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CFD Used to Develop Airflow and Acoustics Control on Fighter Aircraft
Posted Mon May 15, 2006 @03:43PM
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Application by Bob Cramblitt

Combustion Research and Flow Technology, better known as CRAFT Tech, says it likes to swim in data.

Not just any data, of course. CRAFT Tech immerses itself in terabyte-sized datasets used to simulate complex fluid dynamic and combustive problems. To swim in these waters requires large-scale, parallel architecture computers running specialized CFD codes and the most advanced visualization software.

A recently completed project called HiFAST (High Frequency Acoustic Suppression Technology) showcased CRAFT Tech’s ability to take full advantage of CFD visualizations at every phase of weapons system development. The project, conducted for the Air Force Research Laboratory’s Air Vehicles Directorate (AFRL/VAAI), culminated in a successful flight test of a device that controls internal weapons bay airflow and acoustics.

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A new level of knowledge

HiFAST built on knowledge gained from the Active Separation Control (ASC) project, which led to a spoiler successfully tested in a Royal Australian Air Force (RAAF) F-111 fighter jet. The spoiler injects high-pressure air into the F-111’s internal weapons bay to neutralize turbulence and ensure safe weapon separation.

With HiFAST, CRAFT Tech extended ASC research by developing a second-generation device that reduces weapons bay acoustics and requires less airflow than the ASC device. CFD was used throughout the project: from conceptual development, to validating wind tunnel tests, to supporting full-scale hardware testing.

The HiFAST project was a breakthrough because it went beyond the knowledge that a particular control system worked. It created a process for establishing how and why a design works. With that knowledge, engineers can make changes to a design and run “what if” scenarios.

“We can use these high-powered simulations to make design changes without being afraid of inaccuracies,” says Raj Sinha, vice president and technical director at CRAFT Tech. “We don’t do simulation to replace other testing; we use it to develop knowledge of how to make a system behave the way we want it to.”

For the HiFAST project, that behavior involved two factors for the weapons bay: acoustics, and safe weapons separation. Noise from turbulent airflow fatigues metal and electronics systems, and makes it difficult to eject weapons. The need for weapons separation is obvious, but the issue has become more complex over the past decade as weapons have become smaller and more susceptible to aerodynamics.

The opening of an internal weapons bay causes highly sheared, turbulent airflow, according to Roger Birkbeck, senior design engineer at CRAFT Tech and program lead for the HiFAST project.

“Think of what would happen if you were driving in your car at 60 mph and threw a styrofoam cup out the window, or the ‘wap, wap, wap’ effect you get when you roll a car window down. Then, imagine that kind of effect at supersonic speeds.”

Opening weapons bay doors during flight creates an unstable transition area called a shear layer between the high-speed flow outside the bay and the slower flow inside. Pockets of rotating air, called vortices, rebound off the weapons bay walls, adversely affecting weapons pitch and creating acoustic waves that can damage the aircraft, its electronics systems, and even the weapons themselves.

CRAFT Tech’s mission with HiFAST was to simulate airflow and acoustical scenarios, find out what caused problems, and design hardware that would provide a workable solution at supersonic speeds.

Anatomy of a simulation

As with all simulations, CRAFT Tech’s start with a 3D solid model. Although with the HiFAST project, it wasn’t that simple. The only fighter jet with an internal weapons bay is the F-111, still used by the RAAF, but long retired from service in the United States. The F-111 was developed in the mid-1960s, before CAD existed.

CRAFT Tech had to obtain measurements from an F-111 kept in storage by the U.S. Air Force. Based on the measurement data, a 3D model was created in SolidWorks software. The CAD model was imported into Gridgen software for meshing, then into CRAFT Tech’s CFD solvers, CRUNCH and CRAFT.

CRUNCH, a multi-element, unstructured-grid Navier-Stokes code for complex geometries and multi-body problems, was used for solving weapons separation problems. CRAFT, a structured-grid Navier-Stokes code, was used for acoustical datasets that require higher accuracy.

“Acoustic simulation requires a fifth order of accuracy, compared to a second or third order for normal CFD,” says Sinha. “CRAFT enables us to handle 4D data – 3D models with the added dimension of changing phenomena over time.”

CRAFT Tech computed the massive airflow and acoustical problems using a combination of its in-house cluster of 96 Pentium-based PC processors and a dedicated T1 line providing access to DoD/NASA supercomputers.

Swimming in the data

Results from the simulations were imported into CEI’s EnSight Gold visualization software through interfaces to CRAFT and CRUNCH. EnSight Gold’s ability to take advantage of parallel processing – on both supercomputers and clusters – was the key to visualizing the complex datasets.

“Being able to use parallel processing to visualize unsteady phenomena is vital to our work,” says Sinha. “Looking at data at a single point in time is trivial. EnSight Gold enables us to animate the data to look at underlying fluctuations. We can create isosurfaces and vertices and track the evolution of quantities over space and time.”

A user-defined reader within EnSight Gold gave CRAFT Tech access to the variables it wanted to analyze, and cutting planes enabled those variables to be isolated for detailed study. CRAFT Tech also used multiple viewports that gave engineers the ability to look at different flow fields simultaneously.

“Being able to look at a large volume of data on the fly is key,” says Sinha. “That’s what we call ‘swimming in the data’.”

CRAFT Tech’s simulations led to a solution that uses two 16-inch wide spoilers, each with two HiFAST air nozzles. The spoilers are installed in a false bulkhead in the front of the weapons bay. When the weapons bay opens, the spoilers descend a few inches and the HiFAST nozzles blast pressurized air in pulsating frequencies. The air from the HiFAST nozzles neutralizes turbulence within the bay, and reduces acoustics.

Translating into production

With the HiFAST design verified in real-world conditions, CRAFT Tech is working on a project called SEAR (Separation Enhanced Acoustic Reduction), aimed at creating a system that can be adapted to current and future aircraft. As demanding as the HiFAST project was, stakes are higher for SEAR, according to Srini Arunajatesan, the research scientist in charge of visualization at CRAFT Tech.

On the front end, CFD tools need to isolate calculations, and allow for separations and curvature effects. The range of calculations is wider – they must take into account the geometry for different types of aircraft, and then isolate the weapons bay, the weapons themselves, the actuator, and other factors.

Fortunately for CRAFT Tech, the pace of computing power and the software that takes advantage of it continues to advance at a blistering pace. CEI recently announced that it set a new 3D rendering record at Lawrence Livermore National Labs, reaching 3.17 billion polygons per second on a cluster of 76 standard dual-processor PCs.

That kind of computing and visualization power will no doubt make swimming in terabyte-sized datasets a lot easier for Sinha and his team.

F111 weapons bay
Close up picture of an F-111 weapons bay during testing.


weapon seperation
weapon seperation
weapon seperation
weapon seperation
A series of images from the weapon separation simulation.

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