FROM VORTEX LINES TO 3-D WINGS
Such air flow simulations all fall under the general category of computational fluid dynamics (CFD) and computational aerodynamics. Traditional simulation methods, based on lifting-line theory, model the airplane wing as a straight vortex line and the wake as straight lines trailing behind the wing like streamers. At the next level of detail, called lifting-surface theory, the model includes the shape of the wings and a more complex wake; however, the shape of the wake must be specified in advance. Today's workstations permit this second level of simulation, with a plane model that includes 7,000 elements, or panels (Figure 1).
Going one level further requires a supercomputer. Diaz is modifying 3-D fluid modeling software developed originally by NASA, called PMARC, to take advantage of parallel computers. He is parallelizing the code using NPACI's CRAY T3E at the University of Texas and testing it on a model with 10,000 panels. This is better resolution than is practical on workstations, but they still must specify the shape of the wake, according to Diaz.
The problem is the canard. The behavior of air over a single wing is fairly well understood, but the canard adds a new twist. In this case, the placement of the canard on the aircraft, a remotely piloted vehicle (RPV), was determined by other design factors. Therefore, to specify the flight limitations for the RPV, the designers needed to know the angle of attack at which the wake from the canard impinges the wing (Figure 2).
"The sudden change in lift could potentially cause the RPV to become unstable in flight," Diaz said. "Unfortunately, we don't know the shape of the wake a priori. We have to make educated guesses."
Diaz's ultimate goal is to perform time-accurate simulations on high-resolution models. A time-accurate simulation will allow the wake to develop naturally and let them know for certain how the canard wake interacts with the wing. Workstations do not have the computing power to perform so-called time-accurate simulations. In fact, workstation simulations will fail in time-accurate simulations because the resolution is not great enough to permit the wake to pass smoothly over the wing.
Even on the Texas T3E, a model with 10,000 panels does not have enough resolution to allow the wake to develop naturally. To achieve this level of accuracy, Diaz plans to use the 256-processor CRAY T3E at SDSC, once he has finished parallelizing and testing the modified PMARC code, to simulate a plane model with 50,000 panels and a wake that develops naturally over time.
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