Robert L. Meakin

Unsteady aerodynamic simulation of static and moving bodies using scalable computers

Methods to accurately .solve the Navier-Stokes equations for geometrically complex problems that may involve relative motion between configuration components are discussed. General curvilinear structured grids are used to discretize the volume of space in the near vicinity to configuration surfaces. Off-body domain volumes are discretized with uniform Cartesian structured grids of varying levels of refinement. Off-body grid refinement is automatically adapted in response to body movement and estimates of solution error. Near-body and off-body grid components are organized into groups of equal size. Method scalability is discussed. A set of three-dimensional applications are given to demonstrate method attributes on scalable computers and advantages for solving this class of problems.

On Strand Grids for Complex Flows

The need for highly automated and computationally efficient tools for high fidelity simulation of complex flow fields is recognized. A discretization paradigm that holds significant advantages relative to these needs is described. Problem domains are categorized into nearand offbody partitions. Strand grid technology is applied to near-body partitions, while block-structured Cartesian AMR (Adaptive Mesh Refinement) is applied to the off-body partition. The computational advantages and degrees of automation derivable from the approach are reviewed. A set of software tools that have been developed for grid generation and flow solution using both strand grid and block-structured Cartesian AMR are presented. Demonstration of strand grid technology is provided via time-dependent flow simulations and comparison with experimental data. The degree to which strand grid technology expands the spectrum of problems that can be considered via high performance computing is also considered.

CHSSI software for geometrically complex unsteady aerodynamic applications

A comprehensive package of scalable overset grid CFD software is reviewed. The software facilitates accurate simulation of complete aircraft aerodynamics, including viscous effects, unsteadiness, and relative motion between component parts. The software significantly lowers the manpower and computer costs normally associated with such efforts. The software is discussed in terms of current capabilities and planned future enhancements.

An Interface for Specifying Rigid-Body Motions for CFD Applications

An interface for specifying rigid-body motions for CFD applications is presented. This interface provides a means of describing a component hierarchy in a geometric configuration, as well as the motion (prescribed or six-degree-of-freedom) associated with any component. The interface consists of a general set of datatypes, along with rules for their interaction, and is designed to be flexible in order to evolve as future needs dictate. The specification is currently implemented with an XML file format which is portable across platforms and applications. The motion specification is capable of describing general rigid body motions, and eliminates the need to write and compile new code within the application software for each dynamic configuration, allowing client software to automate dynamic simulations. The interface is integrated with a GUI tool which allows rigid body motions to be prescribed and verified interactively, promoting access to non-expert users. Illustrative examples, as well as the raw XML source of the file specifications, are included.

Validation of the Strand Grid Approach

We explore a new approach for automated mesh generation for viscous flows around geometrically complex bodies. A prismatic-like grid using “strands” is grown a short distance from the body surface to capture the viscous boundary layer, and adaptive Cartesian grids are used throughout the rest of the domain. The advantages of this approach are many; nearly automatic grid generation from triangular or quadrilateral surface tessellations, very low memory overhead, and automatic mesh adaptivity for time-dependent problems, and fast and efficient solvers from structured data in both the strand and Cartesian grids. Solvers on the two grid systems are coupled using a Chimera overset approach so the scheme is readily applicable to problems with moving bodies. The paper focuses on validation of the approach for fundamental flow problems, fixed-wing, and rotary-wing applications. Comparison to experiment and to other well-established codes are provided. Results show the approach shows considerable promise, with load computations from the automatically generated strand meshes comparable in accuracy to manually generated fully unstructured meshes, and with excellent resolution of vortex wakes.

Adaptive spatial partitioning and refinement for overset structured grids

The need for adaptive refinement for unsteady aerodynamic applications that may involve relative motion between configuration components is recognized. An efficient means of adaptive refinement within systems of overset structured grids is presented. Problem domains are segregated into near-body and off-body fields. Near-body fields are discretized via overlapping body-fitted grids that extend a short distance from body surfaces. Off-body fields are discretized via systems of overlapping uniform Cartesian (structured) grids of varying levels of refinement. A novel method of adaptive spatial partitioning and refinement that is responsive to evolving off-body flow dynamics and proximity of moving solid bodies is described. Computational advantages of structured data are reviewed. Properties of uniform Cartesian grids that lead to substantial computational advantages are identified. A grid component grouping algorithm is presented. Formal accuracy of the method is considered. The method is demonstrated for three-dimensional unsteady viscous flow applications of practical relevance.

STS-107 Investigation Ascent CFD Support

This paper provides an overview of the computational fluid dynamics analysis of the ascent of the Space Shuttle Launch Vehicle during the investigation of the STS-107 accident. The analysis included both steady-state and unsteady calculations performed with the Overflow and Cart3D flow solvers. The unsteady calculations include moving body, six degree-of-freedom simulations of foam debris shed from the region of the left bipod-ramp of the vehicle. Many such debris trajectories were computed, some of which impacted the vehicle. The analysis provided an estimate of the speed at which such a piece of debris would strike the wing leading edge of the Shuttle Orbiter. These results were supplied to the Columbia Accident Investigation Board, and guided the choice of the impact velocity and foam size selected for the foam-firing test done as part of the investigation. This testing subsequently showed that it was possible for a piece of foam debris to cause massive damage to the Shuttle Orbiter wing Reinforced-Carbon-Carbon panels and T-seals, creating a breach where hot gases could enter the wing structure during reentry.

On Parallel Implementations of Dynamic Overset Grid Methods

This paper explores the parallel performance of structured overset CFD computations for multi-component bodies in which there is relative motion between component parts. The two processes that dominate the cost of such problems are the flow solution on each component and the intergrid connectivity solution. A two-part static-dynamic load balancing scheme is proposed in which the static part balances the load for the flow solution and the dynamic part re-balances, if necessary, the load for the connectivity solution. This scheme is coupled with existing parallel implementations of the OVERFLOW flow solver and DCF3D connectivity routine and used for unsteady calculations about aerodynamic bodies on the IBM SP2 and IBM SP multi-processors. This paper also describes the parallel implementation of a new solution-adaption scheme based on structured Cartesian overset grids.

ADVANCES TOWARDS AUTOMATIC SURFACE DOMAIN DECOMPOSITION AND GRID GENERATION FOR OVERSET GRIDS

An algorithm for surface domain decomposition and grid generation for overset grids is described. A complex surface domain is covered by two types of grids: seam grids and block grids. Seam grids are grids used to wrap around surface crease lines and regions of high surface curvature. Block grids are grids that cover the remaining surface regions not occupied by the seam grids. The seam and block grids form a set of overlapping surface grids that cover the entire surface geometry. Given a set of seam grids, an automated method for generating the block grids is presented in this paper. Examples of seam and block grids are given for several configurations including the V-22 tiltrotor fuselage and the X-CRV Crew Return Vehicle.