Timothy A. Eymann

Rigid and Maneuvering Results with Control Surface and 6DoF Motion for Kestrel v2

This paper documents the second version of a new integrating product that allows cross-over between simulation of aerodynamics, dynamic stability and control, structures, propulsion, and store separation. The Kestrel software product is an integrating product written in modular form with a Python infrastructure to allow growth to additional capabilities as needed. Computational efficiency will also be improved by targeting the next generation peta-flop architectures envisioned for the 2010+ time frame. The need for Kestrel developed from the fact that existing computational resources (hardware and CSE software) are insufficient to generate decision data in a timely enough way to impact early-phase and even many sustainment phase acquisition processes. Kestrel is also targeted to the need of simulating multi-disciplinary physics such as fluid-structure interactions, inclusion of propulsion effects, moving control surfaces, and coupled flight control systems. The Kestrel software product is to address these needs for fixed-wing aircraft in flight regimes ranging from subsonic through supersonic flight, including maneuvers, multi-aircraft configurations, and operational conditions. Version 2.0 adds a 6 Degree of Freedom (6DoF) component to provide a predictive motion capability and a moving control surface capability to allow more realistic aircraft simulations. Version 2.0 also provides parallel scalability performance improvements for moving and deforming mesh use cases. Results of a C-17 cargo aircraft and a GBU-38 Joint Direct Attack Munition (JDAM) are provided for static rigid body conditions. Results are also presented for a NACA0015 wing with a moving flap control surface and a 6DOF predictive motion of a MK-82 gravity bomb in freefall. Finally, parallel performance is demonstrated with a 24 million cell F-16 simulation with up to 2000 cores for static and moving conditions.

Active Flux Schemes.

A class of active flux schemes is developed and employed to solve model problems for the linear advection equation and non-linear Burgers equation. The active flux schemes treat the edge values, and hence the fluxes, as independent variables, doubling the degrees of freedom available to describe the solution without enlarging the stencil. Schemes up to third order accurate are explored. Oscillations generated by the higher-order schemes are controlled through the use of a characteristic-based limiter that preserves true extrema in the solution without excessive clipping.

Multidimensional active flux schemes

We build on active flux (AF) schemes and extend the method to the two-dimensional linear advection, linear acoustics, and linearized Euler equations. The active flux method independently updates edge and centroid values. Because the interface fluxes are calculated from independently updated quantities, we say that they are actively updated, as opposed to a more traditional finite-volume scheme where the fluxes are updated passively from the conserved values. The one-dimensional active flux method is reformulated using Lagrange basis functions and the basic features of the method are reviewed. The two-dimensional formulation also uses standard basis functions, but includes a bubble function to maintain conservation. A novel approach for solving the linear wave system is presented that uses spherical means to compute the edge updates for the flux calculation. We demonstrate that the AF method is third-order accurate for advection, acoustics, and the linearized Euler equations.

Relative Motion Simulations Using an Overset Multi- mesh Paradigm With Kestrel v3

This paper documents the third version of an integrating product that allows cross-over between simulation of aerodynamics, dynamic stability and control, structures, propulsion, and store separation. The Kestrel software product is an integrating product written in modular form with a Python infrastructure to allow growth to additional capabilities as needed. Computational efficiency will also be improved by targeting the next generation peta-flop architectures envisioned for the 2010+ time frame. The need for Kestrel developed from the fact that existing computational resources (hardware and CSE software) are insufficient to generate decision data in a timely enough way to impact early-phase and even many sustainment phase acquisition processes. Kestrel is also targeted to the need of simulating multi-disciplinary physics such as fluid-structure interactions, inclusion of propulsion effects, moving control surfaces, and coupled flight control systems. The Kestrel software product is to address these needs for fixed-wing aircraft in flight regimes ranging from subsonic through supersonic flight, including maneuvers, multi-aircraft configurations, and operational conditions. Version 3.0 adds a relative motion capability for multiple bodies (e.g. aircraft store separation, aircraft cargo release). The relative motion capability utilizes PUNDIT, an implicit hole cutting domain connectivity component to calculate donor and receptor cells, as well as interpolation weights for data transfer between the respective meshes. Preliminary multi-mesh calculations are presented, as well as solutions to demonstrate improvements of the kAVUS solver for steady and unsteady simulations.

