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Aerospace Engineering (ABSTRACT)Several different time-integration algorithms for the Euler equations are investigated on two distributed-memory parallel computers using an explicit message-passing paradigm: these are classic Euler Explicit, four-stage Jameson-style Runge-Kutta, Block Jacobi, Block Gauss-Seidel, and Block Symmetric Gauss-Seidel. A finite-volume formulation is used for the spatial discretization of the physical domain. Both two-and three-dimensional test cases are evaluated against five reference solutions to demonstrate accuracy of the fundamental sequential algorithms. Different schemes for communicating or approximating data that are not available on the local compute node are discussed and it is shown that complete sharing of the evolving solution to the inner matrix problem at every iteration is faster than the other schemes considered. Speedup and efficiency issues pertaining to the various time-integration algorithms are then addressed for each system. Of the algorithms considered, Symmetric Block Gauss-Seidel has the overall best performance. It is also demonstrated that using parallel efficiency as the sole means of evaluating performance of an algorithm often leads to erroneous conclusions; the clock time needed to solve a problem is a much better indicator of algorithm performance. A general method for extending one-dimensional limiter formulations to the unstructured case is also discussed and applied to Van Albada's limiter as well as Roe's Superbee limiter. Solutions and convergence histories for a two-dimensional supersonic ramp problem using these limiters are presented along with computations using the limiters of Barth & Jesperson and Venkatakrishnan -the Van Albada limiter has performance similar to Venkatakrishnan's. IntroductionAlthough sequential computer performance has historically increased exponentially, this trend cannot continue indefinitely. Due to the finite speed of light and other physical limitations, there is an absolute speed limit for sequential computers, and we are rapidly approaching that limit. Bergman and Vos (1991) estimate that computer performance on the order of 10 12 -10 18 floating-point operations per second is necessary to model an entire airplane (including the aerodynamics, structure, and propulsion and control systems) and obtain turnaround times short enough to impact a design. This kind of performance can only be achieved using parallel computers (Simon, et al. 1992, 28). Gropp, Lusk, and Skjellum Gauss-Seidel, and Block Symmetric Gauss-Seidel iterative schemes are also implemented to solve the matrix problem that arises with Euler implicit time integration (Stoer & Bulirsch 1980, 560-562; Golub & Van Loan 1989, 513-514).Several authors have investigated parallel explicit time integration schemes on structured grids (Deshpande et al. 1993;Otto 1993;Underwood et al. 1993;Scherr 1995;Drikakis 1996;Peric & Schreck 1996;Stamatis & Papailiou 1996). Others have used parIntroduction 2 allel explicit time integration schemes on unstructured...

Time-accurate computational fluid dynamics (CFD) calculations were performed to characterize the unsteady nature of the air wake produced by a US Navy LHA-class ship. A series of wind-over-deck (WOD) conditions were examined. It was found that the general character of the airwake changed dramatically as the WOD angle changed. The airwake was also found to be highly unsteady or "turbulent" but also contained some strongly periodic features. The CFD simulations were computed at full-scale conditions with second-order time accuracy using the COBALT CFD code. COBALT is an unstructured grid, Navier-Stokes solver.

Results from an investigation of the predictive capabilities of various two-equation RANS models for the jet-in-crossflow problem are presented. The flow regime consists of a supersonic jet issuing into a transonic cross flow. The parameters varied are the jet momentum ratio, jet inclination angle, and cross flow Mach number. The goal of the investigation is to characterize the behavior of the turbulence models in this flow regimethis has implications for accurate predictions of vortex-fin interactions. The results of this study show that none of the RANS model examined are capable of capturing the vortex location and strength accurately. A detailed analysis of available experimental data shows that the Boussinessq approximation, fundamental to these models, is itself deficient for this category of flows. The analysis shows that vastly different length scales are associated with each component of the Reynolds stress and a single length scale model deficient in capturing this.

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