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CFD Speeds US Navy Aircraft Modifications
Posted Mon July 22, 2002 @04:14PM
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Application Warren H. Davis, Ph.D., Aero CFD Principal Engineer for Northrop Grumman, leads a team responding to continual updates, additions, and enhancements to the U.S. Navy electronics surveillance workhorse, the E-2C Hawkeye. The Hawkeye provided stellar success in Afghanistan missions earlier this year.

Dr. Davis is one of the people who answers the “will it fly right” question when confronted with next generation modifications. He helps assure that any modifications to aero/propulsion capabilities, airframe additions including antennas, and other changes maintain and enhance performance.

Dr. Davis and his team gauge adaptations on the computer, using two remarkable programs (among other tools): Gridgen and Overflow. Gridgen is Pointwise Inc.’s grid generation and pre-processing CFD enabling bridge connecting computer aided design (CAD) software and CFD solvers such as Overflow, Fluent, STAR-CD, and CFX. The program generates hexahedral, tetrahedral, and hybrid (prisms and pyramids) meshes, reads geometry from CAD systems, and includes meshing tools and direct solver interfaces. Overflow is a Navier-Stokes flow solver that uses single block grids or Chimera overset (structured) grid systems. Chimera overset grids are body-fitted hexahedral grids around each component. They can overlap each other instead of requiring exact point-to-point interface matching like traditional structured grids. This makes the engineer’s grid generation task faster and easier while maintaining accuracy.

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“Applied engineers and vehicle designers need solid answers. These are the mechanisms by which we can model handling and performance testing and respond very quickly,” Dr. Davis explains. “Working together, Gridgen and Overflow are the best available tools for over-the-top response and absolutely reliable results.”

Now able to overcome formerly limited abilities to handle complex geometries, Davis and his team mesh, measure, and test scoops, props, the dome, inlets, and other complex configuration components.

On his plate: gauge and analyze prop blade variations, swirl effects, vortices, and incoming flow properties. Also, how would proposed changes to the propellers—changing the four-blade configuration to eight-blade on the E-2C’s co-rotating propellers—affect handling and performance? What would be the wing and tail surface effects? Also, with increased power requirements for an advanced radar installation, the need for alternate radar liquid cooling scoops required designing new, larger scoops to provide cooling via clean airflow. The original inlets were positioned just behind swirling flow from the propellers, engine nacelle and fuselage surfaces. How would additions or modifications to the scoops impact performance? Would a repositioning be advantageous for the larger scoops?

Rather than change the whole wind tunnel model to test these changes, Dr. Davis uses Gridgen’s high quality mesh cells to provide accurate simulations. “Our team can do the CFD calculations before the tunnel, with reliable, verifiable results,” he stressed.

Gridgen’s high accuracy was, is, and will be invaluable as the E-2C undergoes these major upgrades when combined with Overflow and ever-increasing computing power.

“Gridgen provides excellence both in initial surface grid generation and in its flexibility to fix any problems that arise at any stage of the process,” Davis emphasized. He credits Gridgen’s well-organized and intuitive interface and exceptional functionality. “We have the freedom to concentrate on producing the highest-quality grids in the shortest possible time.”

Flexibility is joined by time savings as far as Gridgen’s unique contribution to the process. At Pointwise, based in Fort Worth, Texas, developers recognized that one of the problems in the old days of CFD was that a detailed simulation like Davis’ could actually take longer than building and testing wind tunnel models. Gridgen’s unique ability is that it allows creation of millions of grid points in a relatively short period of time. Davis used 8.1 million grid points in his model. Just a few years ago, building a grid that large would have taken several months. Davis first began working with the early NASA version of Gridgen in 1991. The E-2C work began in 1997 with the early grid generation effort spread over a two-month effort.

With his current full configuration E-2C model variation study, Davis estimates the grid generation portion of the task has taken only three weeks.

Davis says he is continually amazed at the ease of handling and at the ability to fix problems that arise when integrating with other tools (such as CAD, flow solvers, even other grid generators).

Pointwise’s John Chawner sums it up: “We like to think Gridgen’s capabilities render many of the old empirical processes in the design cycle obsolete.”

propeller effects
Simulation of E-2C propeller effects.

scoop modification
Simulation of E-2C radar cooling scoop modification.

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