Papers & Reports 2017

Structured-grid solutions obtained for the NASA Common Research Model for the 6th AIAA CFD Drag Prediction Workshop are detailed. Three different flow solvers were used amongst the contributors: OVERFLOW, ADflow, and elsA. The numerical methodologies and turbulence modeling strategies employed by each code are described. Key results for all authors include grid convergence studies for the drag increment of a nacelle and pylon added to a wing-body configuration and a buffet study accounting for static aeroelastic deformation. Additional studies performed include feature-based adaptive mesh refinement and higher-order convective flux discretization, among others. (Full paper requires login.)
Contributions to the 6th AIAA CFD Drag Prediction Workshop Using Structured Grid Methods,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

It is challenging to review and assimilate large data sets created by Computational Fluid Dynamics (CFD) simulations and wind tunnel tests. Over the past 10 years, NASA Ames Research Center has developed and refined a software tool dubbed the “MiniWall” to increase productivity in reviewing and understanding large CFD-generated data sets. Under the recent NASA ERA project, the application of the tool expanded to enable rapid comparison of experimental and computational data. (Full paper requires login.)
MiniWall Tool for Analyzing CFD and Wind Tunnel Large Data Sets,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

In this study, the ability of a high-order discontinuous-Galerkin spectral element solver to perform accurate wall-modeled large eddy simulation is investigated. Two different wall models are considered. The first approach is based on an equilibrium wall-stress model that uses the flow data away from the wall to reconstruct the wall shear stress. The second approach is a novel method called the slip/penetration wall, derived from the properties of a differential filter. This model allows to define a Robin boundary condition at the wall consistent with a filtered state. (Full paper requires login.)
Assessment of Wall-modeled LES Strategies Within a Discontinuous-Galerkin Spectral-element Framework,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

This paper presents a new wall model to compute turbulent boundary layers using the RANS equations in high Reynolds number flows. The model solves a two-point boundary value problem for a coupled set of equations for the streamwise velocity and the turbulent viscosity. Since it includes both the pressure gradient and the momentum balance of the full Navier-Stokes system, the ODE is valid farther from the wall. (Full paper requires login.)
An ODE-based Wall Model for Turbulent Flow Simulations,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

The meshing procedure for the baseline meshes of the Common Research Model (CRM) geometry for the 3rd AIAA High Lift Prediction Workshop using overset structured grids is described. Four grid systems at different grid resolutions are created for a full flap gap case, and a grid system at the medium resolution level is created for a partially-sealed flap gap case. (Full paper requires login.)
Best Practices on Overset Structured Mesh Generation for the High-Lift CRM Geometry,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

NASA and its industry partners are performing studies of supersonic aircraft concepts with low sonic boom pressure signatures. The interaction of the nozzle jet flow with the aircrafts’ aft components is typically where the greatest uncertainly in the pressure signature is observed with high-fidelity numerical simulations. An extensive wind tunnel test was conducted in February 2016 in the NASA Ames 9- by 7- Foot Supersonic Wind Tunnel to help address the nozzle jet effects on sonic boom. Five test models with a variety of shock generators of differing waveforms and strengths were tested with a convergent-divergent nozzle for a wide range of nozzle pressure ratios. (Full paper requires login.)
Computational Evaluations of Experimental Data for Sonic Boom Models with Nozzle Jet Flow Interactions,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

A wind tunnel test was conducted to investigate the complex interactions between a nozzle plume and shock waves of differing strengths generated from various aft surfaces typical of supersonic aircraft. These analytically-defined aft surfaces consisted of horizontal tails of different sizes and an aft deck. The interactions of the waveforms from these surfaces with the jet exhaust plume can have significant adverse effects on the loudness of the sonic boom if the surfaces are not carefully integrated into an aircraft design. (Full paper requires login.)
Nozzle Plume/Shock Interaction Sonic Boom Test Results from the NASA Ames 9- by 7-Foot Supersonic Wind Tunnel,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

Interactional aerodynamics has been studied for counter-rotating coaxial rotors in hover. The effects of torque balancing on the performance of coaxial-rotor systems have been investigated. The three-dimensional unsteady Navier-Stokes equations are solved on overset grids using high-order accurate schemes, dual-time stepping, and a hybrid turbulence model. Computational results for an experimental model are compared to available data. The results for a coaxial quadcopter vehicle with and without torque balancing are discussed. Understanding interactions in coaxial-rotor flows would help improve the design of next-generation autonomous drones. (Full paper requires login.)
Computations of Torque-Balanced Coaxial Rotor Flows,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

Computational simulations for the Second AIAA Aeroelastic Prediction Workshop using the structured overset grid solver within the Launch Ascent and Vehicle Aerodynamics (LAVA) framework are presented. Three test cases from the workshop are considered: case (1a) steady-state transonic flow over the Benchmark Super Critical Wing (BSCW), case (1b) unsteady forced oscillation of the BSCW, and case (3a) unsteady transonic flow over the BSCW including shock/boundary-layer separation effects. A sequence of three structured overlapping grid systems, including coarse, medium, and fine grids have been generated for the BSCW and applied to the first two test cases. (Full paper requires login.)
Overset Grid Simulations for the Second AIAA Aeroelastic Prediction Workshop,55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, Grapevine, Texas, January 9-13, 2017.

