which acts to slow the aircraft and decrease lift for more effective braking. The reverser is used precisely at the time when high lift devices (i. e., wing leading and trailing edge flaps and slats) are fully deployed. Consequently, the plumes of hot exhaust must be directed so as to not impinge on these devices. Other effects to avoid are reingestion, in which the reversed plume reenters the engine inlet, engine ingestion of debris blown up from the runway, and plume envelopment of the vertical tail, which affects directional control. To avoid these effects, knowledge of exactly where the exhaust plumes go is needed early in the design cycle because it affects such basic decisions as the placement of the engine on the wing.

The CFD process begins with a CAD/ CAM (Computer Aided Design/ Computer Aided Manufacturing) model of the aircraft. In addition to the engine, fuselage, and wing, the CAD/ CAM model includes such devices as flaps, slats, and spoilers. An unstructured mesh is then built around the CAD/ CAM model. For compatibility with other CFD processes at Boeing, a commercial software package from ICEM CFD Engineering is used for mesh generation. Starting from a new airplane CAD geometry, such a mesh, which typically contains from 3 to 8 million cells, can be created in a day or two. Because the grid generation software contains a replay capability, minor changes to the geometry can be remeshed quickly. The mesh is partitioned into sections for parallel computing, and the analysis is completed using FLUENT's flow solver. Depending on the number of CPUs available, a final solution can be obtained within a few hours after the geometry definition and mesh generation are complete.

Because the entire CFD analysis cycle can be completed in about three days, designers can use this tool repeatedly as a way to optimize the design. Wind tunnel testing and expense are reduced, but the key benefits are time and risk mitigation. If a need to change the design should become apparent after the tooling is built and the aircraft is in the test phase, the delay in entry into service and the expense of retooling would be unacceptable. CFD modeling increases early confidence in the design and shortens the development cycle to deliver a quality product on schedule.

FluentNEWS Supplement