Going with the flow in jet engines.

Going with the flow in jet engines

In a jet engine, hot gases sweep past sets of curved blades, some attached to freely rotating hubs and others fixed in position. Now a researcher at the NASA Ames Research Center at Moffett Field, Calif., has completed a computer model that simulates the fluctuating, three-dimensional air flow past such rotor-stator combinations in a turbine. The model allows aircraft engineers to track changes in air pressure, temperature and velocity within a turbine in greater detail than ever before. "With this kind of information, we hope to come up with new engine designs that are safer and more reliable," says NASA's Man Mohan Rai, who developed the computer model.

Rai's simulation required solving complicated mathematical equations describing unsteady fluid flow. In this case, the problem was particularly difficult because some engine parts were in motion with respect to others. This meant the usual practice of drawing a fixed box around the region of interest -- say, a turbine blade -- and computing the value of quantities such as pressure for points within the box wouldn't work. To include both moving and stationary parts, such a box would become more and more distorted as time passed, making the computations trickier to perform. Rai's answer was to treat stationary and moving parts separately by putting them into individual boxes. He developed a scheme for accurately connecting what happens in a stationary box with what happens in an adjacent moving box. To put together a brief movie showing temperature, pressure and velocity fluctuations around a turbine's stator and rotor blades, Rai needed about 100 hours on a Cray-2 supercomputer and sophisticated graphics equipment for handling 2 billion data points.

Engineers are interested in studying where rapid and extreme pressure and temperature fluctuations occur because engine parts buffeted by such forces are more likely to fail. By checking simulations, engineers can determine whether to modify the engine geometry to reduce stress or to strengthen the affected parts by using tougher alloys -- without having to go through the expense of building and testing scale and full-size models. With modifications, Rai's computer program is suitable for modeling a wide variety of turbomachinery, from gas turbines in power plants and pumps in nuclear submarines to helicopter rotors.