A modular procedure is presented to simulate moving control surfaces within an overset grid environment using the Navier–Stokes equations. Gaps are modeled by locally shearing the wing grids instead of using separate grids to model gaps. Grid movements for control surfaces are defined through a separate module, which is driven by an external grid generation tool. Results are demonstrated for a wing with a part-span control surface. Grids for the test case are determined from detailed grid sensitivity studies based on both nonoscillating and oscillating cases. Steady and, for the first time, unsteady pressures from overset grid computations are validated with wind tunnel data. This paper addresses the current needs of high-fidelity flow modeling to design advanced active-controls. (Full paper requires purchase.)
Modeling of Oscillating Control Surfaces Using Overset-Grid-Based Navier–Stokes Equations Solver,Journal of Dynamic Systems, Measurement, and Control, vol. 139, issue 3, March 2017.

We present the results of a search for extremely metal-poor (EMP), carbon-enhanced metal-poor (CEMP), and cataclysmic variable (CV) stars using a new exploration tool based on linked scatter plots (LSPs). Our approach is especially designed to work with very large spectrum data sets such as the SDSS, LAMOST, RAVE, and Gaia data sets, and it can be applied to stellar, galaxy, and quasar spectra. As a demonstration, we conduct our search using the SDSS DR10 data set. We first created a 3326-dimensional phase space containing nearly 2 billion measures of the strengths of over 1600 spectral features in 569,738 SDSS stars.
Exploring the SDSS Data Set with Linked Scatter Plots. I. EMP, CEMP, and CV Stars,The Astrophysical Journal Supplement Series, vol. 228, no. 2, February 9, 2017.

This paper presents results from a thorough validation of a computational approach to predict infrasonic signatures of centimeter-sized meteoroids. This is the first direct comparison of computational results with well-calibrated observations that include trajectories, optical masses, and ground pressure signatures. Overall, the results independently verify energy deposition estimates deduced from optical observations.
Numerical Prediction of Meteoric Infrasound Signatures,Planetary and Space Science, vol. 140, June 2017.

This paper describes computational performance studies of NASA's widely used OVERFLOW computational fluid dynamics code, run on the Pleiades Supercomputer. Two test cases were considered: a full launch vehicle with a grid of 286 million points and a full rotorcraft model with a grid of 614 million points. Computations using up to 8,000 cores were run on Sandy Bridge and Ivy Bridge nodes. Results for two grid topologies are presented and compared in detail. Observations and suggestions for future work are made.
Performance of the Widely-Used CFD Code OVERFLOW on the Pleiades Supercomputer,NASA Technical Memorandum NASA/TM—2017-219510, NASA Ames Research Center, May 2017.

The NASA Tetrahedral Unstructured Software System with the USM3D flow solver was used to compute test cases for the Second AIAA Sonic Boom Prediction Workshop. The intent of this report is to document the USM3D results for SBPW2 test cases. The test cases included an axisymmetric equivalent area body, a JAXA wing body, a NASA low boom supersonic configuration modeled with flow through nacelles and engine boundary conditions. All simulations were conducted for a free stream Mach number of 1.6, zero degrees angle of attack, and a Reynolds number of 5.7 million per meter. (Full paper requires login.)
USM3D Simulations for 2nd Sonic Boom Workshop,35th AIAA Applied Aerodynamics Conference, Denver, Colorado, June 5-9, 2017.

