Andrew M. Wissink

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.

A Multi-Code Python-Based Infrastructure for Overset CFD with Adaptive Cartesian Grids

This paper describes a computational infrastructure that supports Chimera-based interfacing of different CFD solvers a body-fitted unstructured grid solver with a blockstructured adaptive cartesian grid solver to perform time-dependent adaptive movingbody CFD calculations of external aerodynamics. The goal of this infrastructure is to facilitate the use of different solvers in different parts of the computational domain body fitted unstructured to capture viscous near-wall effects, and cartesian adaptive mesh refinement to capture effects away from the wall. The computational infrastructure, written using Python, orchestrates execution of the different solvers and coordinates data exchanges between them, controlling the overall time integration scheme. Details about the infrastructure used to integrate the codes, the parallel implementation, and results from demonstration calculations are presented.

Application of the Helios Computational Platform to Rotorcraft Flowfields

This article describes the architecture, components, capabilities, and validation of the rst version of the Helios platform, targeted towards rotorcraft aerodynamics. Capabilities delivered in the rst version include fuselage aerodynamics with and without momentumdisk rotor models, and isolated rotor dynamics for ideal hover and forward ight coupled with aeroelasticity and trim. Helios is based on an overset framework that employs unstructured mixed-element meshes in the near-body domain combined with high-order Cartesian meshes in the o-body domain. In addition, the aerodynamics solution is coupled with structural dynamics and trim using a delta-coupling algorithm. The near-body CFD, obody CFD, CSD and trim modules are coupled using a Python infrastructure that controls the execution sequence of the solution procedure. Specic validation studies presented include the Slowed Rotor Compound fuselage, Georgia Tech rotor body, TRAM rotor in hover and UH-60A rotor in forward ight. In all cases, Helios predictions are compared with experimental data and other state-of-the-art codes to demonstrate the accuracy, eciency and scalability of the code.

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.

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.

Overview of the Helios Version 2.0 Computational Platform for Rotorcraft Simulations

This article summarizes the capabilities and development of the Helios version 2.0, or Shasta, software for rotary wing simulations. Specific capabilities enabled by Shasta include off-body adaptive mesh refinement and the ability to handle multiple interacting rotorcraft components such as the fuselage, rotors, flaps and stores. In addition, a new run-mode to handle maneuvering flight has been added. Fundamental changes of the Helios interfaces have been introduced to streamline the integration of these capabilities. Various modifications have also been carried out in the underlying modules for near-body solution, off-body solution, domain connectivity, rotor fluid structure interface and comprehensive analysis to accommodate these interfaces and to enhance operational robustness and efficiency. Results are presented to demonstrate the mesh adaptation features of the software for the NACA0015 wing, TRAM rotor in hover and the UH-60A in forward flight.

Parallel Unsteady Overset Mesh Methodology for a Multi-Solver Paradigm with Adaptive Cartesian Grids

Abstract : This paper describes a new domain-connectivity module developed to support Chimera-based interfacing of different CFD solvers for performing time-dependent adaptive moving-body calculations of external aerodynamic flows. The domain-connectivity module coordinates the data transfer between different solvers applied in different parts of the computational domain -- body fitted structured or unstructured to capture viscous near-wall effects, and Cartesian adaptive mesh refinement to capture effects away from the wall. The CFD solvers and the domain-connectivity module are executed within a Python-based computational infrastructure. The domain-connectivity module is fully parallel and performs all its operations (identification of holes and fringe points, donor cell searches and data interpolation) on the partitioned grid data. In addition, the connectivity procedures are completely automated using the implicit hole-cutting methodology such that no user intervention or explicit hole-map specification is necessary. The capabilities and performance of the package are presented for several test problems, including flow over a NACA 0015 wing, AGARD A2 slotted airfoil, hover simulation of scaled V-22 rotor, and a dynamic simulation of UH-60A rotor in forward flight.

Strand Grid Solution Procedures for Sharp Corners

The strand/Cartesian-grid approach provides many advantages for complex moving-body-flow simulations, including fully automatic volume grid generation, highly scalable domain connectivity, and high-order accuracy. In this work, the authors evaluate methods of handling sharp corners with strand grids through combinations of strand vector smoothing, multiple strands emanating from a single surface node, and telescoping Cartesian refinement into corner regions of the near-body grid. A new discretization strategy is introduced to better tolerate mesh skewness induced by strand smoothing. These approaches are tested for unsteady, laminar, and high-Reynolds-number turbulent flows. For standard viscous high-aspect-ratio grids, smoothed strands with telescoping Cartesian refinement provide the most accurate results with the least complexity. Mesh discontinuities associated with the use of multiple strands at sharp corners produce more error than with smoothed strands. With both strand approaches—vector smoothing ...

Rotor Loads Prediction Using Helios: A Multisolver Framework for Rotorcraft Aeromechanics Analysis

This paper documents the prediction of UH-60A Black Hawk aerodynamic loading using the multisolver Computational Fluid Dynamics/Computational Structural Dynamics analysis framework for rotorcraft Helios for a range of critical steady forward flight conditions. Comparisons with available flight test data are provided for all of the predictions. The Helios framework combines multiple solvers and multiple grid paradigms (unstructured and adaptive Cartesian) such that the advantages of each paradigm is preserved. Further, the software is highly automated for execution and designed in a modular fashion to minimize the burden on both the users and developers. The technical approach presented herein provides details of all of the participant modules and the interfaces used for their integration into the software framework. The results composed of sectional aerodynamic loading and wake visualizations are presented. Solution-based adapative mesh refinement, a salient feature of the Helios framework, is explored fo...

Parallelization of a Three-Dimensional Flow Solver for Euler Rotorcraft Aerodynamics Predictions

An approach for parallelizing the three-dimensional Euler/Navier-Stokes rotorcraft computational fluid dynamics flow solver transonic unsteady rotor Navier-Stokes (TURNS) is introduced. Parallelization is performed using a domain decomposition technique that is developed for distributed-memory parallel architectures. Communication between the subdomains on each processor is performed via message passing in the form of message passing interface subroutine calls. The most difficult portion of the TURNS algorithm to implement efficiently in parallel is the implicit time step using the lower-upper symmetric Gauss-Seidel (LU-SGS) algorithm. Two modifications of LUSGS are proposed to improve the parallel performance. First, a previously introduced Jacobi-like method called data-parallel lower upper relaxation (DP-LUR) is used. Second, a new hybrid method is introduced that combines the Jacobi sweeping approach in DP-LUR for interprocessor communications and the symmetric Gauss-Seidel algorithm in LU-SGS for on-processor computations. The parallelized TURNS code with the modified implicit operator is implemented on two distributed-memory multiprocessor, the IBM SP2 and Thinking Machines CM-5, and used to compute the three-dimensional quasisteady and unsteady flowfield of a helicopter rotor in forward flight. Good parallel speedups with a low percentage of communication are exhibited by the code. The proposed hybrid algorithm requires less CPU time than DP-LUR while maintaining comparable parallel speedups and communication costs. Execution rates found on the IBM SP2 are impressive; on 114 processors of the SP2, the solution time of both quasisteady and unsteady calculations is reduced by a factor of about 12 over a single processor of the Cray C-90.