Active Flux Schemes for Systems

We introduce a new formulation of active flux schemes and extend the idea to systems of hyperbolic equations. Active flux schemes treat the edge values, and hence the fluxes, as independent variables, doubling the degrees of freedom available to describe the solution without enlarging the stencil. Schemes up to third order accurate are explored. The limiter employed uses solution characteristics to set the bounds for the edge updates. The process reduces to simply accessing the solution history from memory and ensuring that the updates stay within the bounded range. The limited edge values are then used to construct the fluxes that conservatively update the centroid value. Using data that most closely follows the solution characteristics allows the limiter to better maintain true extrema in the solution. The scheme is used to generate 1-D solutions for the linear advection, Burgers’, and Euler equations.

CREATETM-AV Kestrel Dual-Mesh Simulations of the NASA Common Research Model

We use NASA’s Common Research Model and initial conditions from the sixth Drag Prediction Workshop (DPW VI) to investigate CREATETM-AV Kestrel’s ability to accurately compute the drag coefficient for a transport aircraft. We leverage Kestrel’s enhanced dual mesh capability to wrap the near-body, unstructured mesh with a higher-order Cartesian background. The dual-mesh simulation not only reduces the overall degrees of freedom in the system compared to the fully unstructured mesh but also enables feature-based mesh adaptation. This paper also explores the effects of subset distance and shock-based adaptation on the solution. Kestrel’s drag coefficient values for the configurations with and without the nacelle compare very favorably to results from DPW VI and verify that Kestrel dual-mesh simulations produce accurate answers while minimizing user effort.

Numerical Investigation of Rail Influence on Initial Launch Forces and Moments for Subscale Aerial Drone with Rocket Assisted Take -Off (RATO)

A numerical simulation of the launch rail influence on the initial forces and moments of a subscale aerial target during rocket assisted take -off (RATO) initiation is presented. Typically, t he turbine engine thrust is below the center of gravity of the target which cause s a nose up pitching moment during launch. The RATO angle is set to offset this moment. For launch, the turbine engine is started and the power increased to 95%. Once the turbine engine stabilizes, the RATO is ignited and drone launch is initiated. Achieving the correct geometry for the launch angle of the RATO is impe rative to a successful launch. In order to set the correct RATO angle, other forces and moments acting on the drone during the initial launch phase need to be id entified. Previous efforts identified the source and effect of an external pitching moment (EPM) generated by the turbine engine exhaust impinging on the RATO main body when launched from a symmetrical rail. In addition to identifying the effects of the EPM from a non -symmetrical rail, the effects of the rail on other axes of yaw and roll are also important. This paper presents an analysis of the effects of unsymmetrical rail geometry on the initial forces and moments for a drone during initial launch. Further analyses is also provided by altering the geometry of the rail as well as the height to determine if changes in rail geometry can mitigate any effects on the initial forces and moments.

Numerical Investigation of Launch Dynamics for Subscale Aerial Drone with Rocket Assisted Take-Off (RATO)

A numerical simulation of the trajectory of a subscale aerial target during rocket assisted take-off (RATO) is presented. Typically, the turbine engine thrust is below the center of gravity of the target which causes a nose up pitching moment during launch. The RATO angle is set to offset this moment. For launch, the turbine engine is started and the power increased to 95%. Once the turbine engine stabilizes, the RATO is ignited and drone launch is initiated. Achieving the correct geometry for the launch angle of the RATO is imperative to a successful launch. Previous efforts identified the source and effect of an external pitching moment (EPM) generated by the turbine thrust impinging on the RATO main body. This paper presents an analysis of the wind direction, center of gravity location, and launch rail effects on the first 0.5 seconds of the drone trajectory. Relatively minor changes to the lateral center of gravity location and ambient wind direction are shown to significantly affect the drone’s trajectory. New capabilities of the Beggar (6+)DOF are leveraged to simulate a time accurate trajectory off the BQM-34 launch rail from engine ignition to 0.5 seconds.