Robert Tomaro

Cobalt: a parallel, implicit, unstructured Euler/Navier-Stokes solver

Abstract A parallel, implicit, unstructured Euler/Navier-Stokes solver has been developed that is accurate, robust and user-friendly. The parallel performance of the code is excellent, showing linear, and at times better than linear, scalability on the current parallel architectures. This, combined with the time savings afforded by the implicit algorithm, permits the routine analysis of large viscous problems on workstation-class processors.

A solution on the F-18C for store separation simulation using Cobalt(60)

A demonstration is presented of the ability of Computational Fluid Dynamics (CFD) methods to predict store carriage loads and support store trajectory generation. A complete, complex aircraft, the F/A- 18C, was modeled with actual stores in their carriage positions. Cobalt,, a parallel, implicit unstructured flow solver was used to calculate the flow field and resultant aerodynamic loads on grids composed of tetrahedral cells. Three grids were used to simulate three different flow field approximations. The first grid was a purely inviscid grid containing 3.15 million cells. The second grid was made up of 3.96 million cells clustered to capture viscous effects on only the store components. The third grid was a full viscous grid containing 6.62 million cells. Store carriage loads for two flight conditions were calculated and compared with wind-tunnel measurements and flight-test data for each of the above grids. The resulting carriage loads were used in a separate six degree-of-freedom (6DOF) rigid-body motion code to generate store trajectories. All CFD solutions were second-order accurate and run to steady-state with CFL numbers of one million. Turnaround times ranged from 6 to 21 hours, depending on the number of processors used.

Computation of Compressible Flows Through a Chemical Laser Device with Crossflow Injection

The Air Force Research Laboratory has an ongoing effort to develop an accurate and efe cient computational tool to support the development of advanced chemical oxygen/iodine laser (COIL)devices. In this study, a series of computational simulations have been performed to provide a better understanding of e uid dynamic phenomena withingeometriesassociatedwithCOILe owe elds.Theparallel,implicitunstructuredNavier ‐StokescodeCobalt 60 was used to compute laminar, turbulent, and unsteady e ows of helium within the research assessment and device improvement chemicallaser (RADICL)nozzle. Computational results showing details of thejetmixing interaction and topological structure are presented. The laminar and turbulent results obtained with Cobalt 60 are in excellent agreementwithmeasuredmasse owratesandsurfacepressuredataobtainedfromrecentcold-e owtestsperformed with the RADICL device. Insufe cient experimental measurement prevents the determination of whether or not transition occurs within the injector region. The laminar time-accurate results indicate small-scale unsteadiness in the frequency range of 200 kHz downstream of the nozzle throat.

Static and Dynamic Loads Modeling of an Aerodynamic Control Surface

This study is part of a larger effort to create reduced order models for aerodynamic forces and moments acting on a maneuvering aircraft with moving control surfaces. The methods which use overset grids are illustrated for modeling aerodynamic loads of a jet trainer aircraft with a flap. The static results are compared to experimental data available at different flap deflection angles, with good agreement obtained at low to moderate angles of attack and deflection angles. Results have shown that unsteady Reynolds-Averaged Navier-Stokes simulations are necessary to predict aerodynamic loads due to fast control surface movements. For these unsteady loads, a reduction technique based on Duhamel’s superposition integral was proposed. This model was derived from the aerodynamic responses that were directly calculated from computational fluid dynamics simulations starting from an initial steady-state condition, where the flap deflection angle is zero at t = 0 and is held constant at some flap deflections at all other times. The model predictions were compared with time-accurate simulations and good accuracy found for all motions.