Because future wind tunnel tests associated with the NASA Juncture Flow project are being designed for the purpose of CFD validation, considerable effort is going into the characterization of the wind tunnel boundary conditions, particularly at inflow. This is important not only because wind tunnel flowfield non-uniformities can play a role in integrated testing uncertainties, but also because the better the boundary conditions are known, the better CFD can accurately represent the experiment. This paper describes recent investigative wind tunnel tests involving two methods to measure and characterize the oncoming flow in the NASA Langley 14- by 22-Foot Subsonic Tunnel. (Full paper requires login.)
Boundary Condition Study for the Juncture Flow Experiment in the NASA Langley 14x22-Foot Subsonic Wind Tunnel,47th AIAA Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

An outline of a strategy for automation of overset structured surface grid generation on complex geometries is described. The starting point of the process consists of an unstructured surface triangulation representation of the geometry derived from a native CAD, STEP, or IGES definition, and a set of discretized surface curves that captures all geometric features of interest. The procedure for surface grid generation is decomposed into an algebraic meshing step, a hyperbolic meshing step, and a gap-filling step. This paper will focus primarily on the high-level plan with details on the algebraic step. (Full paper requires login.)
Strategies Toward Automation of Overset Structured Surface Grid Generation,23rd AIAA Computational Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

Simulation results are presented for all test cases prescribed in the Second AIAA Sonic Boom Prediction Workshop. For each of the four nearfield test cases, we compute pressure signatures at specified distances and o↵-track angles, using an inviscid, embedded-boundary Cartesian-mesh flow solver with output-based mesh adaptation. The cases range in complexity from an axisymmetric body to a full low-boom aircraft configuration with a powered nacelle. (Full paper requires login.)
Cart3D Simulations for the Second AIAA Sonic Boom Prediction Workshop,35th AIAA Applied Aerodynamics Conference, Denver, Colorado, June 5-9, 2017.

We present a thorough validation of a computational approach to predict infrasonic signatures of centimeter-sized meteoroids. We assume that the energy deposition along the meteor trail is dominated by atmospheric drag and simulate the steady, inviscid flow of air in thermochemical equilibrium to compute the meteoroid’s near-body pressure signature. This signature is then propagated through a stratified and windy atmosphere to the ground using a methodology adapted from aircraft sonic-boom analysis. An assessment of the numerical accuracy of the nearfield and the farfield solver is presented. (Full paper requires login.)
Computational Modeling of Meteor-Generated Ground Pressure Signatures,23rd AIAA/CEAS Aeroacoustics Conference, Denver, Colorado, June 5-9, 2017.

This work presents an algorithm for generating a high-order, metric-conforming mesh that builds on previous works of Yano1 and Sanjaya–Fidkowski.2 The new contribution is a metric-based optimization approach for determining the location of high-order geometry nodes in a curved mesh. The proposed approach does not require additional infrastructure beyond that already available to support curved elements, and its computational cost is similar to metric-based optimization for linear elements. Furthermore, the process requires minimum tuning and can be automated. This paper focuses on understanding the concept of metric conforming meshes with curved elements (Full paper requires login.)
Error Minimization via Metric-Based Curved-Mesh Adaptation,23rd AIAA Computational Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

Simulating the dynamics of parachute deceleration systems requires solving a coupled multi-physics fluid-structure interaction problem. This work constructs a discrete structural model for parachute fabrics for monolithic coupling to a high-order spectral-element flow solver. The fabric is modeled as a shell structure, because shells are thin curved domains that support bending loads. However, the assumptions inherent to a shell structural model lead to a topologically two-dimensional formulation for the deformation, which depends on the second derivative of the deformation. (Full paper requires login.)
A C1-discontinuous-Galerkin Spectral-element Shell Structural Solver,24th AIAA Aerodynamic Decelerator Systems Technology Conference, Denver, Colorado, June 5-9, 2017.

Three shock-capturing formulations for an entropy-stable discontinuous-Galerkin spectral-element solver are examined: Galeao and Dutra do Carmo's cross-wind stabilization, Barter and Darmofal's differential equation for propagating and diffusing the artificial viscosity, and a novel method based on inter-element jumps of pressure and a modified Ducros-style sensor. (Full paper requires login.)
A DGSEM Shock-capturing Scheme for Scale-resolving Simulations,23rd AIAA Computational Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

Understanding the development of a turbulent mixing layer is essential for various aerospace applications. In particular, experiments found that the flow development is highly sensitive to inflow conditions, which are difficult to reproduce in the flow simulations. In previous direct numerical simulations (DNS) of temporarily and spatially developing turbulent mixing layers, idealized inflow conditions based on mathematical approximations of the mean velocity profile were used to facilitate turbulent flow conditions. The current paper presents results of DNS of a spatially developing turbulent mixing layer, where co-flowing laminar boundary layers over a splitter plate are used as inflow conditions; no artificial perturbations are seeded into the flow. The goal is to closer replicate a naturally developing mixing layer. (Full paper requires login.)
DNS of a Spatially Developing Turbulent Mixing Layer from Co-flowing Laminar Boundary Layers,47th AIAA Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

