SCIENCE & ENGINEERING NEWS

The Department of Defense (DoD) has been providing high performance computing support since 1993. The High Performance Computing Modernization Program was initiated in response to congressional direction to modernize the DoD laboratories’ high performance computing capabilities. Its vision is to enable the DoD to maintain its technological supremacy over adversaries of the United States in weapons systems design.

The program fosters the flow of this technology into warfighting support systems by providing world-class high performance computing capability to the science and technology and test and evaluation communities. A good example of using these capabilities is the DoD Challenge Project Applied Computational Fluid Dynamics (AFCD) in Support of Aircraft and Weapons Integration being conducted by the Air Force at Eglin Air Force Base, Florida. The researchers for this challenge project are Major Mark Lutton and James M. Brock, Jr.

Background

This DoD Challenge Project is being conducted by the Air Force SEEK EAGLE Office. The SEEK EAGLE’s mission is to provide Flight Clearances to enable developmental and operational flight testing, and to provide aircraft/weapons Certification Recommendations to the operational commands, translating to a warfighting capability.

The SEEK EAGLE Office uses Computational Fluid Dynamics (CFD) techniques to obtain quick, cost effective aerodynamic data to support their mission. CFD is a computational method that numerically solves the equations of motion for a variety of aircraft with attached pylons, launchers, weapons, and other stores in support of the aircraft and store certification process.

The SEEK EAGLE Office has been extensively using computational fluid dynamics to provide the high-fidelity aerodynamics necessary to calculate aircraft store carriage loads, to simulate store separation trajectories, and to answer other questions related to aircraft and store compatibility. CFD is unique because it eliminates many of the limitations associated with wind tunnel testing and it has the capability to augment flight testing thereby reducing the weapon certification cost.

Technical Approach

The technical approach for this project is to combine a Euler or Navier-Stokes algorithm with a grid overlapping (Chimera) and embedding capability and a constrained six degree-of-freedom rigid body motion algorithm to reduce and optimize wind tunnel and flight test efforts. A degree of freedom is a displacement quantity, which defines the location and orientation of an object. In three-dimensional space, a rigid object has six degrees of freedom: three translations and three rotations. The code used for the project is the Beggar code, which was developed by the Air Force Research Laboratory, Armament Directorate at Eglin and has been extensively modified by the SEEK EAGLE computational fluid dynamics team.

Beggar provides the flexibility of using blocked grids, patched grids, overlapped grids, or a combination thereof, to decompose complex geometries into subdomains. Domain decomposition is used to ease grid generation tasks and to reduce computational requirements. Beggar has an integrated six degree-of-freedom model that has been coupled with the flow solver to provide a fully time-accurate store separation prediction capability.

When Beggar was first released, it was coded to run in a single-processor mode. However, the SEEK EAGLE Office made extensive modifications to the code so it could be used to solve moving body problems on a parallel computing platform using Chimera techniques. The Chimera scheme combines a system of relatively simple grids defining each geometrical component of the overall configuration to ease the initial grid generation tasks. The individual grids overlap, and once combined, cover the entire computation domain. The grid assembly process within Beggar is the key to successful implementation of a Chimera technique. The process provides grid hole cutting logic for Chimera grid systems, determines the interpolation stencils for these holes and associated boundaries, and establishes communication links between individual grids that are used to pass flow-field information during code execution.

Results/Summary

Over the past year, the Air Force SEEK EAGLE Office has performed various tasks under the Applied Computational Fluid Dynamics (AFCD) in Support of Aircraft and Weapons Integration challenge project. These tasks have ranged from evaluating the aerodynamic effects of canards on a high performance air-to-air missile to calculating and comparing the aerodynamic loads for missile variants mounted on the wingtip of the F-16 aircraft. The following is a brief description of the some of the tasks performed by the SEEK EAGLE Office. All of these tasks required high performance computing resources because of the size of the problems and the computations involved.

High Performance Missile Control.

The purpose of this effort was to evaluate the effects of forward-body canards on a high performance air-to-air missile. At large angles of attack, asymmetric vortices are generated by the missile forebody, which cause large out-of-plane loads on the missile. The addition of the canards was an attempt to alleviate these vortices and reduce the load. By solving the Navier-Stokes equations, the SEEK EAGLE Office predicted that the canards did eliminate the vortices on the forebody of the missile but the vortices were regenerated by the forward fins. The vortices were still asymmetric and produced slightly larger out-of-plane loads than the missiles that did not have the canards.

F-16 Wingtip Missile Carriage Loads.

The purpose of this effort was to calculate and compare the aerodynamic loads for five missile variants mounted on the wingtip of the F-16. These comparisons were used to determine the variation in carriage loads caused by each missile and to determine which missile caused the most critical aerodynamic load. The results showed that one of the missile variants did cause slightly larger loads for all the test conditions. This type of information was also deemed relevant for future use with the SEEK EAGLE Office carriage loads compatibility analyses by minimizing the analysis requirements and reducing or eliminating flight test requirements.

F-16 Fuel Tank Separation.

The purpose of this effort was to predict the separation characteristics of wing mounted fuel tanks when mounted in combination with developmental weapons on neighboring wing stations. The fuel tank pylon has a special release mechanism built into the rear connection point that forces the tank and pylon combination to rotate 13-degrees nose down before it is fully released from the wing. This mechanism is required to ensure that the tank will move away from the wing upon release. A partial separation trajectory using a viscous approach has been generated but the project is still underway and separation trajectories at various test conditions are still being generated.

F-18 Air-to-Ground Separation.

The purpose of this effort was to predict the separation characteristics of an air-to-ground bomb from the wing station of the F-18. Both inviscid and viscous trajectories were generated and compared to flight test telemetry data. As expected, the viscous results compared better from a quantitative standpoint to the flight test results because they take into account the viscous effects. However, the inviscid results predicted the overall separation trends very nicely. This information was important because an inviscid approach allows the computational fluid dynamics team to solve certain problems much cheaper and quicker.

Because the size of their efforts are large and the computations involved are complex, high performance computing resources are necessary for the SEEK EAGLE Office to perform their tasks quickly and efficiently. The SEEK EAGLE Office’s continued use of high performance computing resources will allow them to continue their mission of providing superior systems in support of the warfighter.

The Navier-Stokes equation is the primary equation of computational fluid dynamics, relating pressure and external forces acting on a fluid to the response of the fluid flow. Forms of this equation are used in computations for aircraft and ship design, weather prediction, and climate modeling.

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