Roger C. Strawn

Computational Modeling of Hovering Rotor and Wake Aerodynamics

Steady-state Reynolds-averaged Navier ‐Stokes computations are presented for a range of UH-60A model-rotor testcasesinhover. Thecomputationsaredesignedto assessgrid-related effectson thenumericalresultsandemploy 1)structuredoversetgridswithhighresolution ontherotorblades,2 )asystematicvariationofgridresolutioninthe rotor wake, and 3 ) a systematic variation of outer-boundary locations. Computed rotor performance values agree very well with experimental measurements and show little sensitivity to either grid resolution or outer-boundary locations. However, the computations uniformly overpredict the blade sectional thrust near the rotor tip. This overprediction of blade tip thrust is explained by an analysis of the circulation distribution in the computed rotor wake system.

Tetrahedral and hexahedral mesh adaptation for CFD problems

This paper presents two unstructured mesh adaptation schemes for problems in computational fluid dynamics. The procedures allow localized grid refinement and coarsening to efficiently capture aerodynamic flow features of interest. The first procedure is for purely tetrahedral grids; unfortunately, repeated anisotropic adaptation may significantly deteriorate the quality of the mesh. Hexahedral elements, on the other hand, can be subdivided anisotropically without mesh quality problems. Furthermore, hexahedral meshes yield more accurate solutions than their tetrahedral counterparts for the same number of edges. Both the tetrahedral and hexahedral mesh adaptation procedures use edge-based data structures that facilitate efficient subdivision by allowing individual edges to be marked for refinement or coarsening. However, for hexahedral adaptation, pyramids, prisms, and tetrahedra are used as buffer elements between refined and unrefined regions to eliminate hanging vertices. Computational results indicate that the hexahedral adaptation procedure is a viable alternative to adaptive tetrahedral schemes.

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.

Unstructured Adaptive Mesh Computations of Rotorcraft High-Speed Impulsive Noise

A new method is developed for modeling helicopter high-speed impulsive (HSI) noise. The aerodynamics and acoustics near the rotor blade tip are computed by solving the Euler equations on an unstructured grid. A stationary Kirchhoff surface integral is then used to propagate these acoustic signals to the far field. The near-field Euler solver uses a solution-adaptive grid scheme to improve the resolution of the acoustic signal. Grid points are locally added and/or deleted from the mesh at each adaptive step. An important part of this procedure is the choice of an appropriate error indicator. The error indicator is computed from the flow field solution and determines the regions for mesh coarsening and refinement. Computed results for HSI noise compare favorably with experimental data for three different hovering rotor cases.

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.

HELICOPTER NOISE PREDICTIONS USING KIRCHHOFF METHODS

This paper presents two methods for predicting the noise from helicopter rotors in forward flight. Aerodynamic and acoustic solutions in the near field are computed with a finite-difference solver for the Euler equations. Two different Kirchhoff acoustics methods are then used to propagate the acoustic signals to the far field in a computationally-efficient manner. One of the methods uses a Kirchhoff surface that rotates with the rotor blades. The other uses a nonrotating Kirchhoff surface. Results from both methods are compared to experimental data for both high-speed impulsive noise and blade-vortex interaction noise. Agreement between experimental data and computational results is excellent for both cases. The rotating and nonrotating Kirchhoff methods are also compared for accuracy and efficiency. Both offer high accuracy with reasonable computer resource requirements. The Kirchhoff integrations efficiently extend the near-field finite-difference results to predict the far field helicopter noise.

Parallel Implementation of an Adaptive Scheme for 3D Unstructured Grids on the SP2

Dynamic mesh adaption on unstructured grids is a powerful tool for computing unsteady flows that require local grid modifications to efficiently resolve solution features. For this work, we consider an edge-based adaption scheme that has shown good single-processor performance on the C90. We report on our experience parallelizing this code for the SP2. Results show a 47.0X speedup on 64 processors when 10% of the mesh is randomly refined. Performance deteriorates to 7.7X when the same number of edges are refined in a highly-localized region. This is because almost all the mesh adaption is confined to a single processor. However, this problem can be remedied by repartitioning the mesh immediately after targeting edges for refinement but before the actual adaption takes place. With this change, the speedup improves dramatically to 43.6X.

Computational Modeling of HART II Blade-Vortex Interaction Loading and Wake System

Correlations using a loosely coupled trim methodology of the computational fluid dynamics (OVERFLOW-2) and computational structural dynamics (CAMRAD-II) codes are presented to calculate the helicopter rotor blade-vortex interaction airloads and wake system for the higher-harmonic aeroacoustic rotor test (HART II) rotor at an advance ratio of 0.15. Five different grid models are studied to quantify the effects of grid refinement on rotor-wake resolution. The fine grid model has a total of 113 million grid points and it improves airload predictions compared with the standard grid model for three HART II test cases: baseline, minimum noise, and minimum vibration. The rotor-wake positions are well predicted by this fine grid model. The computed vorticity field for a young vortex using the fine grid model is compared with the measured particle image velocimetry data and the results are good. The fine grid model underpredicts the experimental value for the maximum vorticity by 61%. The predicted vortex core radius is Is % in chord for the fine grid while the measured data show about 5% chord length. The predicted swirl velocity is, however, higher than the measured data for this vortex. The results in this paper provide the first quantitative comparisons between the measured and computational fluid dynamics/computational structural dynamics computed flowfield for a helicopter rotor-wake system.

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.

A Solution Adaptive Structured/Unstructured Overset Grid Flow Solver with Applications to Helicopter Rotor Flows

This paper summarizes a method that solves both the three dimensional thin-layer Navier-Stokes equations and the Euler equations using overset structured and solution adaptive unstructured grids with applications to helicopter rotor flowfields. The overset structured grids use an implicit finite-difference method to solve the thin-layer Navier-Stokes/Euler equations while the unstructured grid uses an explicit finite-volume method to solve the Euler equations. Solutions on a helicopter rotor in hover show the ability to accurately convect the rotor wake. However, isotropic subdivision of the tetrahedral mesh rapidly increases the overall problem size.