NASA through its Transformational Tools and Technologies Project (TTT) under the Advanced Air Vehicle Program, is supporting a substantial effort to investigate the formation and origin of separation bubbles found on wing-body juncture zones. The flow behavior in these regions is highly complex, difficult to measure experimentally, and challenging to model numerically. Multiple wing configurations were designed and evaluated using Computational Fluid Dynamics (CFD), and a series of wind tunnel risk reduction tests were performed to further down-select the candidates for the final experiment. This paper documents the CFD analysis done in conjunction with the 6 percent scale risk reduction experiment performed in NASA Langley’s 14- by 22-Foot Subsonic Tunnel. (Full paper requires login.)
CFD Analysis in Advance of the NASA Juncture Flow Experiment,47th AIAA Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

We present an implementation of a propulsor model based on body force method into the Overflow computational fluid dynamics code to model turbofan engines and propulsors of similar type. The model estimates the forces imparted on the fluid by the blade camber surfaces as a body force source terms in the momentum and energy equations over grids that represent the rotor and stator of a fan stage. We tested the implementation on three cases: 1) Source Diagnostics Test (SDT) fan with R4 rotors, which has extensive test data on aerodynamic performance and rotor wake surveys, 2) A stand-alone Aeronaut TF8000 propulsor which is an off-the-shelf propulsor used on model aircraft and 3) The D8 aircraft model with TF8000 propulsors placed in a wind tunnel. (Full paper requires login.)
Implementation of a Body Force Model in OVERFLOW for Propulsor Simulations,35th AIAA Applied Aerodynamics Conference, Denver, Colorado, June 5-9, 2017.

Hybrid RANS/LES simulations using the structured overset grid approach and a low-dissipation finite-di↵erence method within the Launch Ascent and Vehicle Aerodynamics (LAVA) solver framework are presented for jet noise prediction. The simulations are part of a validation effort to demonstrate jet noise prediction capability, to assess noise characteristics of next generation quiet supersonic commercial jets. Results are compared with experimental data acquired in the Small Hot Jet Acoustic Rig in the Aeroacoustic Propulsion Laboratory at NASA Glenn Research Center. (Full paper requires login.)
Jet Noise Prediction using Hybrid RANS/LES with Structured Overset Grids23rd AIAA Computational Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

Lattice Boltzmann (LB) and compressible Navier-Stokes (NS) equations based computational fluid dynamics (CFD) approaches are compared for simulating airframe noise. Both LB and NS CFD approaches are implemented within the Launch Ascent and Vehicle Aerodynamics (LAVA) framework. Both schemes utilize the same underlying Cartesian structured mesh paradigm with provision for local adaptive grid refinement and sub-cycling in time. (Full paper requires login.)
Lattice Boltzmann and Navier-Stokes Cartesian CFD Approaches for Airframe Noise Predictions,23rd AIAA Computational Fluid Dynamics Conference, Denver, Colorado, June 5-9, 2017.

This paper describes the process of modeling a parachute with payload as a three-degrees-of-freedom system suitable for time-accurate coupling with the Navier–Stokes flow equations. Flow solutions are computed by solving the Reynolds-averaged Navier–Stokes equations with structured overset grids. A time-accurate coupling procedure is validated by comparing the results with a frequency-domain approach. Results are given for a real parachute system, including a comparison with results based on the linear theory. This work advances the fidelity of analysis procedures beyond those in current use based on loose coupling with the Navier–Stokes equations.
Time-Accurate Coupling of Three-Degree-of-Freedom Parachute System with Navier–Stokes Equations,Journal of Spacecraft and Rockets, Vol. 54, No. 6, November–December 2017.

In this paper, the authors discuss the performance characteristics of an enhanced version of the astrophysics code Athena++, which employs an accurate and flexible direct solver for the radiation transport equation. They describe the radiation transport method and its parallelization. Scalability and performance analysis results demonstrate the impact of various parallelization strategies on performance. Results are presented for Intel Xeon and Xeon Phi (KNL) nodes.
Exploiting Multilevel Parallelism in Radiation Transfer Methods for Astrophysics Simulations,NAS Technical Report NAS-2017-01, NASA Ames Research Center, Moffett Field, CA.

Entry and breakup models predict that airburst in the Earth's atmosphere is likely for non-metallic asteroids with diameters up to approximately 200 meters. Objects of this size can deposit over 250 megatons of energy into the atmosphere. Fast-running ground damage prediction codes for such events rely heavily upon methods developed from nuclear weapons research to estimate the damage potential for an airburst at altitude. In particular, these tools rely upon the powerful yield scaling laws developed for point-source blasts that are used in conjunction with a Height of Burst (HOB) map to predict ground damage for an airburst of a specific energy at a given altitude. (Full paper requires login.)
Simulation-Based Height of Burst Map for Asteroid Airburst Damage Prediction,Acta Astronautica, vol. 156, March 2019.